science universe

Magnetism and Electromagnetism

15–1 ELECTRICITY AND MAGNETISM

One of the most familiar and most frequently used effects of electric current is its ability to produce the force we call magnetism. This force is responsible for the operation of motors, generators, electrical measuring instruments, communication equipment, transformers, and a great variety of electrical control devices.

All magnetism is essentially electromagnetic in origin. Electromagnetism results from the energy of motion of electrons. In fact, every time a current flows through a wire, there are magnetic forces at work. Electrical current and its associated magnetic forces are inseparable.

Because of this close relationship between electricity and magnetism, and due to certain similarities, some students tend to confuse one with the other. Some of these pitfalls will be pointed out to you as you progress with this chapter.

To begin our study of magnetism, we will investigate some of the earliest-known properties of magnetism and then explore how these properties can be explained by the action of electrons.

15–2 SIMPLE MAGNETS

A magnet is a piece of material that attracts a number of other materials such as iron, steel, nickel, cobalt, and a few minerals and alloys. Magnets do not attract copper, aluminum, wood, or paper. In fact, magnets have no effect on most substances. Magnetic attraction is quite unlike electrical attraction, which affects all materials.

The force of the magnet is strongest at two areas on the magnet called the poles. If a magnet is supported in the center by a string or is on a pivot, one of its poles turns toward the north and the other pole turns toward the south. Thus the end of the magnet pointing to the north is called the north pole, and the other pole is called the south pole. The needle of a compass is just a lightweight magnet (strip of magnetized steel) mounted on a pivot.

If a compass or magnet is brought near another compass or magnet, the north end of one compass repels the north end of the other compass and attracts its south end, as shown in Figure 15–1. Similarly, the south pole of one magnet (or compass) will repel the south pole of another magnet and attract the north pole. This effect is summarized in

the magnetic attraction and repulsion law: Like poles repel and unlike poles attract. (Even though this law appears to be similar to the electrical attraction and repulsion law, remember that magnets and electrical charges are different.)

The term poles means points where opposite properties exist, such as in the positive and negative poles of a battery or the north and south geographic poles of the Earth. The poles of a magnet could have been assigned names other than north and south. In fact, it would be less confusing if the poles had been given a pair of opposite names such as black and white, or right and left. The geographic poles of the Earth are the ends of the axis on which the Earth turns; they are not areas of magnetic attraction. The Earth does have magnetic poles, however. There is a place in northern Canada that has the same kind of magnetic force as the south pole of a steel magnet; similarly, there is a place in the Antarctic that has the same kind of magnetic force as the north pole of a steel magnet.

15–3 THE MAGNETIC FIELD

You should recall from our discussion of electrical charges that the attraction and repulsion of electrical charges was explained by the existence of an invisible field of force between the charges. The pattern of an electrostatic field was shown in Section 3–8, Figure 3–13.

Similarly, force existing in the space around a magnet is shown by the pattern resulting when iron filings are sprinkled on a card placed over a magnet, as shown in Figure 15–2. Compare the similarities of these phenomena, but keep in mind that we are dealing with two entirely different forces. Magnetism is not the same force as the attraction and repulsion forces caused by static electrical charges.

These lines of force, often referred to as flux lines, have specific characteristics attributed to them.

• Flux lines are directional. They are said to exit from the north pole and enter into the south pole, forming a closed loop through the magnet.

• Flux lines do not cross each other.

• There is no insulator for magnetic flux. It passes through all materials.

• Flux lines act like stretched rubber bands; they tend to contract.

• The flux density, or concentration of flux lines at a point, determines the amount of magnetic force. The greater the concentration of flux lines, the stronger the magnetic field. Flux lines are most densely concentrated at the poles.

• Flux lines facing the same direction attract each other, but flux lines facing opposite directions repel each other.

• The concentration of flux lines, and therefore the strength of a given magnet, is limited. When a magnet achieves maximum flux density, it is said to be saturated.

15–4 FERROMAGNETIC MATERIALS AND THE MAGNETIZING PROCESS

Iron, nickel, cobalt, and some oxides and alloys are called ferromagnetic materials. A magnet is a piece of ferromagnetic material that has magnetic poles developed in it by placing it inside a current-carrying coil of wire or by placing it near another magnet.

Early experimenters found that heating or hammering a magnet causes the mag- net to lose some of its strength. Both of these processes disturb the atoms of the metal. Furthermore, it was found that if an ordinary steel bar magnet (or any magnet) is cut into fragments, each fragment has a north pole and a south pole, as shown in Figure 15–3. If we can continue to cut this material into smaller pieces, eventually we will reach the smallest possible fragment of iron, an atom. Thus, scientists stated that all atoms of magnetic materials are themselves permanent magnets.

In an unmagnetized piece of iron, the atoms of iron are arranged in a disorganized fashion; that is, the north and south poles of these atom-sized permanent magnets point in all directions, as shown in Figure 15–4A. When the iron is magnetized, the atoms are rotated and aligned so that the north pole of each atom faces in the same direction, as shown in Figure 15–4B.

If a magnet is cut without disturbing the atom arrangement, the atomic south poles are exposed on one side of the break and the north poles are exposed on the other side. Before the magnet is cut, these poles exert their attractive forces on each other so that there is no force reaching out into space around them.

Some of the previous conclusions about magnets have changed slightly over the years because of the discovery of a degree of order in an unmagnetized piece of iron. Within a crystal grain of iron, several thousand atoms form a group called a magnetic domain. Within one domain, the atoms are lined up with the north poles all facing in one direction. This group of atoms acts like a minute permanent magnet.

15–5 MAGNETIC MATERIALS AND THE ATOMIC THEORY

Why do atoms of a magnetic material behave like iron magnets? The answer to this question is the result of a long series of complex scientific investigations of the behavior of electrons in atoms. All electrons are constantly spinning on their own axes within an atom. This spin is the reason that each electron is a tiny permanent magnet, as illustrated in Figure 15–5. In most atoms, electrons spinning in opposite directions form pairs. In other words, their north and south poles are so close together that their magnetic effects cancel out, as far as any distant effect is concerned. (Compare this situation with two permanent bar magnets placed together with their north and south poles adjacent to each other.)

An atom of iron contains 26 electrons. Twenty-two of these electrons are paired. Each electron of a pair spins in a direction opposite to that of the other electron so that the external magnetic effect is canceled. In the next-to-the-outermost ring of electrons, 4 electrons are uncanceled. These 4 electrons, because they are spinning in the same direction, are responsible for the magnetic character of the atoms of iron.

There is still a great deal more to discover about electrons in atoms. Electron spin directions in an atom are affected by temperature and by the presence of other atoms. At 1,420°F, iron loses its magnetism due to a rearrangement of electron spin patterns. Strongly magnetic alloys and compounds have been made from elements that are either weakly magnetic or not magnetic at all in their uncombined form.

14–6 MAINTENANCE-FREE BATTERIES

The previous discussions, “Battery Testing” and “Battery Care,” become irrelevant when dealing with maintenance-free batteries, which have become popular in recent years. This kind of battery, although based on the same chemical process as the lead-acid cell, offers distinct ad- vantages over the traditional lead-acid battery. The sealed design prevents leakage of electrolyte and acid fumes, helping to eliminate corrosion problems. Since there is no loss of water, the electrolyte never needs replenishment. In addition, a well-sealed battery can be installed in any position and still function efficiently without danger of electrolyte spillage.

These features are possible because of the unique construction of the cells. The thin, low-volume separators that are normally placed between the electrodes of ordinary lead- acid batteries are replaced with thick, highly absorbent, feltlike, glass fiber mats. These separators absorb all the electrolyte, leaving no excess liquid to slosh around. Such cells are said to operate in a starved electrolyte mode, because there is only enough electrolyte to maintain the rated capacity of the cell. The cell is then sealed.

One may ask what happens to the hydrogen and oxygen gases that are generated during the charging of a lead-acid battery and become trapped inside the gas-tight container. The answer is that the design of the sealed battery allows the gases to recombine internally, with the result that water, rather than being lost, is electrochemically recycled. This is achieved by allowing the oxygen, which is generated at the positive plate, to migrate toward the negative plate, where it combines with the pure lead grids used in this construction. Likewise, the hydrogen will react with the lead dioxide of the positive plate, although at a somewhat lower rate. This recombination of gases is possible because there is just enough electrolyte to cover the plates and the glass mat separators, enabling the gas transfer between the plates. To encourage the recombination of the gases, the battery operates at a slight internal pressure — between 30 and 50 pounds per square inch. This condition is maintained by a pressure relief valve that opens only when the recommended charging rates are greatly exceeded.

It is important to note that not all “maintenance-free” batteries are perfectly sealed; some are vented to allow the escape of gases in the event of an overcharge. Since these gases are derived from the decomposition of the water in the electrolyte, such batteries can lose water, which cannot be replaced, thereby shortening their life span.

Nickel-Cadmium and Nickel-Iron (Edison) Storage Batteries

These truly long-life batteries are unaffected by the mechanical or electrical misuse that spoils a lead battery. But they cost several times as much as lead batteries of similar size. Nickel and cadmium are much more costly than lead. In addition, expensive manufacturing processes are required for these batteries.

The nickel-iron (Edison) cell was originally developed in 1899 for electric motor-driven vehicles. Research was aimed at producing a battery having low weight and volume for a given ampere-hour rating. One that could withstand repeated cycling (frequent charging and complete discharging) was especially desired.

The Edison cell is structurally stronger and lighter in weight than lead cells of the same current rating. The negative plates consist of a nickeled steel grid containing powdered iron, with some FeO and Fe(OH)2. The iron is the source of the electrons, which are

attracted through the external circuit toward nickel ions, Ni11 and Ni111, on the positive

plate. The positive plates are nickel tubes containing a mixture of nickel oxides and hydroxides, with flakes of pure nickel for increased conductivity. The electrolyte solution is 21% KOH (potassium hydroxide, caustic potash), which is chemically a base rather than an acid. The Edison cell is therefore called an alkaline cell.

The Edison cell has two disadvantages: the initial cost is high, and the maximum current is limited by high resistance, especially when the cell is cold. It is not suitable for starting gasoline or diesel engines, because its internal resistance puts a limit on its current output. These disadvantages limit its use. However, the Edison cell has an advantage that makes it useful for specific purposes. It is not damaged by remaining in a discharged condition; there- fore, it is useful in some portable lighting equipment and in a few marine installations, where it neither receives nor needs the attention that lead cells require. It is also appropriate for running traction equipment, such as mine locomotives and forklift trucks.

The nickel-cadmium (ni-cad or Ni-Cd) battery (Junger & Berg, Sweden, 1898) was developed not for frequent cycling but rather for general purposes. It enables the user to draw as many amperes as possible from a battery of given ampere-hour rating, without an excessive decline in voltage.

Both sets of plates are mechanically alike. The active materials are held in finely perforated, thin, flat steel pockets that are locked into a steel frame. The active material put in the positive plate is nickel hydroxide mixed with graphite to improve conductivity. Cadmium oxide is put into the negative plates. The electrolyte is potassium hydroxide (KOH) of about

specific gravity. When the cell is charged, the electrons forced onto the negative plate combine with Cd++ ions of the cadmium oxide (CdO), converting them to uncharged cadmium metal atoms. On the positive plate, electrons are removed from Ni++ ions of Ni(OH)2, converting them into more strongly positively charged Ni+++ ions. (The compound changes to Ni(OH)3.) Therefore, the active materials in the charged cell are

The H2O that is formed stays tied up in combination with the Ni(OH)2 in solid form on the plate and does not dilute the electrolyte; therefore, the density of the electrolyte remains constant, and a hydrometer does not indicate the amount of charge.

The nickel-cadmium battery is similar in its chemical composition to the Edison cell. Both contain alkaline electrolytes. Its electrical characteristics are similar to those of the lead cell; therefore, its applications are competitive with those of the lead cell. Like the Edison cell, the nickel-cadmium battery is chiefly used in industrial service. Its ruggedness, long life, and low-maintenance cost outweigh the high original cost. (Nickel-cadmium batteries cost nearly twice as much as do lead acid batteries of comparable energy ratings.)

Nickel-cadmium cells are marketed in the popular sizes of zinc-carbon cells (sizes AA, A, C, and D) for replacement in toys and consumer appliances. The buyer should con- sider the following potential disadvantages:

1. For any given number of cells, the voltage output is only 80% that of dry cells.

2. Unused nickel-cadmium cells tend to lose a significant amount of their emf when standing idle for several days. This effect is aggravated by higher temperatures.

3. Nickel-cadmium batteries tend to remember repeated demands for low output and sometimes persist in delivering such low levels, even though they are designed to meet the needs of increased output.

4. Nickel-cadmium cells do exhibit one negative characteristic. They remember their charge/discharge cycles. If they are used at only low currents, are permitted to dis- charge through only part of a cycle, and then are recharged, over a period of time they will develop a characteristic curve to match that cycle; see Figure 14–15.

On the positive side, it should be mentioned that nickel-cadmium batteries are good for nearly 2,000 recharge cycles and can be left on a trickle charge for indefinite periods of time.

Commercially, the nickel-cadmium battery is used in railroad signals systems; fire alarm systems; relay and switchgear operation; missile controls; aircraft engines; and diesel engines in locomotives, buses, and oil well pumps. Figure 14–16 shows a large, industrial- type cell. Nickel-cadmium cells in smaller, sealed, cylindrical forms, with no problems of gas or spillage, are used in a great variety of communication equipment and portable appliances.

Nickel-Metal Hydride Cells

Nickel-metal hydride cells (Ni-MH) are similar to nickel-cadmium cells in many respects. Both exhibit a voltage of 1.2 volts per cell and both have very similar charge and discharge curves. Nickel-metal hydride cells exhibit some improved characteristics, however. They have about a 40% higher energy density, and redundant memory accumulation is not as great a problem. Nickel-metal hydride cells are also more environmentally friendly. The posi- tive electrode is made of nickel oxyhydroxide (Ni-OH) and the negative electrode is made of metal hydride. The electrolyte is an aqueous (watery) potassium hydroxide solution. Nickel- metal hydride cells are replacing nickel-cadmium cells in many applications.

Lithium-Ion Cells

Lithium-ion cells are very popular for portable equipment such as notebook computers, video cameras, cell phones, and many others. Lithium-ion cells can be recharged and offer a very high energy density for their size and weight. They exhibit a voltage of 3.6 volts per cell, which is the same voltage that can be obtained by connecting three nickel- cadmium or three nickel-metal hydride cells in series. Lithium-ion cells also exhibit a weight-energy density that is about three times greater than nickel-cadmium cells. Under proper charging conditions, these cells can be recharged about 500 times. They also exhibit

a rather flat discharge curve, as shown in Figure 14–17, which makes them an ideal choice for electronic devices that require a constant voltage. Unlike nickel-cadmium cells and to some extent nickel-metal hydride cells, lithium-ion cells do not have the problem of memory accumulation. They can be recharged to their full capacity each time they are charged.

Lithium-ion cells can be safely recharged because they do not contain metallic lithium. The positive electrode (or cathode) is made of lithium metallic oxide and the negative electrode (or anode) is made of carbon. These cells work by transferring lithium ions between the cathode and the anode during discharging and charging. Although lithium-ion cells are safe to recharge they do require a special chargers that generally produce a constant voltage and constant current. Too much current or voltage during charging can cause early deterioration of the cell.

14–7 MISCELLANEOUS ASPECTS OF BATTERIES

Internal Resistance (R₁) of Cells

Cells themselves have an internal resistance that enters into a circuit calculation. When electrons flow from the negative plate to the positive plate, there is a movement of ions (cur- rent) in the electrolyte in the cell. This movement in the cell, like any current, does not occur with perfect ease; there is some resistance in the internal material of the cell.

This internal resistance of the cell (or battery) is generally shown in series with the load; see Figure 14–18. This means that every time current flows in the circuit, a voltage drop (IR drop) will occur across the internal resistance. This voltage drop subtracts from the battery’s emf and is lost to the load.

Let us assume that a 12-volt car battery has an internal resistance of 0.2 ohms and is connected to a load that draws 5 amperes. The internal voltage drop of the battery will be = I x R = 5 x 0.2 = 1 volt. In other words, the load receives only 11 volts, as shown in Figure 14–19A.

If the current is doubled to 10 amperes, the corresponding voltage drop will be 2 volts, and consequently the load “sees” only 10 volts; see Figure 14–19B. Likewise, if the load current is increased to 20 amperes, the load voltage would be only 8 volts; see Figure 14–19C.

The internal resistance of a cell increases with age, which in turn diminishes the voltage delivered to the load. Such a run-down battery may still yield a good reading when checked with a voltmeter. This happens because high-quality voltmeters have a very high resistance and draw virtually no current from the source. In other words, checking a battery’s condition with a voltmeter can be meaningless unless the test is performed when the battery is loaded down; see Figure 14–20.

Maximum Current from a Cell

A cell produces its greatest current, uselessly, when it is short-circuited. Assume we connect a wire of practically 0 ohms resistance across the terminals of a 1.5-volt, 0.035-ohm dry cell. The amount of current is limited only by the internal resistance of the cell: I = 1.5 ÷ 0.035 = 42.8 amperes. The terminal voltage is now 0, because all of the cell emf is used inside the cell. If this condition exists for more than a few seconds, the cell overheats, gases form in it, the electrolyte starts boiling out the top of the cell, and the cell is destroyed.

If a dry cell is used to produce a moderate current for 10 or 15 minutes, there can be a noticeable drop in terminal voltage and current by the end of this time. This drop is caused by the temporary increase in internal resistance due to the formation of a very small amount of hydrogen around the positive plate. When the cell is allowed to stand on open-circuit, this hydrogen is reconverted to H2O, and the cell is restored to its original low internal resistance.

Dry cells generally fail because they really become internally dry. An unused dry cell on a warm shelf for 2 years may lose its moisture by evaporation, despite the manufacturer’s attempt to seal the top. A cell in use loses its moisture when a hole is finally dissolved in the zinc case. Drying out causes a great increase in internal resistance. A cell with 1 or 2 ohms internal resistance is of no value. It may produce enough current to make a voltmeter read 1.5 volts but not enough current to light a flashlight bulb brightly.

Cells Connected in Series

The voltage output from series-connected power sources can be additive or sub- tractive, depending on the polarity of the power sources. This was fully explained in Section 10–11; review that material, if necessary.

Cells Connected in Parallel

When multiple power sources are connected in parallel, there is no increase in emf. Why, then, would anybody want to connect batteries in parallel?

1. The current capacity of such an arrangement is increased. In other words, three cells can deliver three times as much current as one cell can.

2. The life span of such a parallel arrangement is increased. This is an advantage in that it is more convenient to replace a group of three cells every 30 days than to replace one cell every 10 days.

3. The internal resistance of the parallel group is less than that of a single cell; there- fore, the terminal voltage will be closer to the battery emf.

When batteries or other power sources are connected in parallel, it is important to observe the proper polarity (positive to positive and negative to negative) and to ensure that all batteries have equal voltages.

Cells Connected Series-Parallel

If the current and voltage requirements of a load are higher than those of a single cell, a series-parallel arrangement of multiple cells is in order. An example will make this clear.

EXAMPLE 14–1

Given: A load rated at 6 volts and 2.4 amperes; available cells are rated at 1.5 volts and 800 milliamperes.

Find: A suitable arrangement of series-parallel-connected cells to satisfy the load requirement.

Solution

1. Connect four 1.5-volt cells in series to satisfy the voltage requirement of 6 volts.

Such an arrangement of cells, known as a series-connected bank, illustrated in Figure 14–21A, is capable of delivering no more current than one single cell; in this case, 0.8 ampere.

2. Connect additional banks of series-connected cells to increase the current capacity of the bank. In this case, 2.4 divided by 0.8 5 three banks, as shown in Figure 14–21B.

Note: The current capacity of a battery is equal to that of one cell multiplied by the number of parallel banks.

Advantages and Disadvantages of Batteries

Advantages of batteries are that they are portable, reliable, and self-contained. Sub- marines, automobiles, aircraft, and flashlights for small boys are four examples of devices that need to be independent of power line operation. Power line service can be interrupted by storms. In such situations, batteries can provide emergency lighting and communication service.

The great disadvantage of batteries is that the energy they produce is expensive, so batteries are used only where the convenience outweighs the cost. For example, how much energy is there in an automobile battery? A 6-volt car battery can produce 6 amperes for 20 hours (120 ampere-hours). Watt-hours of energy is volts × amperes × hours = 6 × 6 × 20 = 720 watt-hours, which is about 0.75 kilowatt-hour. Power corporations charge much less for 1 kilowatt-hour of energy. An ordinary flashlight cell (size D) contains a little over 0.5 ounce of zinc. According to the chemistry book, this amount of zinc is 1.5 × 10²³ atoms. Each atom supplies 2 electrons, so if all of the zinc is dissolved usefully, 3 × 10²³ electrons are produced. One coulomb is 6 3 1018 electrons; 3 × 10²³ electrons is equal to 50,000 coulombs.

It takes 3,600 coulombs to equal an ampere-hour, so the flashlight cell, even if 100% efficient, can produce only 14 ampere-hours. Fourteen ampere-hours at 1.5 volts equals 21 watt-hours. Therefore, 50 flashlight cells would be needed to produce 1 kilowatt-hour of energy.

Similar calculations can be made to show even more drastic examples of the cost of energy provided by cells. For example, if one considers the price paid for those tiny silver- oxide cells used to power electronic watches and hearing aids, the cost of energy obtained from such cells might amount to more than $3,000 per kilowatt-hour.

Safety Precautions with Batteries

1. The hydrogen and oxygen released by a battery being charged form an explosive gas mixture that can be ignited by sparks or open flames.

2. If acid needs to be diluted with distilled water (for use as an electrolyte), never pour water into acid. Such action may cause splattering and serious acid burns. Instead, slowly pour the acid into the water.

3. When handling acids, wear safety goggles, rubber gloves, rubber aprons, or similar protection garb.

SUMMARY

• Batteries consist of cells that deliver an emf from chemical energy.

• Primary cells cannot be recharged when run down.

• Secondary cells are rechargeable.

• The emf obtainable from a cell depends on its chemical makeup.

• The current rating, or ampere-hour capacity, of a cell depends on the size of its electrodes.

• Cells have an internal resistance that causes them to drop some of their emf. This IR drop is the difference between their open-circuit and closed-circuit voltage.

• Voltage readings from batteries should be taken under loaded conditions.

• Cells connected in series can deliver more voltage.

• Cells connected in parallel can deliver more ampere-hours.

Achievement Review

1. Define the words primary cell and secondary cell.

2. What is meant by the word electrolyte?

3. Explain the term specific gravity.

4. What kind of instrument is used for testing the specific gravity?

5. Someone reports the specific gravity of a battery to be 1,100. What does that say about the battery?

6. What is meant by the term ampere-hour?

7. Name two common types of secondary cells.

8. A battery is generally considered to be a source. Can it ever be regarded as a

load? Explain your answer.

9. Why do most battery chargers contain a rectifier?

10. Is it advisable to store batteries in a discharged condition? Why or why not?

11. What kind of substance is recommended to clean and neutralize corrosion on the terminals of a battery?

12. Why does a discharged battery freeze more easily than a fully charged battery?

13. Why should lighted matches never be used for illumination when inspecting the electrolyte of a battery?

14. Is the voltage output of a battery dependent on

a. The size of the plates?

b. The number of plates? Explain.

15. Draw four cells connected

a. For highest voltage output

b. For maximum life expectancy and high current output

16. Two 24-volt batteries in series are being charged by a 60-volt generator. Each battery has 0.02-ohm internal resistance. Calculate how much additional resistance is needed in the circuit to limit the current to 6 amperes.

17. Make a drawing to show how seven dry cells (1.5 V each) can be connected to yield 6 volts.

18. Now show seven wet cells (2 V each) connected to yield 10 volts.

19. In the following schematics, you see different circuit arrangements of batteries composed of 1.5-volt dry cells.

a. Check each one of the circuits to see if it is properly or incorrectly connected.

b. Estimate the voltage output of each combination from part a.

20. The sketches below represent various possible ways of connecting two dry cells and a lamp (as viewed from the top). The 1 and – are the terminals of the cells; the resistance represents the lamp. For each: State the voltage at the lamp (0, 1.5, or 3 V). Classify the circuits as good or poor.

21. Eight lead storage cells are arranged as in the drawing below (top view). What is the emf of this battery?

14–6 MAINTENANCE-FREE BATTERIES

The previous discussions, “Battery Testing” and “Battery Care,” become irrelevant when dealing with maintenance-free batteries, which have become popular in recent years. This kind of battery, although based on the same chemical process as the lead-acid cell, offers distinct ad- vantages over the traditional lead-acid battery. The sealed design prevents leakage of electrolyte and acid fumes, helping to eliminate corrosion problems. Since there is no loss of water, the electrolyte never needs replenishment. In addition, a well-sealed battery can be installed in any position and still function efficiently without danger of electrolyte spillage.

These features are possible because of the unique construction of the cells. The thin, low-volume separators that are normally placed between the electrodes of ordinary lead- acid batteries are replaced with thick, highly absorbent, feltlike, glass fiber mats. These separators absorb all the electrolyte, leaving no excess liquid to slosh around. Such cells are said to operate in a starved electrolyte mode, because there is only enough electrolyte to maintain the rated capacity of the cell. The cell is then sealed.

One may ask what happens to the hydrogen and oxygen gases that are generated during the charging of a lead-acid battery and become trapped inside the gas-tight container. The answer is that the design of the sealed battery allows the gases to recombine internally, with the result that water, rather than being lost, is electrochemically recycled. This is achieved by allowing the oxygen, which is generated at the positive plate, to migrate toward the negative plate, where it combines with the pure lead grids used in this construction. Likewise, the hydrogen will react with the lead dioxide of the positive plate, although at a somewhat lower rate. This recombination of gases is possible because there is just enough electrolyte to cover the plates and the glass mat separators, enabling the gas transfer between the plates. To encourage the recombination of the gases, the battery operates at a slight internal pressure — between 30 and 50 pounds per square inch. This condition is maintained by a pressure relief valve that opens only when the recommended charging rates are greatly exceeded.

It is important to note that not all “maintenance-free” batteries are perfectly sealed; some are vented to allow the escape of gases in the event of an overcharge. Since these gases are derived from the decomposition of the water in the electrolyte, such batteries can lose water, which cannot be replaced, thereby shortening their life span.

Nickel-Cadmium and Nickel-Iron (Edison) Storage Batteries

These truly long-life batteries are unaffected by the mechanical or electrical misuse that spoils a lead battery. But they cost several times as much as lead batteries of similar size. Nickel and cadmium are much more costly than lead. In addition, expensive manufacturing processes are required for these batteries.

The nickel-iron (Edison) cell was originally developed in 1899 for electric motor-driven vehicles. Research was aimed at producing a battery having low weight and volume for a given ampere-hour rating. One that could withstand repeated cycling (frequent charging and complete discharging) was especially desired.

The Edison cell is structurally stronger and lighter in weight than lead cells of the same current rating. The negative plates consist of a nickeled steel grid containing powdered iron, with some FeO and Fe(OH)2. The iron is the source of the electrons, which are

attracted through the external circuit toward nickel ions, Ni11 and Ni111, on the positive

plate. The positive plates are nickel tubes containing a mixture of nickel oxides and hydroxides, with flakes of pure nickel for increased conductivity. The electrolyte solution is 21% KOH (potassium hydroxide, caustic potash), which is chemically a base rather than an acid. The Edison cell is therefore called an alkaline cell.

The Edison cell has two disadvantages: the initial cost is high, and the maximum current is limited by high resistance, especially when the cell is cold. It is not suitable for starting gasoline or diesel engines, because its internal resistance puts a limit on its current output. These disadvantages limit its use. However, the Edison cell has an advantage that makes it useful for specific purposes. It is not damaged by remaining in a discharged condition; there- fore, it is useful in some portable lighting equipment and in a few marine installations, where it neither receives nor needs the attention that lead cells require. It is also appropriate for running traction equipment, such as mine locomotives and forklift trucks.

The nickel-cadmium (ni-cad or Ni-Cd) battery (Junger & Berg, Sweden, 1898) was developed not for frequent cycling but rather for general purposes. It enables the user to draw as many amperes as possible from a battery of given ampere-hour rating, without an excessive decline in voltage.

Both sets of plates are mechanically alike. The active materials are held in finely perforated, thin, flat steel pockets that are locked into a steel frame. The active material put in the positive plate is nickel hydroxide mixed with graphite to improve conductivity. Cadmium oxide is put into the negative plates. The electrolyte is potassium hydroxide (KOH) of about

specific gravity. When the cell is charged, the electrons forced onto the negative plate combine with Cd++ ions of the cadmium oxide (CdO), converting them to uncharged cadmium metal atoms. On the positive plate, electrons are removed from Ni++ ions of Ni(OH)2, converting them into more strongly positively charged Ni+++ ions. (The compound changes to Ni(OH)3.) Therefore, the active materials in the charged cell are

The H2O that is formed stays tied up in combination with the Ni(OH)2 in solid form on the plate and does not dilute the electrolyte; therefore, the density of the electrolyte remains constant, and a hydrometer does not indicate the amount of charge.

The nickel-cadmium battery is similar in its chemical composition to the Edison cell. Both contain alkaline electrolytes. Its electrical characteristics are similar to those of the lead cell; therefore, its applications are competitive with those of the lead cell. Like the Edison cell, the nickel-cadmium battery is chiefly used in industrial service. Its ruggedness, long life, and low-maintenance cost outweigh the high original cost. (Nickel-cadmium batteries cost nearly twice as much as do lead acid batteries of comparable energy ratings.)

Nickel-cadmium cells are marketed in the popular sizes of zinc-carbon cells (sizes AA, A, C, and D) for replacement in toys and consumer appliances. The buyer should con- sider the following potential disadvantages:

1. For any given number of cells, the voltage output is only 80% that of dry cells.

2. Unused nickel-cadmium cells tend to lose a significant amount of their emf when standing idle for several days. This effect is aggravated by higher temperatures.

3. Nickel-cadmium batteries tend to remember repeated demands for low output and sometimes persist in delivering such low levels, even though they are designed to meet the needs of increased output.

4. Nickel-cadmium cells do exhibit one negative characteristic. They remember their charge/discharge cycles. If they are used at only low currents, are permitted to dis- charge through only part of a cycle, and then are recharged, over a period of time they will develop a characteristic curve to match that cycle; see Figure 14–15.

On the positive side, it should be mentioned that nickel-cadmium batteries are good for nearly 2,000 recharge cycles and can be left on a trickle charge for indefinite periods of time.

Commercially, the nickel-cadmium battery is used in railroad signals systems; fire alarm systems; relay and switchgear operation; missile controls; aircraft engines; and diesel engines in locomotives, buses, and oil well pumps. Figure 14–16 shows a large, industrial- type cell. Nickel-cadmium cells in smaller, sealed, cylindrical forms, with no problems of gas or spillage, are used in a great variety of communication equipment and portable appliances.

Nickel-Metal Hydride Cells

Nickel-metal hydride cells (Ni-MH) are similar to nickel-cadmium cells in many respects. Both exhibit a voltage of 1.2 volts per cell and both have very similar charge and discharge curves. Nickel-metal hydride cells exhibit some improved characteristics, however. They have about a 40% higher energy density, and redundant memory accumulation is not as great a problem. Nickel-metal hydride cells are also more environmentally friendly. The posi- tive electrode is made of nickel oxyhydroxide (Ni-OH) and the negative electrode is made of metal hydride. The electrolyte is an aqueous (watery) potassium hydroxide solution. Nickel- metal hydride cells are replacing nickel-cadmium cells in many applications.

Lithium-Ion Cells

Lithium-ion cells are very popular for portable equipment such as notebook computers, video cameras, cell phones, and many others. Lithium-ion cells can be recharged and offer a very high energy density for their size and weight. They exhibit a voltage of 3.6 volts per cell, which is the same voltage that can be obtained by connecting three nickel- cadmium or three nickel-metal hydride cells in series. Lithium-ion cells also exhibit a weight-energy density that is about three times greater than nickel-cadmium cells. Under proper charging conditions, these cells can be recharged about 500 times. They also exhibit

a rather flat discharge curve, as shown in Figure 14–17, which makes them an ideal choice for electronic devices that require a constant voltage. Unlike nickel-cadmium cells and to some extent nickel-metal hydride cells, lithium-ion cells do not have the problem of memory accumulation. They can be recharged to their full capacity each time they are charged.

Lithium-ion cells can be safely recharged because they do not contain metallic lithium. The positive electrode (or cathode) is made of lithium metallic oxide and the negative electrode (or anode) is made of carbon. These cells work by transferring lithium ions between the cathode and the anode during discharging and charging. Although lithium-ion cells are safe to recharge they do require a special chargers that generally produce a constant voltage and constant current. Too much current or voltage during charging can cause early deterioration of the cell.

14–7 MISCELLANEOUS ASPECTS OF BATTERIES

Internal Resistance (R₁) of Cells

Cells themselves have an internal resistance that enters into a circuit calculation. When electrons flow from the negative plate to the positive plate, there is a movement of ions (cur- rent) in the electrolyte in the cell. This movement in the cell, like any current, does not occur with perfect ease; there is some resistance in the internal material of the cell.

This internal resistance of the cell (or battery) is generally shown in series with the load; see Figure 14–18. This means that every time current flows in the circuit, a voltage drop (IR drop) will occur across the internal resistance. This voltage drop subtracts from the battery’s emf and is lost to the load.

Let us assume that a 12-volt car battery has an internal resistance of 0.2 ohms and is connected to a load that draws 5 amperes. The internal voltage drop of the battery will be = I x R = 5 x 0.2 = 1 volt. In other words, the load receives only 11 volts, as shown in Figure 14–19A.

If the current is doubled to 10 amperes, the corresponding voltage drop will be 2 volts, and consequently the load “sees” only 10 volts; see Figure 14–19B. Likewise, if the load current is increased to 20 amperes, the load voltage would be only 8 volts; see Figure 14–19C.

The internal resistance of a cell increases with age, which in turn diminishes the voltage delivered to the load. Such a run-down battery may still yield a good reading when checked with a voltmeter. This happens because high-quality voltmeters have a very high resistance and draw virtually no current from the source. In other words, checking a battery’s condition with a voltmeter can be meaningless unless the test is performed when the battery is loaded down; see Figure 14–20.

Maximum Current from a Cell

A cell produces its greatest current, uselessly, when it is short-circuited. Assume we connect a wire of practically 0 ohms resistance across the terminals of a 1.5-volt, 0.035-ohm dry cell. The amount of current is limited only by the internal resistance of the cell: I = 1.5 ÷ 0.035 = 42.8 amperes. The terminal voltage is now 0, because all of the cell emf is used inside the cell. If this condition exists for more than a few seconds, the cell overheats, gases form in it, the electrolyte starts boiling out the top of the cell, and the cell is destroyed.

If a dry cell is used to produce a moderate current for 10 or 15 minutes, there can be a noticeable drop in terminal voltage and current by the end of this time. This drop is caused by the temporary increase in internal resistance due to the formation of a very small amount of hydrogen around the positive plate. When the cell is allowed to stand on open-circuit, this hydrogen is reconverted to H2O, and the cell is restored to its original low internal resistance.

Dry cells generally fail because they really become internally dry. An unused dry cell on a warm shelf for 2 years may lose its moisture by evaporation, despite the manufacturer’s attempt to seal the top. A cell in use loses its moisture when a hole is finally dissolved in the zinc case. Drying out causes a great increase in internal resistance. A cell with 1 or 2 ohms internal resistance is of no value. It may produce enough current to make a voltmeter read 1.5 volts but not enough current to light a flashlight bulb brightly.

Cells Connected in Series

The voltage output from series-connected power sources can be additive or sub- tractive, depending on the polarity of the power sources. This was fully explained in Section 10–11; review that material, if necessary.

Cells Connected in Parallel

When multiple power sources are connected in parallel, there is no increase in emf. Why, then, would anybody want to connect batteries in parallel?

1. The current capacity of such an arrangement is increased. In other words, three cells can deliver three times as much current as one cell can.

2. The life span of such a parallel arrangement is increased. This is an advantage in that it is more convenient to replace a group of three cells every 30 days than to replace one cell every 10 days.

3. The internal resistance of the parallel group is less than that of a single cell; there- fore, the terminal voltage will be closer to the battery emf.

When batteries or other power sources are connected in parallel, it is important to observe the proper polarity (positive to positive and negative to negative) and to ensure that all batteries have equal voltages.

Cells Connected Series-Parallel

If the current and voltage requirements of a load are higher than those of a single cell, a series-parallel arrangement of multiple cells is in order. An example will make this clear.

EXAMPLE 14–1

Given: A load rated at 6 volts and 2.4 amperes; available cells are rated at 1.5 volts and 800 milliamperes.

Find: A suitable arrangement of series-parallel-connected cells to satisfy the load requirement.

Solution

1. Connect four 1.5-volt cells in series to satisfy the voltage requirement of 6 volts.

Such an arrangement of cells, known as a series-connected bank, illustrated in Figure 14–21A, is capable of delivering no more current than one single cell; in this case, 0.8 ampere.

2. Connect additional banks of series-connected cells to increase the current capacity of the bank. In this case, 2.4 divided by 0.8 5 three banks, as shown in Figure 14–21B.

Note: The current capacity of a battery is equal to that of one cell multiplied by the number of parallel banks.

Advantages and Disadvantages of Batteries

Advantages of batteries are that they are portable, reliable, and self-contained. Sub- marines, automobiles, aircraft, and flashlights for small boys are four examples of devices that need to be independent of power line operation. Power line service can be interrupted by storms. In such situations, batteries can provide emergency lighting and communication service.

The great disadvantage of batteries is that the energy they produce is expensive, so batteries are used only where the convenience outweighs the cost. For example, how much energy is there in an automobile battery? A 6-volt car battery can produce 6 amperes for 20 hours (120 ampere-hours). Watt-hours of energy is volts × amperes × hours = 6 × 6 × 20 = 720 watt-hours, which is about 0.75 kilowatt-hour. Power corporations charge much less for 1 kilowatt-hour of energy. An ordinary flashlight cell (size D) contains a little over 0.5 ounce of zinc. According to the chemistry book, this amount of zinc is 1.5 × 10²³ atoms. Each atom supplies 2 electrons, so if all of the zinc is dissolved usefully, 3 × 10²³ electrons are produced. One coulomb is 6 3 1018 electrons; 3 × 10²³ electrons is equal to 50,000 coulombs.

It takes 3,600 coulombs to equal an ampere-hour, so the flashlight cell, even if 100% efficient, can produce only 14 ampere-hours. Fourteen ampere-hours at 1.5 volts equals 21 watt-hours. Therefore, 50 flashlight cells would be needed to produce 1 kilowatt-hour of energy.

Similar calculations can be made to show even more drastic examples of the cost of energy provided by cells. For example, if one considers the price paid for those tiny silver- oxide cells used to power electronic watches and hearing aids, the cost of energy obtained from such cells might amount to more than $3,000 per kilowatt-hour.

Safety Precautions with Batteries

1. The hydrogen and oxygen released by a battery being charged form an explosive gas mixture that can be ignited by sparks or open flames.

2. If acid needs to be diluted with distilled water (for use as an electrolyte), never pour water into acid. Such action may cause splattering and serious acid burns. Instead, slowly pour the acid into the water.

3. When handling acids, wear safety goggles, rubber gloves, rubber aprons, or similar protection garb.

SUMMARY

• Batteries consist of cells that deliver an emf from chemical energy.

• Primary cells cannot be recharged when run down.

• Secondary cells are rechargeable.

• The emf obtainable from a cell depends on its chemical makeup.

• The current rating, or ampere-hour capacity, of a cell depends on the size of its electrodes.

• Cells have an internal resistance that causes them to drop some of their emf. This IR drop is the difference between their open-circuit and closed-circuit voltage.

• Voltage readings from batteries should be taken under loaded conditions.

• Cells connected in series can deliver more voltage.

• Cells connected in parallel can deliver more ampere-hours.

Achievement Review

1. Define the words primary cell and secondary cell.

2. What is meant by the word electrolyte?

3. Explain the term specific gravity.

4. What kind of instrument is used for testing the specific gravity?

5. Someone reports the specific gravity of a battery to be 1,100. What does that say about the battery?

6. What is meant by the term ampere-hour?

7. Name two common types of secondary cells.

8. A battery is generally considered to be a source. Can it ever be regarded as a

load? Explain your answer.

9. Why do most battery chargers contain a rectifier?

10. Is it advisable to store batteries in a discharged condition? Why or why not?

11. What kind of substance is recommended to clean and neutralize corrosion on the terminals of a battery?

12. Why does a discharged battery freeze more easily than a fully charged battery?

13. Why should lighted matches never be used for illumination when inspecting the electrolyte of a battery?

14. Is the voltage output of a battery dependent on

a. The size of the plates?

b. The number of plates? Explain.

15. Draw four cells connected

a. For highest voltage output

b. For maximum life expectancy and high current output

16. Two 24-volt batteries in series are being charged by a 60-volt generator. Each battery has 0.02-ohm internal resistance. Calculate how much additional resistance is needed in the circuit to limit the current to 6 amperes.

17. Make a drawing to show how seven dry cells (1.5 V each) can be connected to yield 6 volts.

18. Now show seven wet cells (2 V each) connected to yield 10 volts.

19. In the following schematics, you see different circuit arrangements of batteries composed of 1.5-volt dry cells.

a. Check each one of the circuits to see if it is properly or incorrectly connected.

b. Estimate the voltage output of each combination from part a.

20. The sketches below represent various possible ways of connecting two dry cells and a lamp (as viewed from the top). The 1 and – are the terminals of the cells; the resistance represents the lamp. For each: State the voltage at the lamp (0, 1.5, or 3 V). Classify the circuits as good or poor.

21. Eight lead storage cells are arranged as in the drawing below (top view). What is the emf of this battery?

14–5 SECONDARY CELLS

In one family of cells, the electrochemical action is reversible; that is, if the discharged cell is connected to another electron source so that electrons can be put back into the negative terminal of the cell, the cell is recharged to its original emf value. Cells of this type are called storage cells or secondary cells. Figure 14–10 lists five types of secondary cells in order of increasing cost.

The Lead Storage Battery

Each lead cell produces 2 volts. Automobile batteries rated at 12 volts contain six cells connected in series; see Figure 14–11.

The action of a storage cell is shown in Figure 14–12. The negative plate consists of metallic lead. When the cell is producing current, lead atoms on the surface of the plate lose two electrons each to become Pb++ ions. These Pb++ ions do not dissolve into the electrolyte but remain on the plate and attract SO422 ions from the sulfuric acid solution. As a result, an invisible thin layer of PbSO4 is formed on the negative lead plate.

The positive plate consists of lead dioxide, PbO2. Each lead particle in the plate lacks four electrons (these electrons were given to the oxygen when the compound PbO2 was formed). Each Pb ++++ ion takes two electrons from the external circuit to become a Pb++ ion.

The energy for the electron transfer results from the tendency of neutral lead atoms to give two electrons each to Pb++++ ions, so that both Pb++++ ions and Pb atoms become Pb++ ions.

When each Pb++++ ion of the lead dioxide takes up two electrons, the ion can no longer hold the oxygen. The oxygen, therefore, goes into the acid solution and combines with the hydrogen ions of the acid to form water molecules. The Pb11 remains on the plate and picks up SO4– from the sulfuric acid to form lead sulfate.

The chemical action in the lead storage battery can occur only where the plates are in contact with the sulfuric acid solution. If a large current is required, the plates are

constructed so that the surface area in contact with the electrolyte solution is large. In a cell, the plates are arranged as shown in Figure 14–13. As indicated in Figure 14–12, the negative plate is made of lead sponge and the positive plate of porous lead dioxide. The porosity of these plates makes it possible for a large surface area of material to be wet by the electrolyte. Separators of wood, glass fibers, or similar porous material keep the plates from touching each other. To provide mechanical strength, both the negative and positive plates consist of an open framework of an alloy of lead-antimony. The active material is pressed into this framework. The electrolyte is dilute sulfuric acid having a specific gravity of 1.28.

Batteries can be shipped wet (filled with the electrolyte) or dry (with the electrolyte packaged in a separate container). The relative packing and shipping costs determine which shipment method is used.

The Ampere-Hour Rating

An ampere-hour is the amount of charge delivered by 1 ampere in 1 hour (1 ampere- hour 5 3,600 coulombs). The ampere-hour rating of a battery is determined from its measured ability to produce current for 20 hours at 80°F; therefore, a battery that produces 6 amperes steadily for 20 hours at 80°F has a rating of 120 ampere-hours. If the discharge rate of the battery is 1 or 2 amperes instead of 6 amperes, a 120-ampere-hour battery can produce more than 120 ampere-hours. The battery cannot produce 120 amperes for 1 hour; the actual value of ampere-hours produced depends on the current.

Battery Charging

A rectifier or DC generator is required to charge a battery. The battery is charged by forcing a current through it in a direction opposite to that of normal battery operation. In the lead cell, this reversal of current reverses the chemical changes that take place in the cell when it furnishes energy. (Recall that in primary cells, the chemical changes cannot be reversed.)

The charging process in the lead cell is a series of chemical reactions. Recall that a current through a solution of sulfuric acid in water produces hydrogen at the negative plate and oxygen at the positive plate. If the plates are already covered with a thin layer of lead sulfate (from the discharging process), then the H1 ions forced toward the

negative plate combine with the SO — ions to re-form sulfuric acid. The electrons forced  onto the negative plate by the generator combine with the Pb++ ions to form lead atoms, as shown in Figure 14–14A.

At the same time, water decomposes at the positive plate. The hydrogen set free by this process combines with the SO x–ns on the plate to form more sulfuric acid. The oxygen resulting from the breakdown of the water combines with the lead to form lead dioxide. The Pb11 ions of the discharged positive plate are converted to Pb++++s as the generator removes electrons; see Figure 14–14B.

The following equation summarizes the charge/discharge process of the battery.

In a charging circuit, such as the one shown in Figure 14–14, the generator voltage must (1) equalize and overcome the battery emf, and (2) have a large enough value to produce a current through the resistance of the circuit.

A charging rate that is too high damages a battery by causing it to overheat. It can also cause gas bubble formation inside the spongy plate material, which forces active material to break away from the plate structure. The safest charging procedure is a 10-ampere rate, or less, requiring about 24 hours. A battery can be charged on a constant voltage circuit in 6 or 8 hours, starting with a 30-ampere or 40-ampere rate, which tapers down as the battery charge builds up. With this method, however, the battery should be checked to see that it is not overheating. The accepted limit is usually 110°F.

Battery Testing

A hydrometer measures the specific gravity of the electrolyte in each cell. Specific gravity is the ratio of the weight of a given volume of a liquid to the weight of an equal volume of water. For example, a sample of pure acid may have a specific gravity of 2. Since the specific gravity of water is defined as 1, it follows that, volume for volume, the acid weighs twice as much as water. A charged battery has enough sulfuric acid in the electrolyte so that its specific gravity is 1.25 to 1.28 (water 5 1.00). As the battery discharges, SO4 ions of the acid are tied up on the plates, and the electrolyte becomes more like plain water, its specific gravity approaching 1.1. Since occasionally a cell may not operate properly even when it has sufficient acid, a better method of battery testing measures the voltage of each cell when it is producing current. Voltage of a good cell will remain at 2 volts even though the cell is producing 20 or 30 amperes. Voltage of a dead cell will be 2 volts when the cell is not producing current but will drop off when it is producing current.

Hydrometer calibrations normally omit the decimal point. For example, the reading for a fully charged cell will be from 1250 to 1280, which is understood to mean the specific gravity is equal to 1.25 to 1.28. The indication for a completely discharged cell will be about 1100 (meaning specific gravity equals 1.1).

Battery Care

Particular care should be taken to avoid getting dirt into a cell. When the cap is re- moved, it should be set in a clean place, if it has to be set down at all. Dirt, especially a few flakes of iron rust, can spoil a cell permanently. A great many automobile batteries probably go bad because of the accidental entry of dirt into a cell.

A battery should not be allowed to remain in a discharged condition. If a battery is completely run down, it should be charged within a few hours at a slow rate. If the discharged battery is allowed to stand discharged, the lead sulfate apparently hardens or crystallizes and is difficult to restore to lead and lead dioxide. Also, the watery solution in a discharged battery can freeze in winter, cracking the battery.

The liquid should be maintained at a level that covers the plates. Distilled water is preferable, but faucet water is better than none at all. (Melted frost from the refrigerator is distilled water.) Water is lost from a battery mainly by evaporation or slightly by hydrogen and oxygen forming during charging. Acid is lost only by cracking the case or tipping it over, so acid seldom is needed.

There is no point in adding various amazing powders and liquids to a battery to improve its performance, life, and complexion. For many, many years it has been known that Epsom salt (MgSO4) or sodium sulfate (Na2SO4) can be added to a lead storage cell electrolyte in small amounts without damage to the battery but without doing it any good either. If acid has been lost from the cell, these materials provide ions that can be useful. But a teaspoonful of sulfuric acid (worth $0.02, like the Epsom salt) is more desirable in such a battery. When new and important discoveries appear, more reliable remedies will be announced in news articles and electrical engineering journals rather than in exclamatory and sensational advertising.

14–5 SECONDARY CELLS

In one family of cells, the electrochemical action is reversible; that is, if the discharged cell is connected to another electron source so that electrons can be put back into the negative terminal of the cell, the cell is recharged to its original emf value. Cells of this type are called storage cells or secondary cells. Figure 14–10 lists five types of secondary cells in order of increasing cost.

The Lead Storage Battery

Each lead cell produces 2 volts. Automobile batteries rated at 12 volts contain six cells connected in series; see Figure 14–11.

The action of a storage cell is shown in Figure 14–12. The negative plate consists of metallic lead. When the cell is producing current, lead atoms on the surface of the plate lose two electrons each to become Pb++ ions. These Pb++ ions do not dissolve into the electrolyte but remain on the plate and attract SO422 ions from the sulfuric acid solution. As a result, an invisible thin layer of PbSO4 is formed on the negative lead plate.

The positive plate consists of lead dioxide, PbO2. Each lead particle in the plate lacks four electrons (these electrons were given to the oxygen when the compound PbO2 was formed). Each Pb ++++ ion takes two electrons from the external circuit to become a Pb++ ion.

The energy for the electron transfer results from the tendency of neutral lead atoms to give two electrons each to Pb++++ ions, so that both Pb++++ ions and Pb atoms become Pb++ ions.

When each Pb++++ ion of the lead dioxide takes up two electrons, the ion can no longer hold the oxygen. The oxygen, therefore, goes into the acid solution and combines with the hydrogen ions of the acid to form water molecules. The Pb11 remains on the plate and picks up SO4– from the sulfuric acid to form lead sulfate.

The chemical action in the lead storage battery can occur only where the plates are in contact with the sulfuric acid solution. If a large current is required, the plates are

constructed so that the surface area in contact with the electrolyte solution is large. In a cell, the plates are arranged as shown in Figure 14–13. As indicated in Figure 14–12, the negative plate is made of lead sponge and the positive plate of porous lead dioxide. The porosity of these plates makes it possible for a large surface area of material to be wet by the electrolyte. Separators of wood, glass fibers, or similar porous material keep the plates from touching each other. To provide mechanical strength, both the negative and positive plates consist of an open framework of an alloy of lead-antimony. The active material is pressed into this framework. The electrolyte is dilute sulfuric acid having a specific gravity of 1.28.

Batteries can be shipped wet (filled with the electrolyte) or dry (with the electrolyte packaged in a separate container). The relative packing and shipping costs determine which shipment method is used.

The Ampere-Hour Rating

An ampere-hour is the amount of charge delivered by 1 ampere in 1 hour (1 ampere- hour 5 3,600 coulombs). The ampere-hour rating of a battery is determined from its measured ability to produce current for 20 hours at 80°F; therefore, a battery that produces 6 amperes steadily for 20 hours at 80°F has a rating of 120 ampere-hours. If the discharge rate of the battery is 1 or 2 amperes instead of 6 amperes, a 120-ampere-hour battery can produce more than 120 ampere-hours. The battery cannot produce 120 amperes for 1 hour; the actual value of ampere-hours produced depends on the current.

Battery Charging

A rectifier or DC generator is required to charge a battery. The battery is charged by forcing a current through it in a direction opposite to that of normal battery operation. In the lead cell, this reversal of current reverses the chemical changes that take place in the cell when it furnishes energy. (Recall that in primary cells, the chemical changes cannot be reversed.)

The charging process in the lead cell is a series of chemical reactions. Recall that a current through a solution of sulfuric acid in water produces hydrogen at the negative plate and oxygen at the positive plate. If the plates are already covered with a thin layer of lead sulfate (from the discharging process), then the H1 ions forced toward the

negative plate combine with the SO — ions to re-form sulfuric acid. The electrons forced  onto the negative plate by the generator combine with the Pb++ ions to form lead atoms, as shown in Figure 14–14A.

At the same time, water decomposes at the positive plate. The hydrogen set free by this process combines with the SO x–ns on the plate to form more sulfuric acid. The oxygen resulting from the breakdown of the water combines with the lead to form lead dioxide. The Pb11 ions of the discharged positive plate are converted to Pb++++s as the generator removes electrons; see Figure 14–14B.

The following equation summarizes the charge/discharge process of the battery.

In a charging circuit, such as the one shown in Figure 14–14, the generator voltage must (1) equalize and overcome the battery emf, and (2) have a large enough value to produce a current through the resistance of the circuit.

A charging rate that is too high damages a battery by causing it to overheat. It can also cause gas bubble formation inside the spongy plate material, which forces active material to break away from the plate structure. The safest charging procedure is a 10-ampere rate, or less, requiring about 24 hours. A battery can be charged on a constant voltage circuit in 6 or 8 hours, starting with a 30-ampere or 40-ampere rate, which tapers down as the battery charge builds up. With this method, however, the battery should be checked to see that it is not overheating. The accepted limit is usually 110°F.

Battery Testing

A hydrometer measures the specific gravity of the electrolyte in each cell. Specific gravity is the ratio of the weight of a given volume of a liquid to the weight of an equal volume of water. For example, a sample of pure acid may have a specific gravity of 2. Since the specific gravity of water is defined as 1, it follows that, volume for volume, the acid weighs twice as much as water. A charged battery has enough sulfuric acid in the electrolyte so that its specific gravity is 1.25 to 1.28 (water 5 1.00). As the battery discharges, SO4 ions of the acid are tied up on the plates, and the electrolyte becomes more like plain water, its specific gravity approaching 1.1. Since occasionally a cell may not operate properly even when it has sufficient acid, a better method of battery testing measures the voltage of each cell when it is producing current. Voltage of a good cell will remain at 2 volts even though the cell is producing 20 or 30 amperes. Voltage of a dead cell will be 2 volts when the cell is not producing current but will drop off when it is producing current.

Hydrometer calibrations normally omit the decimal point. For example, the reading for a fully charged cell will be from 1250 to 1280, which is understood to mean the specific gravity is equal to 1.25 to 1.28. The indication for a completely discharged cell will be about 1100 (meaning specific gravity equals 1.1).

Battery Care

Particular care should be taken to avoid getting dirt into a cell. When the cap is re- moved, it should be set in a clean place, if it has to be set down at all. Dirt, especially a few flakes of iron rust, can spoil a cell permanently. A great many automobile batteries probably go bad because of the accidental entry of dirt into a cell.

A battery should not be allowed to remain in a discharged condition. If a battery is completely run down, it should be charged within a few hours at a slow rate. If the discharged battery is allowed to stand discharged, the lead sulfate apparently hardens or crystallizes and is difficult to restore to lead and lead dioxide. Also, the watery solution in a discharged battery can freeze in winter, cracking the battery.

The liquid should be maintained at a level that covers the plates. Distilled water is preferable, but faucet water is better than none at all. (Melted frost from the refrigerator is distilled water.) Water is lost from a battery mainly by evaporation or slightly by hydrogen and oxygen forming during charging. Acid is lost only by cracking the case or tipping it over, so acid seldom is needed.

There is no point in adding various amazing powders and liquids to a battery to improve its performance, life, and complexion. For many, many years it has been known that Epsom salt (MgSO4) or sodium sulfate (Na2SO4) can be added to a lead storage cell electrolyte in small amounts without damage to the battery but without doing it any good either. If acid has been lost from the cell, these materials provide ions that can be useful. But a teaspoonful of sulfuric acid (worth $0.02, like the Epsom salt) is more desirable in such a battery. When new and important discoveries appear, more reliable remedies will be announced in news articles and electrical engineering journals rather than in exclamatory and sensational advertising.

Batteries

14–1 CHEMICAL ENERGY: A SOURCE OF EMF

In the preceding chapter we learned about the ionization and conductivity of salt or acid solutions called electrolytes. If two electrodes of different metals are immersed into such electrolyte, a chemical reaction between these parts will produce a small emf. Such an arrangement of parts is known as a cell. Cells are the building blocks of batteries. In other words, batteries are combinations of two or more cells for the attainment of higher voltages or currents.

Cells are classified, according to the nature of their chemical activity, as being primary or secondary. A primary cell obtains its energy by consuming one of its electrodes and cannot be restored when its active materials have been depleted. It is discarded at the end of its useful life. The common flashlight battery is an example of a primary cell.

By contrast, a secondary cell may be repetitively recharged after it has run down. The reason for this is that the chemical process within the cell is reversible. The lead-acid cells within a car battery are secondary cells.

The voltage derived from a cell depends solely on the type of materials used in the construction of the cell.

Figure 14–1 lists a number of metals in the order of the ease with which electrons escape into a water solution. Metals at the top of the list lose electrons readily, and those at the bottom of the list lose electrons less readily. The electromotive series list is quite different from a list arranged according to conductivity.

Although it is theoretically possible to make batteries from many combinations of materials, there are actually only a few practical combinations. Many combinations pro- mote undesirable chemical reactions that cause the active metal to corrode rapidly or to interfere with useful current production by building up resistance.

To gain some understanding of the process by which chemical interaction produces an emf, let us consider a simple, though impractical, cell made by placing a strip of zinc and a strip of copper into a weak solution of water and hydrochloric acid; see Figure 14–2. When a voltmeter is connected between the strips, an emf of approximately 1 volt is indicated.

From where does this energy come? The energy is due to the electrons of the zinc atoms and the hydrogen ions (H1) of the acid. All acids consist of positively charged H1 ions plus negative ions. The H1 ions attract electrons. Each zinc atom has two electrons

in its outer orbit that have sufficient energy to leave the zinc atom when they are subjected to the attraction of the H1 ions.

The transfer of electrons from atoms of zinc to H1 ions takes place when a piece of zinc is placed into an acid (whether or not another metal is present). When H1 ions take electrons from the zinc, they become neutral H atoms. Once the atoms are no longer positively charged, they are not attracted to the negative ions in the water. The H atoms then pair to form H2 molecules. These molecules are ordinary hydrogen gas that is visible as it bubbles away. The circle labeled Zn11 in Figure 14–3 represents

a zinc ion (a zinc atom that has lost two electrons). Zinc ions (Zn11) are attracted from the metal into the solution by the negative ions of the acid.

Electron transfer at the surface of the zinc bar does not result in a useful electric cur- rent. To obtain a useful current, a wire is connected between the zinc strip and the copper strip in the acid. The copper in the acid provides another source of electrons that can be attracted by the H1 ions, as shown in Figure 14–4. The electrons taken by the H1 ions at the copper surface are simultaneously replaced by electrons at the wire connection supplied by the zinc. The copper serves as a conductor and carries electrons away from the zinc and to the H1 ions.

Figure 14–4 shows a simple battery cell. However, this type of cell is impractical be- cause of the rapid, noncurrent-producing transfer of electrons at the surface of the zinc. A small strip of zinc gives away all of its energetic electrons and is distributed in the solution as zinc ions after just a few minutes of operation.

The Carbon-Zinc Cell (Leclanché Cell)

Georges Leclanché improved the basic battery cell by using a solution of water and ammonium chloride instead of hydrochloric acid. Ammonium chloride consists of NH41 and Cl2 ions. The ammonium ions, NH41, do not cause the zinc to be consumed as rapidly as the H1 ions of the HCl solution. Instead of a copper strip, Leclanché used a carbon rod surrounded by packed manganese dioxide. Manganese dioxide (MnO2) is a solid that does not dissolve in water. The manganese atoms in manganese dioxide have a strong positive charge and thus provide a strong attraction for electrons.

When a wire is connected between the zinc and the carbon rod, electrons flow from the zinc through the wire and carbon rod and are attracted to the positive manganese. At the same time, zinc ions are attracted into the ammonium chloride solution.

The combination of the carbon powder mixed with the manganese dioxide and the carbon rod acts as an inert conductor to carry electrons to the manganese dioxide in the cell. (The Mn1111 ions gain two electrons each to become Mn11 ions. However, these ions do not have an attraction for electrons that is strong enough for the electrons to be of any further use in the cell.) The typical dry cell used in flashlights and portable radios is a sealed Leclanché cell; see Figure 14–5. The inside of the cell is not dry but rather is a moist paste (when the cell is new). Zinc is often used as the outer container of a cell in which the container is the negative electrode. Dry cells cannot be recharged.

If a cell is to produce a large current in amperes, both the positive and negative terminals must have a large area of metal plate in contact with the electrolyte solution. This large area of contact makes it easy for electron transfers to occur. In other words, the resistance of the cell is low. A small penlight dry cell has just as much emf as the larger #6 dry cell, but the penlight cell cannot produce the same amount of current, because it has more internal resistance (this statement can be verified by Ohm’s law).

14–2 ANODES AND CATHODES

As mentioned in Chapter 13, the terminal by which electrons enter a device is called the cathode; the terminal where electrons leave the device is called the anode. The energy user in Figure 14–6 can be any device (a lamp; an electroplating bath) in

which electrons are forced onto the cathode by some other energy source. Electrons in the device are repelled from the cathode toward the anode, where they are attracted by an external energy source. The cathode of the energy user is negative (containing excess electrons) and the anode is positive. Electrons flow through the device from the cathode (negative) to the anode (positive).

For the energy producer in Figure 14–6, the anode is the metal that is rich in electrons (such as zinc). The anode supplies electrons to the external energy user. Electrons enter the cell at the cathode, since they are attracted to this location by a relatively positive electrode. The anode is negative; the cathode is positive. Thus, the energy in the cell pushes electrons out at the anode ( 2) and attracts them at the cathode ( 1).

The careful reader will observe that this conforms to our theory of current flow, which is that electrons move from negative to positive in the outer circuit (the load), but that in- side the source they move from positive to negative. This explains why the cathode of the source is labeled positive, while the anode is labeled negative.

14–3 PRIMARY CELLS

Each cell listed in Figure 14–7 is called a primary cell. In this type of cell, electron transfer is not readily reversible; that is, the dissolved zinc is not easily returned to its original metallic form. The dry cell cannot be reclaimed. A dry cell is the most commonly used primary cell. (Cells that can be recharged, such as those used in car batteries, are called secondary cells.)

Alkaline dry cells provide a greater current than carbon-zinc cells of equal size. Alkaline cells also provide a moderate current for a much longer time and can provide more power at low temperatures. These cells are useful in portable photoflash equipment, motion picture cameras, robot model planes, and for other applications where a better performance is worth the extra cost. A type of alkaline dry cell is rechargeable.

A mercury cell has a still higher energy potential for its size and weight when com- pared with the cells covered previously. In addition, the cost of this cell is greater. A mercury cell will maintain its 1.35-volt emf steadily for a long time. The mercury cells are used in handheld communication sets, hearing aids, and portable electronic equipment and appliances. This type of cell usually consists of a negative center terminal that is connected

to a zinc cylinder or pellet. The positive case is connected to a mixture of mercuric oxide and graphite. To avoid mercury contamination of soil and water, ordinary methods of trash disposal are not suitable for discarded mercury cells or other devices containing mercury.

The three types of dry cells just covered (Leclanché, alkaline, and mercury) are the most widely used dry cells. The silver-zinc cell, which is similar in construction to the mercury cell, maintains its emf throughout its life. Although the silver-zinc cell is expensive, it has a high ratio of energy to weight. This factor makes it useful in hearing aids, electric watches, and spacecrafts. Some silver cells are rechargeable.

In the zinc-air cell, the negative terminal leads to a porous zinc anode, which is soaked with the electrolyte solution. The cathode is a thin conductive plastic arrangement. The cell has enough wet resistance to prevent slow loss of moisture, yet it is porous enough to permit the entry of oxygen from the air. Finely divided platinum (or a silver alloy) on the cathode promotes an oxygen and water reaction that removes electrons from the cathode to form OH2 ions. Electrons are supplied to the load circuit by zinc atoms as they ionize. Some zinc-air cells are mechanically rechargeable; that is, the oxidized zinc anode (consisting mainly of potassium zincate) is removed, the cell is refilled with water, and a new porous zinc plate containing potassium hydroxide is inserted in the cell. Portable military communication equipment commonly uses zinc-air cells.

Many other primary cell combinations have been investigated, including magnesium- air, iron-air, lithium-nickel, and a 2-volt magnesium cell with magnesium bromide as the electrolyte and a cathode made of manganese dioxide.

The lithium battery, especially, needs to be mentioned here because it has gained wide acceptance in the electronics industry due to its unique properties. In low-current applications, such as the memory-retention circuits of computerized equipment, they often last for many years with virtually undiminished voltage output up to the last moments of life.

Actually, only very little lithium goes into the production of a cell; on average about 1⁄2 gram, which often comprises only about 5% of its total weight. Electrode material LiBF4 is dissolved into a nontoxic electrolyte material called gamma butyrolactone, which has a boiling point of 204°C, hence there is no outgassing during normal operation. In fact, some lithium batteries have operating temperatures from 240°F to 185°F.

Safety precautions must be taken if it becomes necessary to solder wires to the battery. The heat from the soldering iron can cause the battery to explode.

14–4 FUEL CELLS

All of the cells described to this point base their operation on the tendency of energetic electrons to transfer to a material that lacks electrons, such as a positively charged ion. During this process, the anode material itself is used up in the sense that it is converted to a useless low- energy form. The fuel cell also makes use of this same type of electron transfer but with the important difference that the material giving electrons and the material taking electrons are not contained within the cell. The solid electrodes of the cell are not consumed in the process.

The term fuel makes one think of something burning. During the burning (combustion) of any fuel, such as coal, oil, or gas, atoms of the fuel give electrons to oxygen in the air. The energy of the electrons immediately appears as heat. It is correct to think of the gradual consumption of the zinc in a flashlight battery as a slow oxidation (a slow burning process). Research over a number of years has produced very efficient fuel cells. The purpose of such cells is to control the electrons of inexpensive fuels and make them perform useful work as they leave the fuel atoms to join atoms of oxygen or some other electron taker. Fuel cells require high-purity fuel and inexpensive but reliable catalytic surfaces on which the essential reactions take place. Fuel cells also require auxiliary equipment such as gas containers and pressure controls. Research is under way to develop efficient, compact fuel cells for use in electric automobiles and tractors and as a source of power for a residence.

The hydrogen-oxygen fuel cell, as shown in Figure 14–8, has hollow, porous car- bon electrodes immersed in a potassium hydroxide solution. Hydrogen gas is pumped

into one electrode and oxygen gas is pumped into the other. The porous carbon also contains certain metals or metal oxides called catalysts. (A catalyst promotes a chemical re- action.) In this case, the catalyst aids hydrogen molecules (which are pairs of hydrogen atoms) to separate into single atoms that can then combine with the negatively charged hydroxide ions (OH2) of the electrolyte. The combination of H and OH2 forms a molecule of water, H2O, with one electron left over. These surplus electrons are attracted to the electrode supplied by oxygen, as shown in Figure 14–9, where they take part in the reaction of oxygen 1 H2O 1 electrons to form hydroxide ions, OH2. Thus, hydroxide ions are re-formed at the cathode at the same rate as they are used up at the anode. The important resultant change is the conversion of hydrogen and oxygen to water.

The alkaline-hydrogen-oxygen fuel cell just discussed was one of the very early types introduced in the 1960s. There are other types of fuel cells today that offer different characteristics and advantages.

The Polymer Electrolyte Membrane (PEM) Fuel Cell

Polymer electrolyte membrane fuel cells are often called proton exchange membrane fuel cells. These cells offer high-power density combined with low weight and volume compared with other types of fuel cells. PEM fuel cells employ a solid polymer as the electrolyte. The electrodes are porous carbon combined with a platinum catalyst. They require only hydrogen, oxygen from the air, and water to operate. They do not need corrosive fluids like some other fuel cells. PEM fuel cells operate at a relatively low temperature, around 80°C (176°F).

Direct Methanol Fuel Cells

Direct methanol fuel cells are powered by pure methanol mixed with steam. The methanol/steam mixture is fed directly to the fuel cell anode. These fuel cells have an advantage in that they do not have the fuel storage problems of cells that rely on pure hydrogen. Since methanol is a liquid, it is much easier to store. Methanol also has a higher energy density than hydrogen although it does not contain as much energy as gasoline or diesel fuel.

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte. The acid is contained in a Teflon-bonded silicon carbide matrix. The electrodes are composed of porous carbon containing a platinum catalyst. This cell is considered the first generation of modern fuel cells and has been used commercially for stationary power generation and to power large vehicles such as city buses.

Molten Carbonate Fuel Cells

Molten carbonate fuel cells are being developed for natural gas and coal fired power plants. These fuel cells operate at very high temperatures, typically 650°C (1200°F) and above. The high operating temperature has an advantage in that non-precious metals can be used as a catalyst in the anode and cathode electrodes, resulting in much lower cost. Molten carbonate fuel cells can reach efficiencies as high as 60%.

Solid Oxide Fuel Cells

Solid oxide fuel cells use a hard non-porous ceramic compound as the electrolyte. Because the electrolyte is solid, the fuel cell does not have to be constructed in the plate like configuration typical of other types of fuel cells. These fuel cells operate at very high temperatures, 1,000°C (1,830°F). The high operating temperature has an advantage in that non-precious metals can be used as a catalyst in the electrodes. Also, the high operating temperature of the solid oxide fuel cell permits reforming of fuels internally, enabling use of a variety of fuels, which helps reduce the cost of adding a reformer to the system. Solid oxide fuel cells have an efficiency of 50% to 60%.

Batteries

14–1 CHEMICAL ENERGY: A SOURCE OF EMF

In the preceding chapter we learned about the ionization and conductivity of salt or acid solutions called electrolytes. If two electrodes of different metals are immersed into such electrolyte, a chemical reaction between these parts will produce a small emf. Such an arrangement of parts is known as a cell. Cells are the building blocks of batteries. In other words, batteries are combinations of two or more cells for the attainment of higher voltages or currents.

Cells are classified, according to the nature of their chemical activity, as being primary or secondary. A primary cell obtains its energy by consuming one of its electrodes and cannot be restored when its active materials have been depleted. It is discarded at the end of its useful life. The common flashlight battery is an example of a primary cell.

By contrast, a secondary cell may be repetitively recharged after it has run down. The reason for this is that the chemical process within the cell is reversible. The lead-acid cells within a car battery are secondary cells.

The voltage derived from a cell depends solely on the type of materials used in the construction of the cell.

Figure 14–1 lists a number of metals in the order of the ease with which electrons escape into a water solution. Metals at the top of the list lose electrons readily, and those at the bottom of the list lose electrons less readily. The electromotive series list is quite different from a list arranged according to conductivity.

Although it is theoretically possible to make batteries from many combinations of materials, there are actually only a few practical combinations. Many combinations pro- mote undesirable chemical reactions that cause the active metal to corrode rapidly or to interfere with useful current production by building up resistance.

To gain some understanding of the process by which chemical interaction produces an emf, let us consider a simple, though impractical, cell made by placing a strip of zinc and a strip of copper into a weak solution of water and hydrochloric acid; see Figure 14–2. When a voltmeter is connected between the strips, an emf of approximately 1 volt is indicated.

From where does this energy come? The energy is due to the electrons of the zinc atoms and the hydrogen ions (H1) of the acid. All acids consist of positively charged H1 ions plus negative ions. The H1 ions attract electrons. Each zinc atom has two electrons

in its outer orbit that have sufficient energy to leave the zinc atom when they are subjected to the attraction of the H1 ions.

The transfer of electrons from atoms of zinc to H1 ions takes place when a piece of zinc is placed into an acid (whether or not another metal is present). When H1 ions take electrons from the zinc, they become neutral H atoms. Once the atoms are no longer positively charged, they are not attracted to the negative ions in the water. The H atoms then pair to form H2 molecules. These molecules are ordinary hydrogen gas that is visible as it bubbles away. The circle labeled Zn11 in Figure 14–3 represents

a zinc ion (a zinc atom that has lost two electrons). Zinc ions (Zn11) are attracted from the metal into the solution by the negative ions of the acid.

Electron transfer at the surface of the zinc bar does not result in a useful electric cur- rent. To obtain a useful current, a wire is connected between the zinc strip and the copper strip in the acid. The copper in the acid provides another source of electrons that can be attracted by the H1 ions, as shown in Figure 14–4. The electrons taken by the H1 ions at the copper surface are simultaneously replaced by electrons at the wire connection supplied by the zinc. The copper serves as a conductor and carries electrons away from the zinc and to the H1 ions.

Figure 14–4 shows a simple battery cell. However, this type of cell is impractical be- cause of the rapid, noncurrent-producing transfer of electrons at the surface of the zinc. A small strip of zinc gives away all of its energetic electrons and is distributed in the solution as zinc ions after just a few minutes of operation.

The Carbon-Zinc Cell (Leclanché Cell)

Georges Leclanché improved the basic battery cell by using a solution of water and ammonium chloride instead of hydrochloric acid. Ammonium chloride consists of NH41 and Cl2 ions. The ammonium ions, NH41, do not cause the zinc to be consumed as rapidly as the H1 ions of the HCl solution. Instead of a copper strip, Leclanché used a carbon rod surrounded by packed manganese dioxide. Manganese dioxide (MnO2) is a solid that does not dissolve in water. The manganese atoms in manganese dioxide have a strong positive charge and thus provide a strong attraction for electrons.

When a wire is connected between the zinc and the carbon rod, electrons flow from the zinc through the wire and carbon rod and are attracted to the positive manganese. At the same time, zinc ions are attracted into the ammonium chloride solution.

The combination of the carbon powder mixed with the manganese dioxide and the carbon rod acts as an inert conductor to carry electrons to the manganese dioxide in the cell. (The Mn1111 ions gain two electrons each to become Mn11 ions. However, these ions do not have an attraction for electrons that is strong enough for the electrons to be of any further use in the cell.) The typical dry cell used in flashlights and portable radios is a sealed Leclanché cell; see Figure 14–5. The inside of the cell is not dry but rather is a moist paste (when the cell is new). Zinc is often used as the outer container of a cell in which the container is the negative electrode. Dry cells cannot be recharged.

If a cell is to produce a large current in amperes, both the positive and negative terminals must have a large area of metal plate in contact with the electrolyte solution. This large area of contact makes it easy for electron transfers to occur. In other words, the resistance of the cell is low. A small penlight dry cell has just as much emf as the larger #6 dry cell, but the penlight cell cannot produce the same amount of current, because it has more internal resistance (this statement can be verified by Ohm’s law).

14–2 ANODES AND CATHODES

As mentioned in Chapter 13, the terminal by which electrons enter a device is called the cathode; the terminal where electrons leave the device is called the anode. The energy user in Figure 14–6 can be any device (a lamp; an electroplating bath) in

which electrons are forced onto the cathode by some other energy source. Electrons in the device are repelled from the cathode toward the anode, where they are attracted by an external energy source. The cathode of the energy user is negative (containing excess electrons) and the anode is positive. Electrons flow through the device from the cathode (negative) to the anode (positive).

For the energy producer in Figure 14–6, the anode is the metal that is rich in electrons (such as zinc). The anode supplies electrons to the external energy user. Electrons enter the cell at the cathode, since they are attracted to this location by a relatively positive electrode. The anode is negative; the cathode is positive. Thus, the energy in the cell pushes electrons out at the anode ( 2) and attracts them at the cathode ( 1).

The careful reader will observe that this conforms to our theory of current flow, which is that electrons move from negative to positive in the outer circuit (the load), but that in- side the source they move from positive to negative. This explains why the cathode of the source is labeled positive, while the anode is labeled negative.

14–3 PRIMARY CELLS

Each cell listed in Figure 14–7 is called a primary cell. In this type of cell, electron transfer is not readily reversible; that is, the dissolved zinc is not easily returned to its original metallic form. The dry cell cannot be reclaimed. A dry cell is the most commonly used primary cell. (Cells that can be recharged, such as those used in car batteries, are called secondary cells.)

Alkaline dry cells provide a greater current than carbon-zinc cells of equal size. Alkaline cells also provide a moderate current for a much longer time and can provide more power at low temperatures. These cells are useful in portable photoflash equipment, motion picture cameras, robot model planes, and for other applications where a better performance is worth the extra cost. A type of alkaline dry cell is rechargeable.

A mercury cell has a still higher energy potential for its size and weight when com- pared with the cells covered previously. In addition, the cost of this cell is greater. A mercury cell will maintain its 1.35-volt emf steadily for a long time. The mercury cells are used in handheld communication sets, hearing aids, and portable electronic equipment and appliances. This type of cell usually consists of a negative center terminal that is connected

to a zinc cylinder or pellet. The positive case is connected to a mixture of mercuric oxide and graphite. To avoid mercury contamination of soil and water, ordinary methods of trash disposal are not suitable for discarded mercury cells or other devices containing mercury.

The three types of dry cells just covered (Leclanché, alkaline, and mercury) are the most widely used dry cells. The silver-zinc cell, which is similar in construction to the mercury cell, maintains its emf throughout its life. Although the silver-zinc cell is expensive, it has a high ratio of energy to weight. This factor makes it useful in hearing aids, electric watches, and spacecrafts. Some silver cells are rechargeable.

In the zinc-air cell, the negative terminal leads to a porous zinc anode, which is soaked with the electrolyte solution. The cathode is a thin conductive plastic arrangement. The cell has enough wet resistance to prevent slow loss of moisture, yet it is porous enough to permit the entry of oxygen from the air. Finely divided platinum (or a silver alloy) on the cathode promotes an oxygen and water reaction that removes electrons from the cathode to form OH2 ions. Electrons are supplied to the load circuit by zinc atoms as they ionize. Some zinc-air cells are mechanically rechargeable; that is, the oxidized zinc anode (consisting mainly of potassium zincate) is removed, the cell is refilled with water, and a new porous zinc plate containing potassium hydroxide is inserted in the cell. Portable military communication equipment commonly uses zinc-air cells.

Many other primary cell combinations have been investigated, including magnesium- air, iron-air, lithium-nickel, and a 2-volt magnesium cell with magnesium bromide as the electrolyte and a cathode made of manganese dioxide.

The lithium battery, especially, needs to be mentioned here because it has gained wide acceptance in the electronics industry due to its unique properties. In low-current applications, such as the memory-retention circuits of computerized equipment, they often last for many years with virtually undiminished voltage output up to the last moments of life.

Actually, only very little lithium goes into the production of a cell; on average about 1⁄2 gram, which often comprises only about 5% of its total weight. Electrode material LiBF4 is dissolved into a nontoxic electrolyte material called gamma butyrolactone, which has a boiling point of 204°C, hence there is no outgassing during normal operation. In fact, some lithium batteries have operating temperatures from 240°F to 185°F.

Safety precautions must be taken if it becomes necessary to solder wires to the battery. The heat from the soldering iron can cause the battery to explode.

14–4 FUEL CELLS

All of the cells described to this point base their operation on the tendency of energetic electrons to transfer to a material that lacks electrons, such as a positively charged ion. During this process, the anode material itself is used up in the sense that it is converted to a useless low- energy form. The fuel cell also makes use of this same type of electron transfer but with the important difference that the material giving electrons and the material taking electrons are not contained within the cell. The solid electrodes of the cell are not consumed in the process.

The term fuel makes one think of something burning. During the burning (combustion) of any fuel, such as coal, oil, or gas, atoms of the fuel give electrons to oxygen in the air. The energy of the electrons immediately appears as heat. It is correct to think of the gradual consumption of the zinc in a flashlight battery as a slow oxidation (a slow burning process). Research over a number of years has produced very efficient fuel cells. The purpose of such cells is to control the electrons of inexpensive fuels and make them perform useful work as they leave the fuel atoms to join atoms of oxygen or some other electron taker. Fuel cells require high-purity fuel and inexpensive but reliable catalytic surfaces on which the essential reactions take place. Fuel cells also require auxiliary equipment such as gas containers and pressure controls. Research is under way to develop efficient, compact fuel cells for use in electric automobiles and tractors and as a source of power for a residence.

The hydrogen-oxygen fuel cell, as shown in Figure 14–8, has hollow, porous car- bon electrodes immersed in a potassium hydroxide solution. Hydrogen gas is pumped

into one electrode and oxygen gas is pumped into the other. The porous carbon also contains certain metals or metal oxides called catalysts. (A catalyst promotes a chemical re- action.) In this case, the catalyst aids hydrogen molecules (which are pairs of hydrogen atoms) to separate into single atoms that can then combine with the negatively charged hydroxide ions (OH2) of the electrolyte. The combination of H and OH2 forms a molecule of water, H2O, with one electron left over. These surplus electrons are attracted to the electrode supplied by oxygen, as shown in Figure 14–9, where they take part in the reaction of oxygen 1 H2O 1 electrons to form hydroxide ions, OH2. Thus, hydroxide ions are re-formed at the cathode at the same rate as they are used up at the anode. The important resultant change is the conversion of hydrogen and oxygen to water.

The alkaline-hydrogen-oxygen fuel cell just discussed was one of the very early types introduced in the 1960s. There are other types of fuel cells today that offer different characteristics and advantages.

The Polymer Electrolyte Membrane (PEM) Fuel Cell

Polymer electrolyte membrane fuel cells are often called proton exchange membrane fuel cells. These cells offer high-power density combined with low weight and volume compared with other types of fuel cells. PEM fuel cells employ a solid polymer as the electrolyte. The electrodes are porous carbon combined with a platinum catalyst. They require only hydrogen, oxygen from the air, and water to operate. They do not need corrosive fluids like some other fuel cells. PEM fuel cells operate at a relatively low temperature, around 80°C (176°F).

Direct Methanol Fuel Cells

Direct methanol fuel cells are powered by pure methanol mixed with steam. The methanol/steam mixture is fed directly to the fuel cell anode. These fuel cells have an advantage in that they do not have the fuel storage problems of cells that rely on pure hydrogen. Since methanol is a liquid, it is much easier to store. Methanol also has a higher energy density than hydrogen although it does not contain as much energy as gasoline or diesel fuel.

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte. The acid is contained in a Teflon-bonded silicon carbide matrix. The electrodes are composed of porous carbon containing a platinum catalyst. This cell is considered the first generation of modern fuel cells and has been used commercially for stationary power generation and to power large vehicles such as city buses.

Molten Carbonate Fuel Cells

Molten carbonate fuel cells are being developed for natural gas and coal fired power plants. These fuel cells operate at very high temperatures, typically 650°C (1200°F) and above. The high operating temperature has an advantage in that non-precious metals can be used as a catalyst in the anode and cathode electrodes, resulting in much lower cost. Molten carbonate fuel cells can reach efficiencies as high as 60%.

Solid Oxide Fuel Cells

Solid oxide fuel cells use a hard non-porous ceramic compound as the electrolyte. Because the electrolyte is solid, the fuel cell does not have to be constructed in the plate like configuration typical of other types of fuel cells. These fuel cells operate at very high temperatures, 1,000°C (1,830°F). The high operating temperature has an advantage in that non-precious metals can be used as a catalyst in the electrodes. Also, the high operating temperature of the solid oxide fuel cell permits reforming of fuels internally, enabling use of a variety of fuels, which helps reduce the cost of adding a reformer to the system. Solid oxide fuel cells have an efficiency of 50% to 60%.

13–4 GASES AS INSULATORS

At atmospheric pressure, air and other gases are very close to being ideal insulators. When a gas is used as an insulator between two charged plates, as illustrated in Figure 13–7, the current shown by the ammeter is practically zero as long as a moderate voltage is applied. At a sufficiently high voltage, the gas suddenly becomes a conductor, and the current value is limited only by the circuit itself.

At less than atmospheric pressure, this breakdown in the insulating ability of a gas occurs at a lower voltage. However, if the gas pressure is increased, such as in com- pressed air, then the applied voltage necessary to start conduction in the gas must be increased a proportional amount. For example, a higher voltage is required to fire a spark plug under compression in an engine than to create the spark in the open air.

Sparks and Arcs

A spark is a noisy, irregular discharge; successive sparks follow separate paths. This statement refers to sparks that indicate electron paths. The sparks that are visible when

a wire is brushed across the terminals of a car battery are similar to the sparks from a grinding wheel; that is, they are tiny fragments of hot metal being sprayed in all directions.

An arc is quieter than a spark and is a continuous discharge. An arc, in air, consists of a conducting path of highly heated gas or metallic vapor. In this case, the air offers almost no resistance; therefore, an external resistance must be present in the circuit to limit the current. The potential drop across the arc itself is moderately low, in the or- der of 15 to 50 volts. An arc is accompanied by high-intensity light. Most of this light is due to gas or metallic vapor, but some light comes from the hot cathode. The cathode temperature of a carbon arc may reach 9,000°F.

13–5 GASEOUS CONDUCTION BY IONIZATION

Gas can be made to conduct a current if a moderately high voltage of 50 volts or more is applied to a gas at a low pressure inside a glass tube, as shown in Figure 13–8. (Note that the wires from the voltage source are sealed inside the tube.) In air, an arc can be started by touching and then separating a pair of conducting contacts, as in a welding arc or a carbon arc.

“How does the gas conduct?” “What happens in the gas so that it becomes a conductor?” The process of ionization causes a gas to become a conductor. In this process, electrons are removed from gas atoms so they become positively charged ions. The free electrons are highly mobile. As a result, conduction in the gas is due mainly to electron flow (as in metals). Although mobile positive ions do exist (as in liquids), the nature of conduction in a gas differs greatly from conduction in either a solid or a liquid.

How Ionization Occurs

The most important process involved in ionization is electron impact. The ionization process can begin when an electron is freed from the negative wire. More likely sources of free electrons are radiation and heat (to be investigated shortly). Once an electron is set free, it gains speed in the enclosing glass tube or envelope as it is repelled by the negative

(2) wire and attracted toward the positive (1) wire; see Figure 13–9. If the electron hits an atom while it is still moving slowly, it merely bounces off the atom and again accelerates toward the positive terminal. When the electron is moving fast enough, any collision with an atom causes one or more electrons to be knocked free of the atom.

The minimum amount of energy that an electron must possess to cause ionization (removal of electrons from atoms) is called the ionization potential. The amount of

energy required depends on the kind of atom that is to provide the electrons. The ionization potential for sodium vapor is about 5 volts. For most gases, the ionization potential ranges between 10 and 25 volts (for example, mercury vapor requires 10.4 volts; neon,

volts; and helium, 24.5 volts). In other words, to ionize neon, an electron must fall through a potential difference of 21.5 volts.

The ionization process is shown in Figure 13–10. As one can see, the original colliding electron frees another electron. Now there are two free electrons that begin to accelerate toward the positive wire. These electrons collide with more atoms, and each electron frees an additional electron. Now there are four electrons. The subsequent series of collisions between electrons and atoms frees more electrons so that in a thousandth of a second, a million electrons may be released.

Let us compare this action with what happens to a heavily laden apple tree when one apple at the top starts to fall to the ground. The one apple will strike another apple, and the two apples will strike other apples, until finally, by the time the apple fall is completed, there is a bushel of apples on the ground.

Light is produced during gaseous conduction as a result of collisions that are not violent enough to free electrons. The collision of a moderately fast-moving electron with an atom can transfer some of the energy of the electron to the electrons of the atom. This transfer of energy produces an excited state in the atom. In this state, the atom has excess energy and rids itself of this energy by emitting light. The color of the emitted light is characteristic of the energy state of the atom.

A second process that produces ionization in a gas is radiation. Rays striking the earth from space may ionize a few atoms and thus provide the first electrons required

to start the collision process. After conduction begins, electron disturbances in the atoms produce visible light and high-frequency (ultraviolet) radiation. This high-frequency radiation is absorbed by other atoms, thus providing them with enough energy to free electrons.

Still another phenomenon that produces ionization is heat. A hot gas is a better conductor and begins to conduct more readily than a cold gas. Heat is the movement of atoms and molecules. At higher temperatures, collisions between atoms can become violent enough to dislodge electrons. Many of the atoms may have a higher temperature than the average and thus can cause ionizing collisions. An ordinary flame contains many ions and is therefore a poor insulator. This statement may be proved by bringing a match flame near a charged electroscope.

Another source of electrons is the collision of positive ions. Although positive ions have lost one or more electrons, they still have electrons that can be set free. Therefore, when positive ions collide, electrons are released and the ions are even more positively charged. In addition, excited gas atoms can collide with other kinds of atoms, causing these atoms to release electrons. For example, conduction can be maintained more easily in neon gas if a trace of nitrogen gas is added to the neon. Excited neon atoms will collide with the nitrogen atoms. In the process, the nitrogen atoms are given enough energy to cause them to release electrons.

Why Low-Pressure Gas Conducts Better Than High-Pressure Compressed Gas

In gas at atmospheric pressure, a free electron collides with gas atoms so frequently that it never travels long enough to gain the speed required to ionize an atom by collision; see Figure 13–11A. At lower pressures, gas atoms are farther apart; therefore, an electron has the opportunity to gain enough kinetic energy to make an ionizing collision; see Figure 13–11B. As a result, designers of gas tubes must determine the mean free path of electrons to ensure that ionizing collisions will occur. The mean free path is the average distance traveled by a particle between collisions.

Electrons and Ions Formed by Collisions

The electrons that reach the positive terminal accomplish useful conduction. How- ever, in a glass tube, many of the free electrons collect on the walls of the tube. Any positive ions that hit the walls are neutralized by the negative charges collected there.

On the average, approximately 1 atom out of every 1,000 atoms is ionized in a typical gas conduction tube. The glowing discharge in the tube is called plasma and consists of positive ions, electrons, and neutral atoms. The voltage drop in this region is not large; most of the voltage drop occurs near the electrodes. Scientists are still con- ducting experiments to define potential applications of the plasma phenomenon. For example, a number of years ago it was found that bursts of current (of a very high amperage) in hydrogen at low pressure produce plasmas that are contained by their own magnetic fields. The purpose of the experiments was to make the particles in the plasma reach speeds corresponding to temperatures of millions of degrees. It is at such temperatures that hydrogen nuclei fuse into helium. This hydrogen fusion process is the same as that occurring in the sun. Hydrogen fusion happens suddenly when an H-bomb explodes.

13–6 CONDUCTION AND IONS IN NATURE

An occasional evening celestial display in the northern skies, the Northern Lights (aurora borealis) is believed to be the result of conduction in the very thin upper part of the atmosphere. This display appears to be caused by electrons that are given off by disturbances in the sun and then travel through space until they reach the Earth’s atmosphere.

In addition to electrons, high-speed protons also reach Earth from space. These protons strike air molecules and produce intense radiation that ionizes various layers of the atmosphere. These conducting layers have the ability to reflect radio signals; these reflections are often useful.

Lightning is a high-voltage spark. At times atmospheric conditions that are not violent enough to cause lightning produce a continuous discharge that can be seen on steeple tops or the masts of ships.

In the vicinity of pointed electrodes, a strong electric field will ionize the air. The discharge that occurs is visible if other illumination does not interfere. This type of dis- charge is called a corona and occurs frequently in high-voltage equipment if precautions are not taken to prevent it. Corona effects can be a serious source of power loss in high-voltage transmission lines.

13–7 CONDUCTION IN A VACUUM

If gas is pumped from a glass tube until the pressure in the tube is reduced to approximately 0.0001 mm of mercury (760 mm of mercury 5 atmospheric pressure), the brilliant glow of the conducting gas is no longer visible. The gas molecules are so far apart that few ions are formed. In this case, conduction is due almost entirely to electron flow. The glass tubing itself may glow because of electrons striking the glass.

If the gas pressure is reduced even more in a cold cathode tube, the discharge may stop entirely. If the cathode wire is then reheated by another current, conduction through the vacuum continues by means of the electrons that escape from the hot cathode.

Over a century ago, the people who were experimenting with conduction in gases and vacuums did not know of the existence of electrons. These experimenters gave the name cathode ray to the emissions from the cathodes of their tubes. In 1869, the German scientist Johann Hittorf described the glow resulting when glass was struck by these rays. His experiments showed that the cathode rays traveled in straight lines. An English scientist, Wilhelm Crookes, found that the paths of these rays can be bent by a magnet. In addition, he found that these rays can be focused, that they heat the objects they strike, and that their speed depends on the applied voltage. In 1879, Crookes suggested that these rays might be “an ultra-gaseous state of matter.” The French investigator Jean-Baptiste Perrin showed in 1895 that the rays carried a negative charge and that positive ions were also formed.

Wilhelm Roentgen, while experimenting with cathode-ray tubes in 1895, found that when the cathode rays strike metal or glass, a new kind of radiation is produced. He deter- mined that these invisible rays passed readily through air, paper, and wood; through thin metal better than thick; through flesh better than bone; and through aluminum better than lead. The rays caused fluorescence in some minerals, affected photographic plates, and were unaffected by a magnetic field. Roentgen called these rays X rays. Within a few months of his discovery, physicians were using X rays as an aid in setting broken bones.

In 1897, the English scientist J. J. Thomson completed a series of experiments in which he was able to measure the ratio of the weight of the negative particle to the electrical charge; see Figure 13–12. Modern cathode-ray tubes, such as those used in oscilloscopes and TV picture tubes, use the same principles of electron ray deflection that Thomson used in his experiments.

As shown in Figure 13–13, in oscilloscopes, the electron beam is accelerated from the cathode and is deflected horizontally at a known rate. The voltage (whose trace is to be observed) is used to deflect the beam vertically at the same time. The result is a time graph of the observed voltage.

In TV picture tubes, magnetic coils are used to sweep the beam of electrons horizon- tally and vertically. At the same time, the intensity of the beam is changed to produce variations in the lightness and darkness of the picture. There continues to be some concern that harmful X rays are produced when the 20,000-volt electrons strike the face of the TV picture tube. This voltage generally is not great enough to produce the highly energetic and penetrating X rays. Any X rays that may be produced are absorbed in the glass and the few inches of air in front of the tube. A more serious source of X rays is the high-voltage rectifier tube in larger TV sets. In the past, some TV receivers radiated undesirable quantities of X rays through the bottom of the set because insufficient metal shielding was used around the high-voltage supply.

The American experimenter Thomas Alva Edison might have discovered electrons. One day in 1883, one of his assistants accidentally connected a meter to a dead-end wire sealed into one of Edison’s experimental electric lamps. When the lamp filament was heated, a small current was indicated on the meter; see Figure 13–14. The heated lamp filament was certainly emitting electrons. The free electrons in the lamp were attracted to the positive wire through the meter, thus giving rise to the current. This event was reported, but it was

not until 20 years later that J. A. Fleming constructed a two-element valve to use as a rectifier and explained the action taking place in this device. The elements are the filament, which emits electrons, and the plate, which collects the electrons when it is positive. (Valve is the British term for vacuum tube.) In later years when Edison was asked why the third wire was sealed into his lamp bulb, he is reported to have said, “I have forgotten.”

Fleming’s concept of electron conduction in vacuum tubes has been applied to hundreds of devices designed to achieve specific tasks. Research conducted to improve glass- enclosed vacuum tubes led eventually to the discovery that special solid materials have the same characteristics as the simple vacuum tubes. As a result, the field of solid-state electronics was born.

SUMMARY

• Electrical conduction in liquids is the movement of positive and negative ions. There are no free electrons as in metals.

• Positive and negative ions are formed by a type of chemical combination in which one metallic element, or group, transfers electrons to a nonmetallic element or group.

• If a chemical compound can be dissolved in water, the charged ions of the compound become freely movable in the solution.

• In electroplating processes, the solution contains ions of the plating metal. The article to be plated is connected to the negative terminal of the current source. A bar of the plating metal is connected to the positive terminal.

• Metal removed from the plating solution and deposited on the object is replaced in the solution by metal dissolved from the positive bar.

• In liquid conduction, positive ions move in one direction and negative ions move in the other direction, resulting in a permanent separation of the parts of the compound. This decomposition process is called electrolysis.

• Gas is a good insulator until sufficient voltage is applied. The sudden ionization process changes the gas to a conductor.

• Ionization of a gas is the freeing of electrons from the gas molecules. The positively charged gas particles are the positive ions. The electrons are the movable negative particles.

• Conduction in a gas consists mainly of electron movement. As electrons collide with gas atoms, a new supply of electrons is continually set free from the atoms.

• Gases at low pressure conduct more readily than gases at high pressure.

• At pressures close to a vacuum, electrons from the cathode travel in straight paths.

These electrons were called cathode rays. The discovery of electrons consisted of the measurement of their properties in cathode-ray tubes.

Achievement Review

1. How do metals differ from nonmetallic elements in the structure of their atoms?

2. What are ions?

3. Is dry salt a conductor? Why or why not?

4. Can a piece of wood be electroplated? Why or why not?

5. What happens if the wires leading to the generator in Figure 13–5 are reversed?

6. When a car battery is disconnected from the charging line by pulling the clip off the battery post, occasionally the top is blown off the battery. Why does this happen and how can it be avoided?

7. Water is regarded as an insulator. If water is an insulator, why is it that electrocution is possible by contact between a power line and wet earth?

8. Special problems in insulation had to be solved in the development of electrical control systems for high-altitude missiles. Why should any special problems exist?

9. Who discovered electrons?

10. What is the difference between cathode rays and electrons?

11. Name some useful examples of gaseous conduction.

Is it true that practically all of the useful discoveries in this field have already been made? Explain.

13–4 GASES AS INSULATORS

At atmospheric pressure, air and other gases are very close to being ideal insulators. When a gas is used as an insulator between two charged plates, as illustrated in Figure 13–7, the current shown by the ammeter is practically zero as long as a moderate voltage is applied. At a sufficiently high voltage, the gas suddenly becomes a conductor, and the current value is limited only by the circuit itself.

At less than atmospheric pressure, this breakdown in the insulating ability of a gas occurs at a lower voltage. However, if the gas pressure is increased, such as in com- pressed air, then the applied voltage necessary to start conduction in the gas must be increased a proportional amount. For example, a higher voltage is required to fire a spark plug under compression in an engine than to create the spark in the open air.

Sparks and Arcs

A spark is a noisy, irregular discharge; successive sparks follow separate paths. This statement refers to sparks that indicate electron paths. The sparks that are visible when

a wire is brushed across the terminals of a car battery are similar to the sparks from a grinding wheel; that is, they are tiny fragments of hot metal being sprayed in all directions.

An arc is quieter than a spark and is a continuous discharge. An arc, in air, consists of a conducting path of highly heated gas or metallic vapor. In this case, the air offers almost no resistance; therefore, an external resistance must be present in the circuit to limit the current. The potential drop across the arc itself is moderately low, in the or- der of 15 to 50 volts. An arc is accompanied by high-intensity light. Most of this light is due to gas or metallic vapor, but some light comes from the hot cathode. The cathode temperature of a carbon arc may reach 9,000°F.

13–5 GASEOUS CONDUCTION BY IONIZATION

Gas can be made to conduct a current if a moderately high voltage of 50 volts or more is applied to a gas at a low pressure inside a glass tube, as shown in Figure 13–8. (Note that the wires from the voltage source are sealed inside the tube.) In air, an arc can be started by touching and then separating a pair of conducting contacts, as in a welding arc or a carbon arc.

“How does the gas conduct?” “What happens in the gas so that it becomes a conductor?” The process of ionization causes a gas to become a conductor. In this process, electrons are removed from gas atoms so they become positively charged ions. The free electrons are highly mobile. As a result, conduction in the gas is due mainly to electron flow (as in metals). Although mobile positive ions do exist (as in liquids), the nature of conduction in a gas differs greatly from conduction in either a solid or a liquid.

How Ionization Occurs

The most important process involved in ionization is electron impact. The ionization process can begin when an electron is freed from the negative wire. More likely sources of free electrons are radiation and heat (to be investigated shortly). Once an electron is set free, it gains speed in the enclosing glass tube or envelope as it is repelled by the negative

(2) wire and attracted toward the positive (1) wire; see Figure 13–9. If the electron hits an atom while it is still moving slowly, it merely bounces off the atom and again accelerates toward the positive terminal. When the electron is moving fast enough, any collision with an atom causes one or more electrons to be knocked free of the atom.

The minimum amount of energy that an electron must possess to cause ionization (removal of electrons from atoms) is called the ionization potential. The amount of

energy required depends on the kind of atom that is to provide the electrons. The ionization potential for sodium vapor is about 5 volts. For most gases, the ionization potential ranges between 10 and 25 volts (for example, mercury vapor requires 10.4 volts; neon,

volts; and helium, 24.5 volts). In other words, to ionize neon, an electron must fall through a potential difference of 21.5 volts.

The ionization process is shown in Figure 13–10. As one can see, the original colliding electron frees another electron. Now there are two free electrons that begin to accelerate toward the positive wire. These electrons collide with more atoms, and each electron frees an additional electron. Now there are four electrons. The subsequent series of collisions between electrons and atoms frees more electrons so that in a thousandth of a second, a million electrons may be released.

Let us compare this action with what happens to a heavily laden apple tree when one apple at the top starts to fall to the ground. The one apple will strike another apple, and the two apples will strike other apples, until finally, by the time the apple fall is completed, there is a bushel of apples on the ground.

Light is produced during gaseous conduction as a result of collisions that are not violent enough to free electrons. The collision of a moderately fast-moving electron with an atom can transfer some of the energy of the electron to the electrons of the atom. This transfer of energy produces an excited state in the atom. In this state, the atom has excess energy and rids itself of this energy by emitting light. The color of the emitted light is characteristic of the energy state of the atom.

A second process that produces ionization in a gas is radiation. Rays striking the earth from space may ionize a few atoms and thus provide the first electrons required

to start the collision process. After conduction begins, electron disturbances in the atoms produce visible light and high-frequency (ultraviolet) radiation. This high-frequency radiation is absorbed by other atoms, thus providing them with enough energy to free electrons.

Still another phenomenon that produces ionization is heat. A hot gas is a better conductor and begins to conduct more readily than a cold gas. Heat is the movement of atoms and molecules. At higher temperatures, collisions between atoms can become violent enough to dislodge electrons. Many of the atoms may have a higher temperature than the average and thus can cause ionizing collisions. An ordinary flame contains many ions and is therefore a poor insulator. This statement may be proved by bringing a match flame near a charged electroscope.

Another source of electrons is the collision of positive ions. Although positive ions have lost one or more electrons, they still have electrons that can be set free. Therefore, when positive ions collide, electrons are released and the ions are even more positively charged. In addition, excited gas atoms can collide with other kinds of atoms, causing these atoms to release electrons. For example, conduction can be maintained more easily in neon gas if a trace of nitrogen gas is added to the neon. Excited neon atoms will collide with the nitrogen atoms. In the process, the nitrogen atoms are given enough energy to cause them to release electrons.

Why Low-Pressure Gas Conducts Better Than High-Pressure Compressed Gas

In gas at atmospheric pressure, a free electron collides with gas atoms so frequently that it never travels long enough to gain the speed required to ionize an atom by collision; see Figure 13–11A. At lower pressures, gas atoms are farther apart; therefore, an electron has the opportunity to gain enough kinetic energy to make an ionizing collision; see Figure 13–11B. As a result, designers of gas tubes must determine the mean free path of electrons to ensure that ionizing collisions will occur. The mean free path is the average distance traveled by a particle between collisions.

Electrons and Ions Formed by Collisions

The electrons that reach the positive terminal accomplish useful conduction. How- ever, in a glass tube, many of the free electrons collect on the walls of the tube. Any positive ions that hit the walls are neutralized by the negative charges collected there.

On the average, approximately 1 atom out of every 1,000 atoms is ionized in a typical gas conduction tube. The glowing discharge in the tube is called plasma and consists of positive ions, electrons, and neutral atoms. The voltage drop in this region is not large; most of the voltage drop occurs near the electrodes. Scientists are still con- ducting experiments to define potential applications of the plasma phenomenon. For example, a number of years ago it was found that bursts of current (of a very high amperage) in hydrogen at low pressure produce plasmas that are contained by their own magnetic fields. The purpose of the experiments was to make the particles in the plasma reach speeds corresponding to temperatures of millions of degrees. It is at such temperatures that hydrogen nuclei fuse into helium. This hydrogen fusion process is the same as that occurring in the sun. Hydrogen fusion happens suddenly when an H-bomb explodes.

13–6 CONDUCTION AND IONS IN NATURE

An occasional evening celestial display in the northern skies, the Northern Lights (aurora borealis) is believed to be the result of conduction in the very thin upper part of the atmosphere. This display appears to be caused by electrons that are given off by disturbances in the sun and then travel through space until they reach the Earth’s atmosphere.

In addition to electrons, high-speed protons also reach Earth from space. These protons strike air molecules and produce intense radiation that ionizes various layers of the atmosphere. These conducting layers have the ability to reflect radio signals; these reflections are often useful.

Lightning is a high-voltage spark. At times atmospheric conditions that are not violent enough to cause lightning produce a continuous discharge that can be seen on steeple tops or the masts of ships.

In the vicinity of pointed electrodes, a strong electric field will ionize the air. The discharge that occurs is visible if other illumination does not interfere. This type of dis- charge is called a corona and occurs frequently in high-voltage equipment if precautions are not taken to prevent it. Corona effects can be a serious source of power loss in high-voltage transmission lines.

13–7 CONDUCTION IN A VACUUM

If gas is pumped from a glass tube until the pressure in the tube is reduced to approximately 0.0001 mm of mercury (760 mm of mercury 5 atmospheric pressure), the brilliant glow of the conducting gas is no longer visible. The gas molecules are so far apart that few ions are formed. In this case, conduction is due almost entirely to electron flow. The glass tubing itself may glow because of electrons striking the glass.

If the gas pressure is reduced even more in a cold cathode tube, the discharge may stop entirely. If the cathode wire is then reheated by another current, conduction through the vacuum continues by means of the electrons that escape from the hot cathode.

Over a century ago, the people who were experimenting with conduction in gases and vacuums did not know of the existence of electrons. These experimenters gave the name cathode ray to the emissions from the cathodes of their tubes. In 1869, the German scientist Johann Hittorf described the glow resulting when glass was struck by these rays. His experiments showed that the cathode rays traveled in straight lines. An English scientist, Wilhelm Crookes, found that the paths of these rays can be bent by a magnet. In addition, he found that these rays can be focused, that they heat the objects they strike, and that their speed depends on the applied voltage. In 1879, Crookes suggested that these rays might be “an ultra-gaseous state of matter.” The French investigator Jean-Baptiste Perrin showed in 1895 that the rays carried a negative charge and that positive ions were also formed.

Wilhelm Roentgen, while experimenting with cathode-ray tubes in 1895, found that when the cathode rays strike metal or glass, a new kind of radiation is produced. He deter- mined that these invisible rays passed readily through air, paper, and wood; through thin metal better than thick; through flesh better than bone; and through aluminum better than lead. The rays caused fluorescence in some minerals, affected photographic plates, and were unaffected by a magnetic field. Roentgen called these rays X rays. Within a few months of his discovery, physicians were using X rays as an aid in setting broken bones.

In 1897, the English scientist J. J. Thomson completed a series of experiments in which he was able to measure the ratio of the weight of the negative particle to the electrical charge; see Figure 13–12. Modern cathode-ray tubes, such as those used in oscilloscopes and TV picture tubes, use the same principles of electron ray deflection that Thomson used in his experiments.

As shown in Figure 13–13, in oscilloscopes, the electron beam is accelerated from the cathode and is deflected horizontally at a known rate. The voltage (whose trace is to be observed) is used to deflect the beam vertically at the same time. The result is a time graph of the observed voltage.

In TV picture tubes, magnetic coils are used to sweep the beam of electrons horizon- tally and vertically. At the same time, the intensity of the beam is changed to produce variations in the lightness and darkness of the picture. There continues to be some concern that harmful X rays are produced when the 20,000-volt electrons strike the face of the TV picture tube. This voltage generally is not great enough to produce the highly energetic and penetrating X rays. Any X rays that may be produced are absorbed in the glass and the few inches of air in front of the tube. A more serious source of X rays is the high-voltage rectifier tube in larger TV sets. In the past, some TV receivers radiated undesirable quantities of X rays through the bottom of the set because insufficient metal shielding was used around the high-voltage supply.

The American experimenter Thomas Alva Edison might have discovered electrons. One day in 1883, one of his assistants accidentally connected a meter to a dead-end wire sealed into one of Edison’s experimental electric lamps. When the lamp filament was heated, a small current was indicated on the meter; see Figure 13–14. The heated lamp filament was certainly emitting electrons. The free electrons in the lamp were attracted to the positive wire through the meter, thus giving rise to the current. This event was reported, but it was

not until 20 years later that J. A. Fleming constructed a two-element valve to use as a rectifier and explained the action taking place in this device. The elements are the filament, which emits electrons, and the plate, which collects the electrons when it is positive. (Valve is the British term for vacuum tube.) In later years when Edison was asked why the third wire was sealed into his lamp bulb, he is reported to have said, “I have forgotten.”

Fleming’s concept of electron conduction in vacuum tubes has been applied to hundreds of devices designed to achieve specific tasks. Research conducted to improve glass- enclosed vacuum tubes led eventually to the discovery that special solid materials have the same characteristics as the simple vacuum tubes. As a result, the field of solid-state electronics was born.

SUMMARY

• Electrical conduction in liquids is the movement of positive and negative ions. There are no free electrons as in metals.

• Positive and negative ions are formed by a type of chemical combination in which one metallic element, or group, transfers electrons to a nonmetallic element or group.

• If a chemical compound can be dissolved in water, the charged ions of the compound become freely movable in the solution.

• In electroplating processes, the solution contains ions of the plating metal. The article to be plated is connected to the negative terminal of the current source. A bar of the plating metal is connected to the positive terminal.

• Metal removed from the plating solution and deposited on the object is replaced in the solution by metal dissolved from the positive bar.

• In liquid conduction, positive ions move in one direction and negative ions move in the other direction, resulting in a permanent separation of the parts of the compound. This decomposition process is called electrolysis.

• Gas is a good insulator until sufficient voltage is applied. The sudden ionization process changes the gas to a conductor.

• Ionization of a gas is the freeing of electrons from the gas molecules. The positively charged gas particles are the positive ions. The electrons are the movable negative particles.

• Conduction in a gas consists mainly of electron movement. As electrons collide with gas atoms, a new supply of electrons is continually set free from the atoms.

• Gases at low pressure conduct more readily than gases at high pressure.

• At pressures close to a vacuum, electrons from the cathode travel in straight paths.

These electrons were called cathode rays. The discovery of electrons consisted of the measurement of their properties in cathode-ray tubes.

Achievement Review

1. How do metals differ from nonmetallic elements in the structure of their atoms?

2. What are ions?

3. Is dry salt a conductor? Why or why not?

4. Can a piece of wood be electroplated? Why or why not?

5. What happens if the wires leading to the generator in Figure 13–5 are reversed?

6. When a car battery is disconnected from the charging line by pulling the clip off the battery post, occasionally the top is blown off the battery. Why does this happen and how can it be avoided?

7. Water is regarded as an insulator. If water is an insulator, why is it that electrocution is possible by contact between a power line and wet earth?

8. Special problems in insulation had to be solved in the development of electrical control systems for high-altitude missiles. Why should any special problems exist?

9. Who discovered electrons?

10. What is the difference between cathode rays and electrons?

11. Name some useful examples of gaseous conduction.

Is it true that practically all of the useful discoveries in this field have already been made? Explain.

Conduction in Liquids and Gases

13–1 THE IONIZATION PROCESS

In solid conductors, the transfer of electrical charges is made by the movement of electrons. In liquids and gases, the transfer of electrical charges depends on particles

called ions. Ions were defined in Chapter 1 as atoms that have become electrically unbalanced by virtue of having lost or gained electrons.

To briefly review this concept, consider the model of a magnesium atom alongside a chlorine atom, as shown in Figure 13–1.

You should recall that magnesium is classified as a metal because it has only two valence electrons. By contrast, chlorine gas has seven valence electrons and is, therefore, classified as a nonconductor.

When a piece of magnesium is heated in the presence of chlorine gas, the two substances will chemically interact to form a new compound: a metallic salt called magnesium chloride. During this chemical process, each magnesium atom gives up its two valence electrons, which are then transferred to two neighboring atoms, as shown in Figure 13–2. These atoms are no longer known as atoms, since they now have been electrically charged. After an originally neutral atom becomes electrically charged, it is called an ion.

The magnesium atom has lost electrons (negative charges) and is now positively charged. Thus, it is called a positive ion. Likewise, the chlorine atoms have gained additional electrons and have been transformed into negative ions. Notice in Figure 13–2 that each ion has eight electrons in its outer shell.

This newly formed substance, magnesium chloride, is similar to table salt. The chemical name for table salt is sodium chloride; that is, the elements that make up table salt are a metal, sodium, and a nonmetal, chlorine.

The conducting ability of a solution of salt in water can be demonstrated with the equipment shown in Figure 13–3. First, fill the glass with kerosene, and then connect the circuit to the 120-volt line. Nothing happens to the lamp as a result of this step. Now add some table salt or magnesium chloride to the kerosene. The neutral molecules of the kerosene do not attract the undissolved charged ions of the salt. Again, there is no current through the lamp.

Fill another glass with pure distilled water (if available) and connect the circuit. Note that nothing happens. If water from the faucet is used in the glass, the lamp may light very dimly. When salt is added to the water and is stirred to dissolve, the lamp brightens. What causes this increased conductivity when magnesium chloride or sodium

chloride is dissolved in water? The dissolved substance does not retain its molecular structure but breaks up into its ion particles, which are free to move through the liquid.

Figure 13–4 shows a water solution consisting of negative chlorine ions and positive magnesium ions. When two electrically charged wires are submerged into the solution, a two-way movement of ions takes place. The negative chlorine ions are being attracted by the positive wire, and the positive magnesium ions are pulled toward the negative wire. When the ions reach their respective wires, the resulting action depends on the kinds of ions and the type of wire used.

In this manner, water solutions of acids, bases, and metallic salts can conduct large amounts of current. Such solutions are known as electrolytes.

The following points summarize the information presented so far:

• Metals and nonmetals unite to form compounds by transferring electrons from the metallic atoms to the nonmetallic atoms.

• As a result of the electron transfer, the compound consists of positively charged metallic

ions and negatively charged nonmetallic ions.

• Some compounds dissolve in water and dissociate; that is, the ions become separated and move freely. Ions are much larger and heavier than electrons.

• Conduction in metals consists of the movement of free (negative) electrons.

• Conduction in solutions, known as electrolytes, consists of the movement of free positive and negative ions in opposite directions.

13–2 USEFUL CHEMICAL COMPOUNDS

The ions covered so far are formed by the electrical charging (either positive or negative) of a single atom. More complicated ions, however, do exist. These ions consist of charged groups of atoms. The following four compounds, each of which is useful in electrical processes, are formed, in part, of these larger ions.

Sulfuric Acid. This compound is used as the electrolyte in automobile batteries. The chemical formula for sulfuric acid is H2SO4. In other words, one molecule of the acid contains two atoms of hydrogen, one atom of sulfur, and four atoms of oxygen. When mixed with water, each molecule can separate into the ions shown: H1 H1 SO422. The 1 sign on the H indicates a hydrogen ion (a hydrogen atom that has lost its one electron). The is called a sulfate ion and consists of one sulfur atom, four oxygen atoms, and two extra electrons. (Notice that an ion can be either one electrically charged atom or a group of such atoms.) The two electrons are necessary to hold the SO4 group together. The electrons are part of an electron-sharing arrangement that keeps the electron rings of the sulfur and oxygen atoms tied together.

Ammonium Chloride. The electrolyte in a dry cell is a solution of ammonium chloride dissolved in water. Pure ammonium chloride is a white solid. The formula for ammonium

chloride is NH4Cl. Ammonium chloride separates into two ions when it dissolves: NH41 and Cl2. The symbol NH41 means that one nitrogen atom and four hydrogen atoms are grouped together with the loss of one electron. The electron lost by the NH4 group is taken by the chlorine atom to form a negatively charged chlorine ion. The NH41 group is called an ammonium ion.

Cuprous Cyanide. The commercial process of electroplating copper on iron uses a poi- sonous solid called cuprous cyanide as the plating solution. The formula for cuprous cyanide is CuCN. In solution, the compound separates into the ions Cu1 and CN2. Cu is the symbol for copper. Cu1 represents the cuprous ion (a copper atom that has lost its outer- most electron). CN2 is one atom of carbon and one atom of nitrogen held together with the help of one electron taken from the copper atom. CN2 is called the cyanide ion.

Copper Sulfate. Some copper-plating processes use a copper compound called copper sulfate, CuSO4. Copper sulfate is a solid in the form of blue crystals. When copper sulfate dissolves in water, it forms Cu11 and SO422 ions. Under some conditions, a copper atom can lose two of its electrons to form an ion called a cupric ion. SO422 is the same sulfate ion that appears in sulfuric acid.

13–3 ELECTROPLATING AND ELECTROLYSIS

To illustrate the electrical process that occurs in the plating of metals, let us examine the plating of an object with copper from a cyanide solution. The object to be plated, Figure 13–5, must be an electrical conductor. It is connected to the negative terminal of a battery or DC generator and is covered by the cuprous cyanide solution. The positive terminal of the supply is connected to a copper bar, which is also covered by the solution. When the circuit is complete, positively charged copper ions in the liquid move toward the object to be plated. When the copper ions touch the negatively charged object, the ions pick up electrons and become neutral atoms of copper. As the ions become atoms at the surface of the negatively charged object, they form a copper coating over the object. As long as the generator pushes electrons onto the object to be plated so that its negative charge is maintained, a copper coating of increasing thickness will be deposited on the object.

When the cyanide ions hit the positive copper bar, copper atoms on the surface of the bar lose electrons and become ions. In other words, the negative CN ion causes a copper atom to lose an electron and become a positive ion. The copper ion is attracted into the solution by the negative ions present in the solution. The electrons lost by the copper atoms drift toward the generator. Just as many copper ions are liberated from the copper bar as are de- posited on the object to be plated; therefore, the solution stays at a constant strength.

In any electroplating process, the solution must contain the ions of the metal that is to form the coating. Since metal ions are all positively charged, the object to be plated is connected to the negative wire. The positive terminal normally is made of the same metal that is to form the coating. Plating solutions also contain ingredients other than the dissolved metal compound. These other ingredients are added to prevent corrosion of the object to be plated, to prevent poisonous fumes, and to aid in forming a smooth coating.

The purification of metals can be accomplished through a large-scale plating process. Almost all commercial copper is refined by plating in a copper sulfate solution.

Pure copper is deposited on the negative plate. Impurities from the crude copper on the positive terminal either stay in the solution or never leave the copper bar at all. Impure copper has a high resistance. Copper wire manufacturers require electrolytically purified copper in the production of low-resistance conductor wire. Zinc and other metals are purified in a similar fashion.

The term electrolysis refers to the process of separating elements by the use of electrical energy. The commercial production of metallic and nonmetallic elements is often a matter of separating the element from others with which it is chemically combined. In 1885, aluminum was a rare and precious metal because it was difficult to separate from its abundant compound, aluminum oxide. Aluminum metal became inexpensive when a process was discovered for separating the aluminum from the oxygen. The extraction of aluminum is an electrical process in which electrons are removed from oxygen ions and are returned to aluminum ions. Magnesium metal is extracted from magnesium chloride (from seawater) by electrically separating the magnesium ions from the chlorine ions.

One process that has an important application in batteries is the electrolysis of water containing sulfuric acid. Hydrogen and oxygen gases are produced in this process; however, the commercial production of large amounts of these gases is achieved more economically by other processes.

Electrolysis of Sulfuric Acid in Water

Two new terms are shown in Figure 13–6: the cathode is the terminal or electrode where electrons enter the cell, and the anode is the electrode that carries electrons away from the cell.

Assume that the cathode and anode are made of materials that are not affected by the sulfuric acid solution or by the hydrogen or oxygen released in the process. When hydrogen ions touch the cathode, they pick up electrons from it and become neutral atoms. These atoms of hydrogen form hydrogen gas, which escapes from the solution as bubbles. When the sulfate ions approach the anode, they cause the water molecules to break up. The positive anode requires electrons, and the water molecules part with electrons more readily than the SO422 ions. Therefore, the removal of two electrons from the H2O molecule leaves two H1 ions and one oxygen atom. The oxygen atoms bubble away as gas at the anode, and the H1 ions stay in the solution. For each pair of hydrogen ions formed at the anode, a pair of hydrogen ions is discharged as gas at the cathode. The amount of acid in the solution remains constant, and the water is gradually consumed.

If lead plates are used for the cathode and anode, then not all of the oxygen bubbles away. A portion of the oxygen combines with the lead anode to form lead dioxide. The hydrogen does not affect the cathode, which remains as pure lead.

Applications of plates of lead and lead dioxide in a solution of sulfuric acid relates to the working principles of storage batteries. More detailed information about this principle is offered in Section 14–5.

Electrolytic Corrosion

Corrosion is the rusting or wearing away of a metal by the formation of a chemical compound of the metal. In some instances, the corrosion process is desirable. For example, in copper plating, the copper anode dissolves and in that way maintains the necessary copper ions in the plating solution. An example of an undesirable type of corrosion is shown when there is an electrical current through the earth that can follow buried water pipes for a portion of its path. At any points where the buried metal is positive (anodic) as compared to the earth, the attack on the pipe by negative ions converts the metal atoms into metal ions; in other words, iron pipe is converted into iron rust.