science universe

5–4 SIGNIFICANT DIGITS

Most practical calculations are generally limited to only three significant digits. What does this statement mean? An example will make this clear.

Assume that some friends you know want to brag about the performance of their new automobile. They tell you that they clocked their speed at 104.6784935 miles per hour. (That is downhill with a tailwind blowing.) How would you react to such information, short of calling them liars? Obviously, speedometers do not give 10-digit readouts. Neither do electrical measuring instruments. Most such instruments (called meters) are limited to three-digit accuracy.

Does this mean that electricity is a sloppy science? Quite the contrary! Electronic measurements can be extremely precise; however, practical considerations would recommend that all numbers be limited to three-digit accuracy. Thus, your friends should have reported their speed as 105 miles per hour.

5–5 ROUNDING OFF TO THREE SIGNIFICANT DIGITS

The last statement may have puzzled you. “Why 105 miles per hour?” you might ask. “Why not 104 miles per hour?” The answer to this is determined by the fourth digit. If the fourth digit is 5 or greater, the third digit should be rounded off to the next higher number. On the other hand, if the fourth digit is smaller than 5, it would be dropped along with all successive digits.

It must be pointed out that 0’s are not considered to be significant unless they are preceded by a number other than 0. Let us consider, for instance, the number 0.00020543. In rounding off such a number, it is important to recognize that the numeral 2 is the first significant digit to be retained. Thus, the answer would be reported as 0.000205, or in scientific notation as

2.05 x10^–4

5–6 METRIC PREFIXES

As stated earlier, in electrical theory we are often confronted with very large or very small numbers. To overcome the inherent inconvenience of dealing with such awkward numbers, it is customary to modify the basic measuring units by attaching a prefix to their multiples or submultiples.

For instance, 1,000 volts can be called 1 kilovolt. This example shows that the word kilo stands for a multiple of l,000, or 103. In other words, 1 kilo means 1,000 times a unit.

Can you see that with this convention we can express a number like 27,000 ohms as 27x 10³ = 27 kilohms, or just 27 kΩ?

Now that we understand the word kilo, let us have a look at Figure 5–2 to become acquainted with some other prefixes. Note that these prefixes, commonly called engineering units or engineering notation, are in steps of 1,000 instead of 10. Engineering units are commonly used throughout the electrical field. Many scientific calculators have a function indicated as ENG that displays the answers in engineering units. Scientific notation units will be given in steps of 1,000 instead of 10 when this function is activated. The advantage of using engineering notation is that the calculator will display scientific notation in steps of 1,000 only, which corresponds to common engineering prefixes. When a calculator is not set for engineering notation, an answer may appear as

Note that the decimal point in the answer has moved and the exponent is now 20³ instead of 20². The scientific notation unit 203 corresponds to milli, so the answer can be read as 12.689764 milli.

Note that both uppercase letters and lowercase letters are used for letter symbols. It is especially important to distinguish between M for mega and m for milli. Note also the letter symbol for micro—the Greek letter μ (pronounced: myoo).

EXAMPLE 5–5

Given:

More examples are shown in conjunction with the Achievement Review practice problems at the end of the chapter.

5–7 MULTIPLICATION AND DIVISION WITH POWERS OF 10

Even though it is assumed that all students of electricity have a basic knowledge of algebra, it may be helpful to briefly refresh our memory.

Rule #1 (Multiplication): When powers of 10 are to be multiplied, add their exponents. For instance,

SUMMARY

• Powers of 10 are useful in making computations involving very large or very small numbers.

• Scientific notation mandates that a decimal point be placed behind the first significant

digit of a number expressed in powers of 10.

• Three-digit accuracy is generally sufficient for quantities related to practical applications. Numbers should be rounded off accordingly.

• Rounding off to three digits demands a look at the fourth digit to determine whether to round up or down.

• Metric prefixes are used in the electrical trades to describe multiples or submultiples of basic units. Examples are kilovolt, milliampere, and megohm.

• In multiplying powers of 10, the exponents are added.

• In dividing powers of 10, the exponent of the divisor is subtracted from the exponent of the dividend.

Achievement Review

PART I ROUNDING OFF MATHEMATICAL ANSWERS

Key Ideas

1. It will be assumed that electric circuit quantities are sufficiently accurate to justify solutions containing three significant digits.

2. A sequence of significant figures never begins with 0.

3. The fourth significant digit determines whether the third figure should be rounded off.

4. If a number has less than three digits, fill in the rest with 0’s. Thus, 75 5 75.0.

Examples

PART IV ELECTRICAL UNITS, SCIENTIFIC NOTATION, CONVERSION, AND ABBREVIATIONS

Express each of the following electrical terms, units, or numbers as integers or decimal numbers, as well as powers of 10 (scientific notation).

Note: For this task, we have included some electrical units we have not explained yet but that are important for the aspiring electrical worker to know. These are the units of:

• Electrical energy—called the watt-hour (Wh)

• Frequency—called the hertz (Hz)

• Electrical capacity—called the farad (F)

• Inductance—called the henry (H)

Some answers have been provided throughout this assignment so that you, the student, may have guidelines for this task. Be sure to study all these examples before you begin.

A.

5–4 SIGNIFICANT DIGITS

Most practical calculations are generally limited to only three significant digits. What does this statement mean? An example will make this clear.

Assume that some friends you know want to brag about the performance of their new automobile. They tell you that they clocked their speed at 104.6784935 miles per hour. (That is downhill with a tailwind blowing.) How would you react to such information, short of calling them liars? Obviously, speedometers do not give 10-digit readouts. Neither do electrical measuring instruments. Most such instruments (called meters) are limited to three-digit accuracy.

Does this mean that electricity is a sloppy science? Quite the contrary! Electronic measurements can be extremely precise; however, practical considerations would recommend that all numbers be limited to three-digit accuracy. Thus, your friends should have reported their speed as 105 miles per hour.

5–5 ROUNDING OFF TO THREE SIGNIFICANT DIGITS

The last statement may have puzzled you. “Why 105 miles per hour?” you might ask. “Why not 104 miles per hour?” The answer to this is determined by the fourth digit. If the fourth digit is 5 or greater, the third digit should be rounded off to the next higher number. On the other hand, if the fourth digit is smaller than 5, it would be dropped along with all successive digits.

It must be pointed out that 0’s are not considered to be significant unless they are preceded by a number other than 0. Let us consider, for instance, the number 0.00020543. In rounding off such a number, it is important to recognize that the numeral 2 is the first significant digit to be retained. Thus, the answer would be reported as 0.000205, or in scientific notation as

2.05 x10^–4

5–6 METRIC PREFIXES

As stated earlier, in electrical theory we are often confronted with very large or very small numbers. To overcome the inherent inconvenience of dealing with such awkward numbers, it is customary to modify the basic measuring units by attaching a prefix to their multiples or submultiples.

For instance, 1,000 volts can be called 1 kilovolt. This example shows that the word kilo stands for a multiple of l,000, or 103. In other words, 1 kilo means 1,000 times a unit.

Can you see that with this convention we can express a number like 27,000 ohms as 27x 10³ = 27 kilohms, or just 27 kΩ?

Now that we understand the word kilo, let us have a look at Figure 5–2 to become acquainted with some other prefixes. Note that these prefixes, commonly called engineering units or engineering notation, are in steps of 1,000 instead of 10. Engineering units are commonly used throughout the electrical field. Many scientific calculators have a function indicated as ENG that displays the answers in engineering units. Scientific notation units will be given in steps of 1,000 instead of 10 when this function is activated. The advantage of using engineering notation is that the calculator will display scientific notation in steps of 1,000 only, which corresponds to common engineering prefixes. When a calculator is not set for engineering notation, an answer may appear as

Note that the decimal point in the answer has moved and the exponent is now 20³ instead of 20². The scientific notation unit 203 corresponds to milli, so the answer can be read as 12.689764 milli.

Note that both uppercase letters and lowercase letters are used for letter symbols. It is especially important to distinguish between M for mega and m for milli. Note also the letter symbol for micro—the Greek letter μ (pronounced: myoo).

EXAMPLE 5–5

Given:

More examples are shown in conjunction with the Achievement Review practice problems at the end of the chapter.

5–7 MULTIPLICATION AND DIVISION WITH POWERS OF 10

Even though it is assumed that all students of electricity have a basic knowledge of algebra, it may be helpful to briefly refresh our memory.

Rule #1 (Multiplication): When powers of 10 are to be multiplied, add their exponents. For instance,

SUMMARY

• Powers of 10 are useful in making computations involving very large or very small numbers.

• Scientific notation mandates that a decimal point be placed behind the first significant

digit of a number expressed in powers of 10.

• Three-digit accuracy is generally sufficient for quantities related to practical applications. Numbers should be rounded off accordingly.

• Rounding off to three digits demands a look at the fourth digit to determine whether to round up or down.

• Metric prefixes are used in the electrical trades to describe multiples or submultiples of basic units. Examples are kilovolt, milliampere, and megohm.

• In multiplying powers of 10, the exponents are added.

• In dividing powers of 10, the exponent of the divisor is subtracted from the exponent of the dividend.

Achievement Review

PART I ROUNDING OFF MATHEMATICAL ANSWERS

Key Ideas

1. It will be assumed that electric circuit quantities are sufficiently accurate to justify solutions containing three significant digits.

2. A sequence of significant figures never begins with 0.

3. The fourth significant digit determines whether the third figure should be rounded off.

4. If a number has less than three digits, fill in the rest with 0’s. Thus, 75 5 75.0.

Examples

PART IV ELECTRICAL UNITS, SCIENTIFIC NOTATION, CONVERSION, AND ABBREVIATIONS

Express each of the following electrical terms, units, or numbers as integers or decimal numbers, as well as powers of 10 (scientific notation).

Note: For this task, we have included some electrical units we have not explained yet but that are important for the aspiring electrical worker to know. These are the units of:

• Electrical energy—called the watt-hour (Wh)

• Frequency—called the hertz (Hz)

• Electrical capacity—called the farad (F)

• Inductance—called the henry (H)

Some answers have been provided throughout this assignment so that you, the student, may have guidelines for this task. Be sure to study all these examples before you begin.

A.

Scientific Notation and Metric Prefixes

5–1 RATIONALE FOR STUDYING THIS UNIT

Personnel working in the field of electricity and electronics are frequently called upon to perform mathematical calculations in order to predict or confirm proper circuit operation. The numbers involved in such calculations are often extremely large or extremely small. For example, you may remember the electrical unit coulomb (defined in Section 4–3), consisting of 6,250,000,000,000,000,000 electrons. Many people feel ill at ease about pronouncing such numbers, let alone manipulating the numbers by multiplication or division.

For more than a generation, electrical technicians made use of a tool known as the slide rule, which requires knowledge and application of the powers of 10 principle. The invention and widespread use of modern electronic calculators has not relieved us of this need to manipulate numbers in scientific notation. In fact, the ability of a handheld

calculator to process and display very large (or very small) numbers depends on the use of scientific notation.

It is assumed that all students of this chapter will use an electronic calculator in working out the various exercises. If you own a calculator but do not fully understand all of its operations, you should consult your calculator’s instruction manual or seek help from someone within your study group.

A word of encouragement: If the topics of this chapter are new to you and you suffer frustration in your attempt to solve the assigned problems, do not despair! You are not expected to become an expert at this merely by finishing just one more chapter. The proficiency will come after repeated exposure to the ideas and principles presented here. You may have to refer back to this chapter many times as you progress in your studies.

5–2 POWERS OF 10

Surely, you know that 102 means 10 3 10, or 100. Likewise, 103 means 10 3 10 3 10 5 1,000, and so forth. Armed with this knowledge, you should easily construct a chart like the one shown in Figure 5–1.

Let us work a few examples to show how this knowledge can be used to express any number of powers of 10.

EXAMPLE 5–1

Given:

5–3 SCIENTIFIC NOTATION

The term scientific notation refers to a special way of using powers of 10. The method demands that a decimal point be placed behind the first significant digit when the number is changed into powers of 10. Let us use Example 5–4 to demonstrate this principle.

In fact, we have many more ways to write our answer. Here is a list of alternatives.

Of these choices, only the last number qualifies as scientific notation, because the decimal point appears behind the first significant digit.

Suggested Procedure

Count the number of spaces that the decimal point has to be moved in order to appear behind the first digit. This count, the number of spaces moved, becomes your exponent in the power of 10.

Let us express the number 6,750.0 as scientific notation. Note that the decimal point must be moved three places to the left in order to appear behind the numeral 6. The three places moved determines the exponent. Thus, we write our answer

6.75 x 10³

Scientific Notation and Metric Prefixes

5–1 RATIONALE FOR STUDYING THIS UNIT

Personnel working in the field of electricity and electronics are frequently called upon to perform mathematical calculations in order to predict or confirm proper circuit operation. The numbers involved in such calculations are often extremely large or extremely small. For example, you may remember the electrical unit coulomb (defined in Section 4–3), consisting of 6,250,000,000,000,000,000 electrons. Many people feel ill at ease about pronouncing such numbers, let alone manipulating the numbers by multiplication or division.

For more than a generation, electrical technicians made use of a tool known as the slide rule, which requires knowledge and application of the powers of 10 principle. The invention and widespread use of modern electronic calculators has not relieved us of this need to manipulate numbers in scientific notation. In fact, the ability of a handheld

calculator to process and display very large (or very small) numbers depends on the use of scientific notation.

It is assumed that all students of this chapter will use an electronic calculator in working out the various exercises. If you own a calculator but do not fully understand all of its operations, you should consult your calculator’s instruction manual or seek help from someone within your study group.

A word of encouragement: If the topics of this chapter are new to you and you suffer frustration in your attempt to solve the assigned problems, do not despair! You are not expected to become an expert at this merely by finishing just one more chapter. The proficiency will come after repeated exposure to the ideas and principles presented here. You may have to refer back to this chapter many times as you progress in your studies.

5–2 POWERS OF 10

Surely, you know that 102 means 10 3 10, or 100. Likewise, 103 means 10 3 10 3 10 5 1,000, and so forth. Armed with this knowledge, you should easily construct a chart like the one shown in Figure 5–1.

Let us work a few examples to show how this knowledge can be used to express any number of powers of 10.

EXAMPLE 5–1

Given:

5–3 SCIENTIFIC NOTATION

The term scientific notation refers to a special way of using powers of 10. The method demands that a decimal point be placed behind the first significant digit when the number is changed into powers of 10. Let us use Example 5–4 to demonstrate this principle.

In fact, we have many more ways to write our answer. Here is a list of alternatives.

Of these choices, only the last number qualifies as scientific notation, because the decimal point appears behind the first significant digit.

Suggested Procedure

Count the number of spaces that the decimal point has to be moved in order to appear behind the first digit. This count, the number of spaces moved, becomes your exponent in the power of 10.

Let us express the number 6,750.0 as scientific notation. Note that the decimal point must be moved three places to the left in order to appear behind the numeral 6. The three places moved determines the exponent. Thus, we write our answer

6.75 x 10³

4–6 A SIMPLE ELECTRIC CIRCUIT

Figure 4–7 represents a simple, functional circuit. The necessary elements of such a circuit are described as source, load, conductors, and control. The first three of these terms are closely linked with the four important concepts discussed in Sections 4–2 to 4–5. The source represents the voltage that forces the electrons through the circuit. The battery, in this example, represents the source. DC electrical sources are very often color coded to designate polarity of the terminals, positive or negative. By convention (widespread agreement and usage) the negative terminal of a source will be identified by the color black and the positive terminal will be marked by red. The load (or load resistance) utilizes the electrical energy. The lamp represents the load. The conductors provide the path for the electron current; and the control is provided by the switch, which can be operated to turn the lamp on or off.

4–7 OPEN CIRCUITS AND CLOSED CIRCUITS

Careful comparison of the two circuits shown in Figure 4–7 will reveal the differ- ence between open and closed circuits. You see, if electrons are to start moving and keep moving, a complete path for them to follow must exist. In other words, a closed circuit is required to allow the current to flow whenever electrical energy is needed by the load. Figure 4–7B represents this condition.

Now, let us look at Figure 4–7A. The switch has been opened. This simple switch is known as a knife switch. Its operation resembles that of a drawbridge. When the drawbridge is opened, the infinite resistance of the air between the switch contacts stops all current motion. This condition is known as an open circuit. This term applies to a circuit containing an open switch, a burned-out fuse, or any separation of wires that prevents current from flowing. If an appliance fails to operate when connected but does not blow a fuse, an open circuit in the device may be the problem.

4–8 THE SCHEMATIC DIAGRAM

Pictures of electric circuits and their components, such as that shown in Figure 4–7, are seldom used in the practice of electrical trades. It is more practical to convey ideas by means of graphic symbols known as schematic diagrams, or schematics. Figure 4–8 is a schematic representation of the circuit shown in Figure 4–7A. Compare the two drawings carefully. It is important that you learn how to draw and interpret such schematic diagrams. This type of skill requires some time, of course, and necessitates that you learn many graphic symbols. A chart of such symbols is shown in the Appendix, in Figures A–12 and A–13. It is suggested that you frequently refer to this list as you progress in your studies.

4–9 BASIC CIRCUIT NOTATION

In the preceding sections you became acquainted with some of the basic units of electrical measurement. At this time you should recall that the voltage (also called emf ) is measured in units called volts.

Let us assume that the battery symbol in Figure 4–8 represents a 12-volt car bat- tery. We might then use a circuit notation to show this specific value, as illustrated in Figure 4–9, where the notation reads E = 12 V. The letter E, in this case, stands for electromotive force (emf ).

Alongside the wire in Figure 4–9, you see the notation I 5 3 A. The letter I stands for intensity and is used to represent current in electrical notations and formulas. Thus, since current is measured in amperes, or amps, the equation states that the current, I, in the circuit is equal to 3 amperes.

In the same circuit diagram, the symbol for a resistor has been used to replace the symbol of a lamp. This symbol is often used to indicate the general resistance of any load, without specifying its exact nature. The adjoining legend, R 5 4 Ω, means this load has a resistance of 4 ohms. The letter R represents resistance, and the word ohm is denoted by the Greek letter omega (Ω).

4–10 THE SHORT CIRCUIT

A short circuit is a parallel path of extremely low resistance, often caused accidentally; see Figures 4–10 and 4–11. In Figure 4–11, the frayed insulation on the lamp cord may permit the two wires to touch each other. If the wires do touch, they form a path of nearly 0 ohms resistance. As a result, a large amount of current appears in the wires leading to the place of contact. The wires can overheat and start a fire. To prevent such an outcome, fuses or circuit breakers are used in series with each house circuit. If a short circuit occurs, the excessive current melts the fuse wire or trips the circuit breaker, opening the circuit.

4–11 ELECTRICAL SAFETY

All personnel working with electric circuits must be made aware of the special hazards they may encounter when they come in contact with live circuit components. A firm knowledge of electrical principles and safety practices will aid in combating irrational fear and help develop good safety attitudes.

Consider the following safety precautions:

1. Electrical shock can be harmful, if not fatal. Voltage levels are not the only deter- mining factor. One hundred twenty volts is sufficient to kill. On the other hand, one can experience high voltages, say 25,000 volts, without lasting damage. Instead, it is the effect of current through the body that is harmful. Generally, currents as low as

0.15 ampere are fatal.

2. Therefore, to avoid or minimize electrical shock, one should:

a. Never touch two wires at the same time.

b. Never touch one wire and ground (earth) at the same time.

3. Instead, be sure to:

a. Shut power off and, if possible, lock it out before working on a circuit.

b. Insulate the body from the ground.

c. Properly ground all metal enclosures of electrical equipment. A green-colored wire usually serves this purpose. This green wire is the equipment ground and attaches to the third prong of a male attachment cap (plug).

d. Discharge capacitors before touching their terminals.

e. Avoid use of metal measuring tapes in the vicinity of live conductors.

f. Use a type C extinguisher to combat electrical fires. Any other type of fire extinguisher may cause fatal shock.

4. At times it may be necessary to test live circuits in operation. Under such conditions it is advisable to put one hand in your pocket or behind your back. This prevents contact between two hot wires or between one hot wire and ground. Remove metal jewelry, such as rings and watches.

5. Hot soldering irons can cause severe burns. Be sure hot soldering irons have sufficient time to cool before you store them.

6. Hot solder splatters easily. Keep this in mind before you shake your soldering tool.

Molten solder on someone’s body may cause serious injury.

7. Wear safety glasses for the following activities:

a. Using rotating power tools, such as grinders and drills

b. Chiseling or chipping

c. Handling chemical electrolytes

8. When charging batteries, remember that hydrogen gas is highly volatile. A small electrical spark from a wire is sufficient to start an explosion.

9. Ventilate batteries when you are charging them. Be sure to loosen the cell caps to allow gases to escape.

10. To prevent oxidation, make electrical connections and splices well and be sure they are tight. Poor connections introduce resistance at the junction, which inevitably causes heat and fire hazards.

11. Do not walk away from a machine that is in motion. When shutting down a machine, stay with it until it has completely stopped.

12. Report all injuries to your instructor, regardless of how slight or unimportant you think they might be.

13. Some hand tools, such as pliers or screwdrivers, have insulated handles. Such insulation may not always be sufficient to protect you from accidental shock. You may have to take additional safety precautions.

14. If contact is made with metal parts, causing a short circuit, live wires may cause serious burns and blinding flashes.

15. Some electrical components, especially resistors, develop considerable heat. Allow them to cool before touching.

There are a few more specialized safety precautions that will be pointed out to you as you progress in your studies.

4–12 NATIONAL ELECTRICAL CODE ®

The foremost guide to electrical safety is the National Electrical Code® (NEC®). The Code® is a compilation of recommendations and regulations for the safeguarding of people and property with relation to electricity.

This book, first printed in 1897, has been revised every three years. These revisions have been sponsored since 1911 by the National Fire Protection Association. Electrical fires due to inadequate or overloaded wiring systems present real hazards that prompted the original group effort of insurance and architectural interests to develop the Code® in the first place. The book deals with rules and regulations pertaining to the safe installation of electrical conductors and equipment in private and public buildings.

Every state has governmental bodies that have legal jurisdiction over enforcement of the Code® . All cities and townships throughout the country have offices of building safety with inspectors who enforce the provisions of the Code®. Local authorities may waive specific requirements in the Code® or permit alternate methods as long as the desired objectives are achieved within safety guidelines. Many local codes are more stringent in setting minimum standards. Where such codes exist they supersede the provision of the NEC®.

To become a licensed electrician demands a thorough understanding of the codes as well as passing licensing exams. Licensing procedures are generally administered by the office of electrical inspection at local city of township halls.

SUMMARY

• The four key concepts of an electric circuit are voltage, current, resistance, and watts.

• Voltage may be thought of as a pressure (or force) that exists between two points of different potential.

• Voltage is also known as electromotive force (emf ).

• Voltage is measured in units called volts.

• Electron current is a motion of free electrons from negative to positive. This is the standard adopted for this book.

• Conventional current is said to flow from positive to negative.

• Current is measured in units called amperes.

• When measuring amperes, we measure the rate of flow of electrons (1 ampere = 1 coulomb per second).

• The effect of the electrical impulse travels with the speed of light, while individual electrons move relatively slowly.

• The opposition to current flow is called resistance.

• Resistance is measured in units called ohms.

• Resistance always generates heat when a current flows through it.

• Electricity is a form of pure energy that is converted to some other form, usually referred to as electrical power.

• Electrical power is measured in watts.

• An electric circuit consists of a load connected to a source by means of conductors. An boptional control is often added to such a circuit.

• Schematic diagrams are the blueprints of the electrical trades.

• Closed circuits provide a complete path for the electrons.

• There can be no current flow in an open circuit.

• Short circuits are caused when a low-resistance path bypasses the load.

• Short circuits cause excessive current flow and create potential fire hazards.

• Fuses and circuit breakers are used to protect against excessive currents.

• Electrical safety rules must be followed to ensure the safety of personnel and equipment.

• The NEC® governs the safety standards for electrical installations throughout the United States.

• Local codes may supersede the NEC® with more stringent requirements.

• Licensing procedures for electricians require extensive knowledge of the Code®.

Achievement Review

1. What is the difference between the negative pole and the positive pole of a battery or power source?

2. What colors are used to identify (a) the negative pole and (b) the positive pole?

3. What is voltage? Define the word.

4. What are some other names for voltage?

5. What letter symbol is used for voltage?

6. Define the word current.

7. What letter symbol is used for current?

8. Explain the difference between conventional current flow and electron current flow.

9. What is an ohm, and what is its letter symbol?

10. What is the unit of measurement for current?

11. What kind of instrument is used for measuring resistance?

12. What are amperemeters used for?

13. What is an emf? Explain.

14. How fast does the electron current travel?

15. What is meant by potential or potential difference?

16. Draw a simple schematic with a DC source of 6 volts connected to a 1.5-ohm load. Indicate a current of 4 A flowing, as well as the voltage and resistance values.

17. Explain the purpose of fuses and circuit breakers.

18. Explain how fuses differ from circuit breakers.

19. Voltage, current, resistance, and watts are the basic concepts underlying all electrical and electronic principles.

a. Write brief but concise definitions for the following terms: voltage, current, resistance, and power.

b. Complete the chart below.

4–6 A SIMPLE ELECTRIC CIRCUIT

Figure 4–7 represents a simple, functional circuit. The necessary elements of such a circuit are described as source, load, conductors, and control. The first three of these terms are closely linked with the four important concepts discussed in Sections 4–2 to 4–5. The source represents the voltage that forces the electrons through the circuit. The battery, in this example, represents the source. DC electrical sources are very often color coded to designate polarity of the terminals, positive or negative. By convention (widespread agreement and usage) the negative terminal of a source will be identified by the color black and the positive terminal will be marked by red. The load (or load resistance) utilizes the electrical energy. The lamp represents the load. The conductors provide the path for the electron current; and the control is provided by the switch, which can be operated to turn the lamp on or off.

4–7 OPEN CIRCUITS AND CLOSED CIRCUITS

Careful comparison of the two circuits shown in Figure 4–7 will reveal the differ- ence between open and closed circuits. You see, if electrons are to start moving and keep moving, a complete path for them to follow must exist. In other words, a closed circuit is required to allow the current to flow whenever electrical energy is needed by the load. Figure 4–7B represents this condition.

Now, let us look at Figure 4–7A. The switch has been opened. This simple switch is known as a knife switch. Its operation resembles that of a drawbridge. When the drawbridge is opened, the infinite resistance of the air between the switch contacts stops all current motion. This condition is known as an open circuit. This term applies to a circuit containing an open switch, a burned-out fuse, or any separation of wires that prevents current from flowing. If an appliance fails to operate when connected but does not blow a fuse, an open circuit in the device may be the problem.

4–8 THE SCHEMATIC DIAGRAM

Pictures of electric circuits and their components, such as that shown in Figure 4–7, are seldom used in the practice of electrical trades. It is more practical to convey ideas by means of graphic symbols known as schematic diagrams, or schematics. Figure 4–8 is a schematic representation of the circuit shown in Figure 4–7A. Compare the two drawings carefully. It is important that you learn how to draw and interpret such schematic diagrams. This type of skill requires some time, of course, and necessitates that you learn many graphic symbols. A chart of such symbols is shown in the Appendix, in Figures A–12 and A–13. It is suggested that you frequently refer to this list as you progress in your studies.

4–9 BASIC CIRCUIT NOTATION

In the preceding sections you became acquainted with some of the basic units of electrical measurement. At this time you should recall that the voltage (also called emf ) is measured in units called volts.

Let us assume that the battery symbol in Figure 4–8 represents a 12-volt car bat- tery. We might then use a circuit notation to show this specific value, as illustrated in Figure 4–9, where the notation reads E = 12 V. The letter E, in this case, stands for electromotive force (emf ).

Alongside the wire in Figure 4–9, you see the notation I 5 3 A. The letter I stands for intensity and is used to represent current in electrical notations and formulas. Thus, since current is measured in amperes, or amps, the equation states that the current, I, in the circuit is equal to 3 amperes.

In the same circuit diagram, the symbol for a resistor has been used to replace the symbol of a lamp. This symbol is often used to indicate the general resistance of any load, without specifying its exact nature. The adjoining legend, R 5 4 Ω, means this load has a resistance of 4 ohms. The letter R represents resistance, and the word ohm is denoted by the Greek letter omega (Ω).

4–10 THE SHORT CIRCUIT

A short circuit is a parallel path of extremely low resistance, often caused accidentally; see Figures 4–10 and 4–11. In Figure 4–11, the frayed insulation on the lamp cord may permit the two wires to touch each other. If the wires do touch, they form a path of nearly 0 ohms resistance. As a result, a large amount of current appears in the wires leading to the place of contact. The wires can overheat and start a fire. To prevent such an outcome, fuses or circuit breakers are used in series with each house circuit. If a short circuit occurs, the excessive current melts the fuse wire or trips the circuit breaker, opening the circuit.

4–11 ELECTRICAL SAFETY

All personnel working with electric circuits must be made aware of the special hazards they may encounter when they come in contact with live circuit components. A firm knowledge of electrical principles and safety practices will aid in combating irrational fear and help develop good safety attitudes.

Consider the following safety precautions:

1. Electrical shock can be harmful, if not fatal. Voltage levels are not the only deter- mining factor. One hundred twenty volts is sufficient to kill. On the other hand, one can experience high voltages, say 25,000 volts, without lasting damage. Instead, it is the effect of current through the body that is harmful. Generally, currents as low as

0.15 ampere are fatal.

2. Therefore, to avoid or minimize electrical shock, one should:

a. Never touch two wires at the same time.

b. Never touch one wire and ground (earth) at the same time.

3. Instead, be sure to:

a. Shut power off and, if possible, lock it out before working on a circuit.

b. Insulate the body from the ground.

c. Properly ground all metal enclosures of electrical equipment. A green-colored wire usually serves this purpose. This green wire is the equipment ground and attaches to the third prong of a male attachment cap (plug).

d. Discharge capacitors before touching their terminals.

e. Avoid use of metal measuring tapes in the vicinity of live conductors.

f. Use a type C extinguisher to combat electrical fires. Any other type of fire extinguisher may cause fatal shock.

4. At times it may be necessary to test live circuits in operation. Under such conditions it is advisable to put one hand in your pocket or behind your back. This prevents contact between two hot wires or between one hot wire and ground. Remove metal jewelry, such as rings and watches.

5. Hot soldering irons can cause severe burns. Be sure hot soldering irons have sufficient time to cool before you store them.

6. Hot solder splatters easily. Keep this in mind before you shake your soldering tool.

Molten solder on someone’s body may cause serious injury.

7. Wear safety glasses for the following activities:

a. Using rotating power tools, such as grinders and drills

b. Chiseling or chipping

c. Handling chemical electrolytes

8. When charging batteries, remember that hydrogen gas is highly volatile. A small electrical spark from a wire is sufficient to start an explosion.

9. Ventilate batteries when you are charging them. Be sure to loosen the cell caps to allow gases to escape.

10. To prevent oxidation, make electrical connections and splices well and be sure they are tight. Poor connections introduce resistance at the junction, which inevitably causes heat and fire hazards.

11. Do not walk away from a machine that is in motion. When shutting down a machine, stay with it until it has completely stopped.

12. Report all injuries to your instructor, regardless of how slight or unimportant you think they might be.

13. Some hand tools, such as pliers or screwdrivers, have insulated handles. Such insulation may not always be sufficient to protect you from accidental shock. You may have to take additional safety precautions.

14. If contact is made with metal parts, causing a short circuit, live wires may cause serious burns and blinding flashes.

15. Some electrical components, especially resistors, develop considerable heat. Allow them to cool before touching.

There are a few more specialized safety precautions that will be pointed out to you as you progress in your studies.

4–12 NATIONAL ELECTRICAL CODE ®

The foremost guide to electrical safety is the National Electrical Code® (NEC®). The Code® is a compilation of recommendations and regulations for the safeguarding of people and property with relation to electricity.

This book, first printed in 1897, has been revised every three years. These revisions have been sponsored since 1911 by the National Fire Protection Association. Electrical fires due to inadequate or overloaded wiring systems present real hazards that prompted the original group effort of insurance and architectural interests to develop the Code® in the first place. The book deals with rules and regulations pertaining to the safe installation of electrical conductors and equipment in private and public buildings.

Every state has governmental bodies that have legal jurisdiction over enforcement of the Code® . All cities and townships throughout the country have offices of building safety with inspectors who enforce the provisions of the Code®. Local authorities may waive specific requirements in the Code® or permit alternate methods as long as the desired objectives are achieved within safety guidelines. Many local codes are more stringent in setting minimum standards. Where such codes exist they supersede the provision of the NEC®.

To become a licensed electrician demands a thorough understanding of the codes as well as passing licensing exams. Licensing procedures are generally administered by the office of electrical inspection at local city of township halls.

SUMMARY

• The four key concepts of an electric circuit are voltage, current, resistance, and watts.

• Voltage may be thought of as a pressure (or force) that exists between two points of different potential.

• Voltage is also known as electromotive force (emf ).

• Voltage is measured in units called volts.

• Electron current is a motion of free electrons from negative to positive. This is the standard adopted for this book.

• Conventional current is said to flow from positive to negative.

• Current is measured in units called amperes.

• When measuring amperes, we measure the rate of flow of electrons (1 ampere = 1 coulomb per second).

• The effect of the electrical impulse travels with the speed of light, while individual electrons move relatively slowly.

• The opposition to current flow is called resistance.

• Resistance is measured in units called ohms.

• Resistance always generates heat when a current flows through it.

• Electricity is a form of pure energy that is converted to some other form, usually referred to as electrical power.

• Electrical power is measured in watts.

• An electric circuit consists of a load connected to a source by means of conductors. An boptional control is often added to such a circuit.

• Schematic diagrams are the blueprints of the electrical trades.

• Closed circuits provide a complete path for the electrons.

• There can be no current flow in an open circuit.

• Short circuits are caused when a low-resistance path bypasses the load.

• Short circuits cause excessive current flow and create potential fire hazards.

• Fuses and circuit breakers are used to protect against excessive currents.

• Electrical safety rules must be followed to ensure the safety of personnel and equipment.

• The NEC® governs the safety standards for electrical installations throughout the United States.

• Local codes may supersede the NEC® with more stringent requirements.

• Licensing procedures for electricians require extensive knowledge of the Code®.

Achievement Review

1. What is the difference between the negative pole and the positive pole of a battery or power source?

2. What colors are used to identify (a) the negative pole and (b) the positive pole?

3. What is voltage? Define the word.

4. What are some other names for voltage?

5. What letter symbol is used for voltage?

6. Define the word current.

7. What letter symbol is used for current?

8. Explain the difference between conventional current flow and electron current flow.

9. What is an ohm, and what is its letter symbol?

10. What is the unit of measurement for current?

11. What kind of instrument is used for measuring resistance?

12. What are amperemeters used for?

13. What is an emf? Explain.

14. How fast does the electron current travel?

15. What is meant by potential or potential difference?

16. Draw a simple schematic with a DC source of 6 volts connected to a 1.5-ohm load. Indicate a current of 4 A flowing, as well as the voltage and resistance values.

17. Explain the purpose of fuses and circuit breakers.

18. Explain how fuses differ from circuit breakers.

19. Voltage, current, resistance, and watts are the basic concepts underlying all electrical and electronic principles.

a. Write brief but concise definitions for the following terms: voltage, current, resistance, and power.

b. Complete the chart below.

Basic Circuit Concepts

4–1 FOUR MEASURABLE CIRCUIT QUANTITIES

In dealing with any useful quantity, whether vegetables, steel bars, or electrons, a system of measurement must be used to keep track of the production, transfer, and use of the commodity. So it is with electric circuits that are arranged to obtain practical use of electrical energy. There are four fundamental concepts that constitute the elements of an electric circuit, namely

Voltage, Current, Resistance, and Watts

Since these words represent abstractions, quantities that cannot be directly perceived by one’s senses, it is very important that you develop a correct mental image of the key concepts. The following sections are designed to help you gain a clear understanding of these terms.

4–2 VOLTAGE

Let us consider a simple source of DC, a common flashlight battery, which is technically known as a dry cell; see Figure 4–1. The little minus signs in the drawing represent a huge quantity of electrons. This should suggest that the shell, accessible at the bottom plate, has a vast surplus of electrons as compared with the number of electrons at the top cap of the cell.

Remember that the electrons have a negative charge, making the bottom plate with its surplus of electrons more negative than the top cap with its electron deficiency. It is this difference in potential (difference in the number of electrical charges) that is known as voltage. Frequently, you will find the term electromotive force (emf ) used to describe the same condition.

It is important to note that protons do not enter into our discussion when we de- fine the positive pole of the dry cell in Figure 4–1. (Remember, protons are the carriers of positive charges.) Both the positive and the negative poles can be defined in terms of their relative number of electrons. In this context the word positive simply means less negative, as compared to some other reference point.

Voltage is always measured between two points of different potential. The unit of measure used for this is called the volt (V), named after the Italian physicist Alessandro Volta.

Thus we can say that our dry cell has an emf of 1.5 volts. Compare this with the two terminals of a car battery. Between these two points we would expect to measure 12 volts. To measure such a voltage, we use an instrument known as a voltmeter.

Another common concept of voltage is electrical pressure. Voltage is the force that pushes the electrons through a circuit in much the same way that pressure pushes liquid through a pipe. The higher the voltage, the greater the force moving the electrons. Electric circuits are often compared to a water system because the concepts are similar. In the illustration shown in Figure 4–2, a pump is used to supply the pressure necessary to cause water to flow through the water system. In like manner, a battery supplies the electrical pressure (voltage) necessary to cause electrons to flow through the electric circuit. In the water sys- tem a flowmeter measures the amount of flow in gallons per minute. In the electric circuit an ammeter measures the flow of current. In the water system a pressure gauge measures the difference in pressure produced by the pump in pounds per square inch. In the electric circuit a voltmeter measures the voltage of the battery. A reducer is used in the water system to limit the flow of water. A resistive device is employed in the electric circuit to limit the flow of current. It is important to understand that neither pressure nor voltage flows. Pressure causes water to flow and voltage causes electrons to flow. Water can flow and current can flow, but pressure and voltage cannot.

4–3 CURRENT

For discussion purposes, let us pretend that we connect a heavy wire between the poles of our battery. In reality this must never be done because it constitutes an undesirable condition known as a short circuit. Figure 4–3 shows how such a wire provides a path by which the electron surplus can drain off toward the point of electron deficiency. It is this motion of electrons that we refer to as current. Notice that this definition of current considers the motion of free electrons from a point of electron surplus to a point of electron shortage. In other words, the electron theory defines current flow as a motion of free electrons from the negative pole of a source, through the outer circuit path, and back to the positive pole. This is the definition of electron current that we will use throughout this book.

However, there is yet another theory that leads to the conclusion that current flows from positive to negative. This is known as conventional current flow. The concept of conventional current can be very useful and will be reintroduced to you at a later date when you begin your study of electronics and semiconductors.

Current as a Rate of Flow

In measuring the rate of electron flow, we are concerned with a quantity rate rather than simple speed. In ordinary electrical devices, the number of electrons passing through the device each second is the important consideration, not their speed in miles per hour. Water pumps are rated in gallons per minute, ventilation fans in cubic feet per minute, and grain-handling equipment in bushels per hour. All of these are quantity rates.

To establish such a rate for electrons, we must first decide on a measure of quan- tity. We could use the number of electrons passing a point per second, but so many pass by a point in one second that the number is too great to use. Instead, we lump together 6,250,000,000,000,000,000 (6.25 3 1018) electrons and call this quantity a coulomb of elec- trons (in honor of Charles Coulomb, a French scientist). To show just how large a number 6,250,000,000,000,000,000 is, assume that this many flies are in New York State in the sum- mer. If all of the flies were killed, they would cover the land area of the state to a depth of 51⁄2 feet, so densely packed that 500 fly cadavers would be compressed into each cubic inch.

We can measure the rate of electron flow in coulombs per second. This is comparable to measuring the flow of traffic in cars per hour or measuring air current in cubic feet per sec- ond. It can also be compared to measuring water flow in gallons per second; see Figure 4–4.

To express the rate of electron flow, the phrase coulombs per second is seldom used. Rather, we use one word that means coulombs per second. This word is ampere (A), again named in honor of a French scientist, André Ampère (1775–1836).

One ampere is a flow rate of 1 coulomb/second.

To measure the current flow in an electric circuit, we use an amperemeter, or ammeter.

Current Speed

People sometimes get into discussions of what is meant by the speed of electricity. A more exact term than speed of electricity is needed to distinguish between (1) the aver- age speed of individual electrons as they drift through the wire and (2) the speed of the impulse. We realize that when we turn on a light, the light is on immediately. In a house

wired with #12-gauge wire, calculations show that there are so many electrons in the wire that the average speed of individual electrons is only about 3 inches per hour when the cur- rent is 1 ampere. Three inches per hour is the speed of the electron drift through the copper wire. However, keep in mind that since the wire is full of electrons to begin with, they start moving everywhere at once when the switch is turned on. The actual speed of this impulse depends on the arrangement of the wires and may be anything from a few thousand miles per second to the speed of light, 186,000 miles per second, as a theoretical top limit.

4–4 RESISTANCE

A single stroke of the oars will not keep a rowboat moving indefinitely at the same rate of speed. Neither will voltage keep electrons moving indefinitely at the same rate. Friction slows the movement. This internal friction, which retards the flow of current (electrons) through a material, is called electrical resistance.

Electrons slide through a copper wire easily, like a boat through water. Electrons also move through iron and some metal alloys fairly easily, although not as easily as they do through copper. But there are many materials through which electrons can move hardly at all, even if a lot of pressure (high voltage) is applied. Trying to move electrons through sulfur, glass, plastic, or porcelain, for example, is about as effective as trying to row a boat on a concrete road or on plowed ground.

The list in Figure 4–5 compares the resistance of common materials. Those of highest resistance (so high it is difficult even to measure) are the best insulators. Those of lowest resistance are the best conductors. In the range between the two extremes are the materials that are poor conductors yet do not have quite enough resistance to be called insulators.

Recall from Chapter 1 that elements whose atoms have only one, two, or three electrons in the outer shell (or orbit) are conductors, because these electrons are free to move. Elements whose atoms have only five, six, or seven electrons in the outer shell are insula- tors, because there are no free electrons.

This unit of measure of resistance is the ohm, which is denoted by the Greek let- ter omega (Ω), named after a German scientist, Georg Ohm. Electrical resistance can be measured with an ohmmeter. Many multimeters (multipurpose instruments), such as a VOM, have an ohmmeter function built in.

The thickness of a conductor also affects the resistance of a circuit. Assume that the heavy wire we placed across the dry cell in Figure 4–3 has a short section of very thin wire inserted, as shown in Figure 4–6. This thin section represents a resistance to the flow of current in the same manner that the bottleneck of a one-lane detour represents a reduction in the flow of traffic on a broad freeway.

It is important to note that heat is being developed whenever an electrical current is forced through a resistance. It is conceivable that a resistance wire, like the one shown in Figure 4–6, becomes so hot that it begins to glow intensely and gives off light. This is called incandescence. Ordinary lightbulbs, called incandescent lamps, operate on this principle.

Most of the heat-generating appliances that you know contain an electrical resistance wire. Think, for instance, of the cigarette lighter in the dashboard of your car. It, too, has a built-in resistance wire that glows from heat whenever a voltage forces current through the resistance.

4–5 WATTS

Electrical power is measured in watts. Electricity is a form of pure energy and in accord with basic physical laws can be neither created nor destroyed. The form of energy can be changed, however. The watt is a measure of the amount of electrical energy that is changed to some other form. As we have discussed, when current flows through a resistor, the resistor becomes hot; this is an example of electrical energy being converted to thermal energy. When electricity is used to power a motor, electrical energy is converted to kinetic (moving) energy. The quantity watts is often called power and is generally represented by the letter P in electrical formulas. Although the letter P is used to represent power, the letter W denotes the quantity watts. Power in an electric circuit is measured with a wattmeter.

Basic Circuit Concepts

4–1 FOUR MEASURABLE CIRCUIT QUANTITIES

In dealing with any useful quantity, whether vegetables, steel bars, or electrons, a system of measurement must be used to keep track of the production, transfer, and use of the commodity. So it is with electric circuits that are arranged to obtain practical use of electrical energy. There are four fundamental concepts that constitute the elements of an electric circuit, namely

Voltage, Current, Resistance, and Watts

Since these words represent abstractions, quantities that cannot be directly perceived by one’s senses, it is very important that you develop a correct mental image of the key concepts. The following sections are designed to help you gain a clear understanding of these terms.

4–2 VOLTAGE

Let us consider a simple source of DC, a common flashlight battery, which is technically known as a dry cell; see Figure 4–1. The little minus signs in the drawing represent a huge quantity of electrons. This should suggest that the shell, accessible at the bottom plate, has a vast surplus of electrons as compared with the number of electrons at the top cap of the cell.

Remember that the electrons have a negative charge, making the bottom plate with its surplus of electrons more negative than the top cap with its electron deficiency. It is this difference in potential (difference in the number of electrical charges) that is known as voltage. Frequently, you will find the term electromotive force (emf ) used to describe the same condition.

It is important to note that protons do not enter into our discussion when we de- fine the positive pole of the dry cell in Figure 4–1. (Remember, protons are the carriers of positive charges.) Both the positive and the negative poles can be defined in terms of their relative number of electrons. In this context the word positive simply means less negative, as compared to some other reference point.

Voltage is always measured between two points of different potential. The unit of measure used for this is called the volt (V), named after the Italian physicist Alessandro Volta.

Thus we can say that our dry cell has an emf of 1.5 volts. Compare this with the two terminals of a car battery. Between these two points we would expect to measure 12 volts. To measure such a voltage, we use an instrument known as a voltmeter.

Another common concept of voltage is electrical pressure. Voltage is the force that pushes the electrons through a circuit in much the same way that pressure pushes liquid through a pipe. The higher the voltage, the greater the force moving the electrons. Electric circuits are often compared to a water system because the concepts are similar. In the illustration shown in Figure 4–2, a pump is used to supply the pressure necessary to cause water to flow through the water system. In like manner, a battery supplies the electrical pressure (voltage) necessary to cause electrons to flow through the electric circuit. In the water sys- tem a flowmeter measures the amount of flow in gallons per minute. In the electric circuit an ammeter measures the flow of current. In the water system a pressure gauge measures the difference in pressure produced by the pump in pounds per square inch. In the electric circuit a voltmeter measures the voltage of the battery. A reducer is used in the water system to limit the flow of water. A resistive device is employed in the electric circuit to limit the flow of current. It is important to understand that neither pressure nor voltage flows. Pressure causes water to flow and voltage causes electrons to flow. Water can flow and current can flow, but pressure and voltage cannot.

4–3 CURRENT

For discussion purposes, let us pretend that we connect a heavy wire between the poles of our battery. In reality this must never be done because it constitutes an undesirable condition known as a short circuit. Figure 4–3 shows how such a wire provides a path by which the electron surplus can drain off toward the point of electron deficiency. It is this motion of electrons that we refer to as current. Notice that this definition of current considers the motion of free electrons from a point of electron surplus to a point of electron shortage. In other words, the electron theory defines current flow as a motion of free electrons from the negative pole of a source, through the outer circuit path, and back to the positive pole. This is the definition of electron current that we will use throughout this book.

However, there is yet another theory that leads to the conclusion that current flows from positive to negative. This is known as conventional current flow. The concept of conventional current can be very useful and will be reintroduced to you at a later date when you begin your study of electronics and semiconductors.

Current as a Rate of Flow

In measuring the rate of electron flow, we are concerned with a quantity rate rather than simple speed. In ordinary electrical devices, the number of electrons passing through the device each second is the important consideration, not their speed in miles per hour. Water pumps are rated in gallons per minute, ventilation fans in cubic feet per minute, and grain-handling equipment in bushels per hour. All of these are quantity rates.

To establish such a rate for electrons, we must first decide on a measure of quan- tity. We could use the number of electrons passing a point per second, but so many pass by a point in one second that the number is too great to use. Instead, we lump together 6,250,000,000,000,000,000 (6.25 3 1018) electrons and call this quantity a coulomb of elec- trons (in honor of Charles Coulomb, a French scientist). To show just how large a number 6,250,000,000,000,000,000 is, assume that this many flies are in New York State in the sum- mer. If all of the flies were killed, they would cover the land area of the state to a depth of 51⁄2 feet, so densely packed that 500 fly cadavers would be compressed into each cubic inch.

We can measure the rate of electron flow in coulombs per second. This is comparable to measuring the flow of traffic in cars per hour or measuring air current in cubic feet per sec- ond. It can also be compared to measuring water flow in gallons per second; see Figure 4–4.

To express the rate of electron flow, the phrase coulombs per second is seldom used. Rather, we use one word that means coulombs per second. This word is ampere (A), again named in honor of a French scientist, André Ampère (1775–1836).

One ampere is a flow rate of 1 coulomb/second.

To measure the current flow in an electric circuit, we use an amperemeter, or ammeter.

Current Speed

People sometimes get into discussions of what is meant by the speed of electricity. A more exact term than speed of electricity is needed to distinguish between (1) the aver- age speed of individual electrons as they drift through the wire and (2) the speed of the impulse. We realize that when we turn on a light, the light is on immediately. In a house

wired with #12-gauge wire, calculations show that there are so many electrons in the wire that the average speed of individual electrons is only about 3 inches per hour when the cur- rent is 1 ampere. Three inches per hour is the speed of the electron drift through the copper wire. However, keep in mind that since the wire is full of electrons to begin with, they start moving everywhere at once when the switch is turned on. The actual speed of this impulse depends on the arrangement of the wires and may be anything from a few thousand miles per second to the speed of light, 186,000 miles per second, as a theoretical top limit.

4–4 RESISTANCE

A single stroke of the oars will not keep a rowboat moving indefinitely at the same rate of speed. Neither will voltage keep electrons moving indefinitely at the same rate. Friction slows the movement. This internal friction, which retards the flow of current (electrons) through a material, is called electrical resistance.

Electrons slide through a copper wire easily, like a boat through water. Electrons also move through iron and some metal alloys fairly easily, although not as easily as they do through copper. But there are many materials through which electrons can move hardly at all, even if a lot of pressure (high voltage) is applied. Trying to move electrons through sulfur, glass, plastic, or porcelain, for example, is about as effective as trying to row a boat on a concrete road or on plowed ground.

The list in Figure 4–5 compares the resistance of common materials. Those of highest resistance (so high it is difficult even to measure) are the best insulators. Those of lowest resistance are the best conductors. In the range between the two extremes are the materials that are poor conductors yet do not have quite enough resistance to be called insulators.

Recall from Chapter 1 that elements whose atoms have only one, two, or three electrons in the outer shell (or orbit) are conductors, because these electrons are free to move. Elements whose atoms have only five, six, or seven electrons in the outer shell are insula- tors, because there are no free electrons.

This unit of measure of resistance is the ohm, which is denoted by the Greek let- ter omega (Ω), named after a German scientist, Georg Ohm. Electrical resistance can be measured with an ohmmeter. Many multimeters (multipurpose instruments), such as a VOM, have an ohmmeter function built in.

The thickness of a conductor also affects the resistance of a circuit. Assume that the heavy wire we placed across the dry cell in Figure 4–3 has a short section of very thin wire inserted, as shown in Figure 4–6. This thin section represents a resistance to the flow of current in the same manner that the bottleneck of a one-lane detour represents a reduction in the flow of traffic on a broad freeway.

It is important to note that heat is being developed whenever an electrical current is forced through a resistance. It is conceivable that a resistance wire, like the one shown in Figure 4–6, becomes so hot that it begins to glow intensely and gives off light. This is called incandescence. Ordinary lightbulbs, called incandescent lamps, operate on this principle.

Most of the heat-generating appliances that you know contain an electrical resistance wire. Think, for instance, of the cigarette lighter in the dashboard of your car. It, too, has a built-in resistance wire that glows from heat whenever a voltage forces current through the resistance.

4–5 WATTS

Electrical power is measured in watts. Electricity is a form of pure energy and in accord with basic physical laws can be neither created nor destroyed. The form of energy can be changed, however. The watt is a measure of the amount of electrical energy that is changed to some other form. As we have discussed, when current flows through a resistor, the resistor becomes hot; this is an example of electrical energy being converted to thermal energy. When electricity is used to power a motor, electrical energy is converted to kinetic (moving) energy. The quantity watts is often called power and is generally represented by the letter P in electrical formulas. Although the letter P is used to represent power, the letter W denotes the quantity watts. Power in an electric circuit is measured with a wattmeter.

Electrostatics

3–1 STATIONARY ELECTRONS

The word static means that something is at rest; it is not going anywhere. There- fore, static electricity is a charge that is stationary; it is not a current.

A good deal can be learned about electrons when they are stationary. Even though they are not traveling, electrons do not sit still but constantly jump from one place to another. The sparks and arcs they produce are often very important.

The word static in a discussion of electricity may first remind one of noise in a radio. When lightning flashes or an electric spark occurs, vibrating electrons broadcast some energy that is received as crackling in a radio receiver. The term static is now applied to any unexplained radio interference, whether the interference is of true electro- static origin or not.

The material covered in this chapter is an extension of ideas introduced in Chapter 1. The Summary of Chapter 1 should be reviewed at this point, particularly these useful facts:

• Any object that has a negative charge has received some electrons that normally do not belong to it, and they will try to escape.

• Any object that has a positive charge has lost some of its electrons. When a positively charged object has electrons returned to it, it goes back to its original neutral state. The returned electrons do not have to be the same ones that were lost. (Remember that electrons are all alike, regardless of where they come from.) If a million electrons are lost and a different million electrons return, everything is still back to neutral.

Lists A and B in Figure 1–1 introduced insulators (nonconductors). Insulators easily acquire a static electric charge, because they are materials in which electrons cannot move readily. If a spot on the insulating material has a surplus of electrons, it keeps the electrons. If at another location on the material there is a lack of electrons, however, that surplus finds it difficult to slide over and fill the lack.

Materials in which electrons move freely are called conductors. Metals, with their loose electrons in the outer rings of the atoms, are good conductors. Conductors can be charged only if they are insulated from their surroundings by some nonconducting material.

3–2 ELECTROSCOPES

The electroscope was once widely used to compare electrical charges and served as a crude voltmeter. It was also used in the detection of radioactive minerals. The device was an important aid in experiments that led to the discovery of electrons. For present-day use, it has been replaced by devices that are more accurate and sensitive but that are also more complex.

The electroscope, Figure 3–1, must first be charged using a known charge. The charging procedure is simple: (1) A hard rubber rod is charged by rubbing it on wool; (2) then the hard rubber rod is slid along the knob of the electroscope. Electrons from the negative hard rubber are transferred from the rod to the electroscope, where they repel each other and scatter all over the metal knob, metal rod, and metal leaves. The leaves repel each other because both are charged alike, in this case, negatively, as shown in Figure 3–1A.

The electroscope can also be charged positively, using a charged glass rod instead of the hard rubber rod. As the glass rod is wiped along the knob of a neutral electroscope, the rod attracts electrons from the electroscope. The electroscope now has a lack of electrons (since they have been transferred to the glass rod), and the electroscope is positively charged. The leaves again spread apart, as shown in Figure 3–1B, because the like positive charges repel.

Unknown Charges

A charged electroscope will indicate whether an unknown charge is positive or negative. For example, a pen is charged by rubbing it on a sleeve. The question now is whether the pen is negatively or positively charged. With a hard rubber rod, the electro- scope is charged negatively, as described before. As shown in Figure 3–2A, the pen is then brought slowly toward the top of the electroscope, and the leaves are observed closely for their motion. The leaves may repel even more. Why?

The leaves were made negative with the rubber rod. If they repel more, they must now be more negative. The leaves can become more negative only by having more electrons forced onto them. The unknown charge on the pen, therefore, must be negative, because more of the electroscope’s electrons were repelled to the leaves.

If the pen is charged positively, as in Figure 3–2B, what will happen? The positively charged pen will attract some of the excess electrons from the leaves to the knob. As these repelling electrons are removed, the leaves will fall together.

A positively charged electroscope can also be used to identify unknown charges. Keep in mind that the positively charged electroscope has relatively few of its electrons removed; there are still many electrons there. When a negative object comes near, it will repel the electrons in the electroscope toward the leaves. These electrons will neutralize the repelling positive charges on the leaves, causing the leaves to fall together. An approaching positive object will pull even more electrons from the positively charged leaves, causing them to repel each other more strongly.

In these instances, all of the electron motion is within the electroscope itself. The charged pen is not brought close enough to the knob to permit electrons to jump the gap between the two objects.

3–3 ELECTROSTATIC INDUCTION

A charged object always affects other objects in its neighborhood either by at- tracting or repelling electrons within the nearby object. Electrostatic induction is the process by which the neighboring object acquires a charge. In unusual circumstances, electrostatic induction can cause accidents. For example, on a warm, dry day in south- ern Oregon (where there are no self-serve gas stations), a truck driver speaks to the new gas station attendant, Joe, as shown in Figure 3–3. Joe lets go of the iron post he was sitting on near the gas pump. As the truck drives away, he steps over to remove the hose from another customer’s gas tank. At this point, the gasoline catches fire. What causes this to happen?

A strongly charged truck, Joe’s new rubber-soled shoes, the removal of his hand from the grounded post, electrostatic induction, and a spark in the presence of gasoline fumes all contributed to the fire. Because the truck was negatively charged, it repelled electrons from Joe through the post to ground (earth). Having lost some electrons, Joe was then positively charged. When he removed his hand from the post, Joe did not immediately regain the lost electrons since he was insulated by his rubber-soled shoes and the dry air. Electrons, attracted to his positive hand from the gasoline tank, jumped the gap, causing a spark. The spark ignited the gasoline vapor.

Let us illustrate this electrostatic induction process using the electroscope shown in Figure 3–4.

1. Bring a negatively charged rod (truck) near the electroscope (Joe). The experimenter’s hand touching the electroscope corresponds to Joe’s connection to the ground through the post. The rod will repel electrons from the electroscope knob. These electrons will move from the electroscope to the experimenter’s hand touching the knob.

2. Remove the hand from the knob. This leaves the electroscope with a positive charge (lack of electrons).

3. Remove the negative rod.

If these steps are performed in the correct order, the electroscope will be left in a charged condition, shown by its repelling leaves. The fact that the charge is positive can be checked by its behavior when the negative rod is slowly brought near it again. The charge on the electroscope and the charge on Joe are examples of induced charges.

An induced charge on one object is caused by the approach of another charged object, without contact between the two objects.

3–4 LIGHTNING

Benjamin Franklin showed that lightning is a large-scale performance of ordinary electrostatic behavior. (A few people who tried his kite experiment were killed by it.)

The distribution of electrical charges in thunderclouds has been mapped, but as yet no one knows just how the charges are formed. Most of the lightning flashes occur within the cloud itself. Whether electrons go from cloud to ground or from ground to cloud depends on the part of the storm area that is over a particular location; see Figure 3–5.

Objects, such as a house, that are under the small, positively charged area at the center of the storm are hit by electrons moving from the earth to the cloud. Objects, such as trees, under the large, negatively charged part of the cloud are hit by electrons driving from the cloud to the earth.

It is not possible to insulate against lightning. Protection can be provided only by giving it an easy path to ground. Well-grounded lightning rods are very effective. Ammunition sheds have been protected simply by a steel cable supported along the ridge of the shed and grounded at each end. Steel frames of large buildings, grounded during construction, serve as lightning protection for the building.

An automobile may be struck by lightning, but its occupants are not harmed because the steel body conducts the high-voltage electrons away from them. (Crawl- ing under the car is definitely unsafe!) A lightning arrester for TV and radio antennas is simply a grounded wire brought close to, but not touching, the antenna. Lightning arresters for power lines are made of materials with specialized resistance properties. The materials permit the high-voltage lightning discharge to pass to ground but stop the lower- voltage energy of the power line from being grounded.

3–5 NUISANCE STATIC CHARGES

Besides lightning and the radio interference it causes, static charges are responsible for a variety of other nuisances and hazards.

• Power belting readily becomes charged. Grounded pulleys, combs or tinsel bars close to the belt, conductive belt dressing, and the use of conductive belt materials help pre- vent the buildup of static electricity.

• Charges that have accumulated on trucks and cars are usually grounded by a wire on the appro chway to a tollgate to prevent electrical discharges (shocks) when coins change hands.

• Anesthetic gases are combustible. Precautions are taken to avoid static sparks, which may cause explosions during surgical operations. Grounded equipment, moist air, and conductive rubber help prevent the accumulation of charges.

• Grain dust, flour, wood dust, and cotton lint have produced disastrous explosions. Prevention of static charges helps avoid such dust explosions.

• In printing, sheets of paper may fail to feed into the press or to stack properly if they repel each other or are attracted to nearby objects. When printing is done from a continuous sheet, the sheet may become charged and ignite if combustible solvent vapors are present. Similar problems occur in the cloth and plastic industries. Some devices that ground or neutralize these charges are the tinsel bar, flame, or a long metal comb connected to an alternating current power supply.

3–6 USEFUL STATIC CHARGES

Painting

Although some industries must prevent it, other industries use the static charge as a tool. In paint-spraying applications, paint particles, given a charge after they leave the spray gun, are attracted to the oppositely charged object receiving the paint, as illustrated in Figure 3–6. This method produces an even coat without wasting paint, and paint particles

do not accumulate in holes or openings since the charge there is no greater than that at the outer surface.

Sandpaper Manufacture

Sandpaper grit can be made to stand up as it is being applied to the paper backing by giving the sand particles a positive charge and the glue-covered paper backing a negative charge; see Figure 3–7. Since the individual sand grains have like charges, they repel each other and stand apart. This method produces a sharper sandpaper.

Smoke Precipitators; Rug Manufacture

Smoke precipitators charge the smoke particles, which are then collected on oppositely charged screens, so that the particles are not released to pollute the atmosphere. Similarly, charged rug fibers are attracted to oppositely charged, glue-covered backings to form new types of rugs and fabrics.

Xerography (Dry Copying)

Chester F. Carlson (1906–1968), the inventor of the process known as xerography, or dry copying, had worked for nearly 15 years on the application of electrostatic charges for office copiers before such machines appeared on the market around 1950. The core of the machine is a rotating aluminum drum coated with a thin film of the element selenium, a semiconducting material; see Figure 3–8. The conductivity of selenium is increased by light. The machine operates in the following manner:

1. As the selenium surface rotates past a positively charged wire, it loses electrons to the wire. This charging of the selenium is done in darkness.

2. Light, reflected from the sheet of material to be copied, is projected by lenses and mirrors onto the positively charged selenium on the drum. The black form of the letter R, for example, reflects no light to the drum. However, the white paper back- ground does reflect. This reflection forms an image there, just as a camera or projector forms an image. Where the light surrounding the letter strikes the selenium, the selenium becomes a better conductor. Electrons flow from the aluminum drum to this lighted area and neutralize its positive charge. The dark area of the drum, where the letter R was, remains positively charged.

3. A small conveyor belt, not shown in Figure 3–8, pours a mixture of positively charged glass beads and negatively charged black powder over the previously illuminated surface of the drum. The black particles are attracted to the positively charged areas left on the drum in Step 2. The black particles are not attracted to the neutralized areas of the polished selenium surface. Any excess black-powder toner and glass beads fall away.

4. The piece of paper that is to receive the finished print now passes under the drum.

The paper has been positively charged by a wire so that it attracts the negatively charged black powder from the surface of the drum.

5. Finally, the paper carrying the black-powder image passes either under a radiant heater or over a heated roller, where the black particles are melted into the paper to form a permanent copy. A brush (not shown) removes any particles still on the drum, and the selenium surface is ready for recharging.

Electrostatic Generators

Many types of static generators constructed in the past produced tiny currents at high voltage. A much more efficient device is the Van de Graaff generator, shown in Figure 3–9, developed by the American physicist Robert J. Van de Graaff (1901–1967). This device is used to create high voltages for speeding up charged particles in atom-smashing experiments. This generator can also be used to test lightning protection equipment. Small models of the Van de Graaff generator are available for small-scale laboratory work.

In Figure 3–9, the sphere at the top of the device charges to a few hundred thousand volts, but the number of electrons accumulated there is small enough so that the spark is harmless.

3–7 POTENTIAL ENERGY OF ELECTRONS

An electron forcibly taken away from one neutral object and put on another neutral object has gained potential energy; see Figure 3–10. Force is needed to pull the electron away from Object A because the electron is attracted back to A by the positive charge that remains on A. If several electrons are transferred to Object B, they repel additional electrons from A. These electrons possess potential energy. Given the opportunity, the electrons will return to A, producing heat as they return. If these electrons are permitted to return to A through an electric motor, their potential energy is converted to mechanical energy.

This concept may be compared to another example of potential energy, as illustrated in Figure 3–11. Water can be taken from the pond and put into the tank, but it cannot get there by itself. In other words, energy must be used to carry the water up to the tank. The water, however, then has potential energy. Given the opportunity, the water in the tank will run back down to the ground, releasing energy on whatever it hits; that is, it can do useful work if it passes over a water wheel or drives a water turbine.

The energy used to carry the pond water up to the tank is not wasted. Each gallon of water in the tank has more potential energy than a gallon of water in the pond. There is a potential energy difference between the water in the tank and the water in the pond.

In a similar manner, there is a potential energy difference between the electrons in Object B in Figure 3–10 and the electrons in Object A. In other words, there is a potential difference between B and A. It is precisely this difference in potential energy that is expressed by the word voltage, a very common term in our electrical vocabulary that is more fully explained in Section 4–2.

3–8 ELECTROSTATIC LINES OF FORCE

Many years ago, people were puzzled as to just how two objects could exert force on each other when they were some distance apart with no material connection between them. How do two oppositely charged objects attract each other in a vacuum in the absence of light and heat? Even today, this question is still puzzling. We have no evidence that any small particles pass back and forth between them to pull them together or push them apart. There is no evidence that any sort of wave passes from one object to another.

Years ago, a solution to this puzzle was proposed by picturing invisible lines of force, like ropes or rubber bands, pulling two opposite charges together, as shown in Figure 3–12. The pattern of these lines may actually be seen by scattering splinters or dust of some non- conducting material between two strongly charged objects, as illustrated by Figure 3–13. Shredded wheat, short fibers, grass seed, or wood splinters can be used. The short fibers become charged by induction and tend to become aligned in patterns like those shown in Figure 3–12. These patterns, referred to as electrostatic lines of force, show the existence of a very real force between two electrically charged objects. Later, as you proceed with your studies, you will learn that similar force fields exist between magnetic poles as well

(see Chapter 15). The graphic representation of such a magnetic force field may look similar, but it is a distinctly different subject. Do not confuse one with the other.

SUMMARY

• A negatively charged object has a surplus of electrons; a positively charged object lacks some electrons.

• A charged electroscope identifies unknown charges.

• Objects may become charged by being near a charged object, without contact. This is known as electrostatic induction.

• Potential difference is measured in volts. It is the energy difference between electrons in different locations.

• Charged objects are surrounded by an electric field that consists of electrostatic lines of force extending from the charged object.

Achievement Review

1. What is static electricity?

2. Is static electricity more often noticed on conductors or insulators? Why?

3. Can conductors be charged?

4. Describe what happens when a positively charged object comes near but does not touch a positively charged electroscope.

5. What happens when a negatively charged object comes near a positively charged electroscope?

6. What is electrostatic induction?

7. When a negatively charged object approaches a neutral object, what sort of charge is induced on the neutral object?

8. Charged objects are said to possess energy. Explain.

9. What are electrostatic lines of force?

10. How does a lightning arrester stop lightning?

11. State a few industrial uses of static charge.

12. What industries try to avoid static charges?

Electrostatics

3–1 STATIONARY ELECTRONS

The word static means that something is at rest; it is not going anywhere. There- fore, static electricity is a charge that is stationary; it is not a current.

A good deal can be learned about electrons when they are stationary. Even though they are not traveling, electrons do not sit still but constantly jump from one place to another. The sparks and arcs they produce are often very important.

The word static in a discussion of electricity may first remind one of noise in a radio. When lightning flashes or an electric spark occurs, vibrating electrons broadcast some energy that is received as crackling in a radio receiver. The term static is now applied to any unexplained radio interference, whether the interference is of true electro- static origin or not.

The material covered in this chapter is an extension of ideas introduced in Chapter 1. The Summary of Chapter 1 should be reviewed at this point, particularly these useful facts:

• Any object that has a negative charge has received some electrons that normally do not belong to it, and they will try to escape.

• Any object that has a positive charge has lost some of its electrons. When a positively charged object has electrons returned to it, it goes back to its original neutral state. The returned electrons do not have to be the same ones that were lost. (Remember that electrons are all alike, regardless of where they come from.) If a million electrons are lost and a different million electrons return, everything is still back to neutral.

Lists A and B in Figure 1–1 introduced insulators (nonconductors). Insulators easily acquire a static electric charge, because they are materials in which electrons cannot move readily. If a spot on the insulating material has a surplus of electrons, it keeps the electrons. If at another location on the material there is a lack of electrons, however, that surplus finds it difficult to slide over and fill the lack.

Materials in which electrons move freely are called conductors. Metals, with their loose electrons in the outer rings of the atoms, are good conductors. Conductors can be charged only if they are insulated from their surroundings by some nonconducting material.

3–2 ELECTROSCOPES

The electroscope was once widely used to compare electrical charges and served as a crude voltmeter. It was also used in the detection of radioactive minerals. The device was an important aid in experiments that led to the discovery of electrons. For present-day use, it has been replaced by devices that are more accurate and sensitive but that are also more complex.

The electroscope, Figure 3–1, must first be charged using a known charge. The charging procedure is simple: (1) A hard rubber rod is charged by rubbing it on wool; (2) then the hard rubber rod is slid along the knob of the electroscope. Electrons from the negative hard rubber are transferred from the rod to the electroscope, where they repel each other and scatter all over the metal knob, metal rod, and metal leaves. The leaves repel each other because both are charged alike, in this case, negatively, as shown in Figure 3–1A.

The electroscope can also be charged positively, using a charged glass rod instead of the hard rubber rod. As the glass rod is wiped along the knob of a neutral electroscope, the rod attracts electrons from the electroscope. The electroscope now has a lack of electrons (since they have been transferred to the glass rod), and the electroscope is positively charged. The leaves again spread apart, as shown in Figure 3–1B, because the like positive charges repel.

Unknown Charges

A charged electroscope will indicate whether an unknown charge is positive or negative. For example, a pen is charged by rubbing it on a sleeve. The question now is whether the pen is negatively or positively charged. With a hard rubber rod, the electro- scope is charged negatively, as described before. As shown in Figure 3–2A, the pen is then brought slowly toward the top of the electroscope, and the leaves are observed closely for their motion. The leaves may repel even more. Why?

The leaves were made negative with the rubber rod. If they repel more, they must now be more negative. The leaves can become more negative only by having more electrons forced onto them. The unknown charge on the pen, therefore, must be negative, because more of the electroscope’s electrons were repelled to the leaves.

If the pen is charged positively, as in Figure 3–2B, what will happen? The positively charged pen will attract some of the excess electrons from the leaves to the knob. As these repelling electrons are removed, the leaves will fall together.

A positively charged electroscope can also be used to identify unknown charges. Keep in mind that the positively charged electroscope has relatively few of its electrons removed; there are still many electrons there. When a negative object comes near, it will repel the electrons in the electroscope toward the leaves. These electrons will neutralize the repelling positive charges on the leaves, causing the leaves to fall together. An approaching positive object will pull even more electrons from the positively charged leaves, causing them to repel each other more strongly.

In these instances, all of the electron motion is within the electroscope itself. The charged pen is not brought close enough to the knob to permit electrons to jump the gap between the two objects.

3–3 ELECTROSTATIC INDUCTION

A charged object always affects other objects in its neighborhood either by at- tracting or repelling electrons within the nearby object. Electrostatic induction is the process by which the neighboring object acquires a charge. In unusual circumstances, electrostatic induction can cause accidents. For example, on a warm, dry day in south- ern Oregon (where there are no self-serve gas stations), a truck driver speaks to the new gas station attendant, Joe, as shown in Figure 3–3. Joe lets go of the iron post he was sitting on near the gas pump. As the truck drives away, he steps over to remove the hose from another customer’s gas tank. At this point, the gasoline catches fire. What causes this to happen?

A strongly charged truck, Joe’s new rubber-soled shoes, the removal of his hand from the grounded post, electrostatic induction, and a spark in the presence of gasoline fumes all contributed to the fire. Because the truck was negatively charged, it repelled electrons from Joe through the post to ground (earth). Having lost some electrons, Joe was then positively charged. When he removed his hand from the post, Joe did not immediately regain the lost electrons since he was insulated by his rubber-soled shoes and the dry air. Electrons, attracted to his positive hand from the gasoline tank, jumped the gap, causing a spark. The spark ignited the gasoline vapor.

Let us illustrate this electrostatic induction process using the electroscope shown in Figure 3–4.

1. Bring a negatively charged rod (truck) near the electroscope (Joe). The experimenter’s hand touching the electroscope corresponds to Joe’s connection to the ground through the post. The rod will repel electrons from the electroscope knob. These electrons will move from the electroscope to the experimenter’s hand touching the knob.

2. Remove the hand from the knob. This leaves the electroscope with a positive charge (lack of electrons).

3. Remove the negative rod.

If these steps are performed in the correct order, the electroscope will be left in a charged condition, shown by its repelling leaves. The fact that the charge is positive can be checked by its behavior when the negative rod is slowly brought near it again. The charge on the electroscope and the charge on Joe are examples of induced charges.

An induced charge on one object is caused by the approach of another charged object, without contact between the two objects.

3–4 LIGHTNING

Benjamin Franklin showed that lightning is a large-scale performance of ordinary electrostatic behavior. (A few people who tried his kite experiment were killed by it.)

The distribution of electrical charges in thunderclouds has been mapped, but as yet no one knows just how the charges are formed. Most of the lightning flashes occur within the cloud itself. Whether electrons go from cloud to ground or from ground to cloud depends on the part of the storm area that is over a particular location; see Figure 3–5.

Objects, such as a house, that are under the small, positively charged area at the center of the storm are hit by electrons moving from the earth to the cloud. Objects, such as trees, under the large, negatively charged part of the cloud are hit by electrons driving from the cloud to the earth.

It is not possible to insulate against lightning. Protection can be provided only by giving it an easy path to ground. Well-grounded lightning rods are very effective. Ammunition sheds have been protected simply by a steel cable supported along the ridge of the shed and grounded at each end. Steel frames of large buildings, grounded during construction, serve as lightning protection for the building.

An automobile may be struck by lightning, but its occupants are not harmed because the steel body conducts the high-voltage electrons away from them. (Crawl- ing under the car is definitely unsafe!) A lightning arrester for TV and radio antennas is simply a grounded wire brought close to, but not touching, the antenna. Lightning arresters for power lines are made of materials with specialized resistance properties. The materials permit the high-voltage lightning discharge to pass to ground but stop the lower- voltage energy of the power line from being grounded.

3–5 NUISANCE STATIC CHARGES

Besides lightning and the radio interference it causes, static charges are responsible for a variety of other nuisances and hazards.

• Power belting readily becomes charged. Grounded pulleys, combs or tinsel bars close to the belt, conductive belt dressing, and the use of conductive belt materials help pre- vent the buildup of static electricity.

• Charges that have accumulated on trucks and cars are usually grounded by a wire on the appro chway to a tollgate to prevent electrical discharges (shocks) when coins change hands.

• Anesthetic gases are combustible. Precautions are taken to avoid static sparks, which may cause explosions during surgical operations. Grounded equipment, moist air, and conductive rubber help prevent the accumulation of charges.

• Grain dust, flour, wood dust, and cotton lint have produced disastrous explosions. Prevention of static charges helps avoid such dust explosions.

• In printing, sheets of paper may fail to feed into the press or to stack properly if they repel each other or are attracted to nearby objects. When printing is done from a continuous sheet, the sheet may become charged and ignite if combustible solvent vapors are present. Similar problems occur in the cloth and plastic industries. Some devices that ground or neutralize these charges are the tinsel bar, flame, or a long metal comb connected to an alternating current power supply.

3–6 USEFUL STATIC CHARGES

Painting

Although some industries must prevent it, other industries use the static charge as a tool. In paint-spraying applications, paint particles, given a charge after they leave the spray gun, are attracted to the oppositely charged object receiving the paint, as illustrated in Figure 3–6. This method produces an even coat without wasting paint, and paint particles

do not accumulate in holes or openings since the charge there is no greater than that at the outer surface.

Sandpaper Manufacture

Sandpaper grit can be made to stand up as it is being applied to the paper backing by giving the sand particles a positive charge and the glue-covered paper backing a negative charge; see Figure 3–7. Since the individual sand grains have like charges, they repel each other and stand apart. This method produces a sharper sandpaper.

Smoke Precipitators; Rug Manufacture

Smoke precipitators charge the smoke particles, which are then collected on oppositely charged screens, so that the particles are not released to pollute the atmosphere. Similarly, charged rug fibers are attracted to oppositely charged, glue-covered backings to form new types of rugs and fabrics.

Xerography (Dry Copying)

Chester F. Carlson (1906–1968), the inventor of the process known as xerography, or dry copying, had worked for nearly 15 years on the application of electrostatic charges for office copiers before such machines appeared on the market around 1950. The core of the machine is a rotating aluminum drum coated with a thin film of the element selenium, a semiconducting material; see Figure 3–8. The conductivity of selenium is increased by light. The machine operates in the following manner:

1. As the selenium surface rotates past a positively charged wire, it loses electrons to the wire. This charging of the selenium is done in darkness.

2. Light, reflected from the sheet of material to be copied, is projected by lenses and mirrors onto the positively charged selenium on the drum. The black form of the letter R, for example, reflects no light to the drum. However, the white paper back- ground does reflect. This reflection forms an image there, just as a camera or projector forms an image. Where the light surrounding the letter strikes the selenium, the selenium becomes a better conductor. Electrons flow from the aluminum drum to this lighted area and neutralize its positive charge. The dark area of the drum, where the letter R was, remains positively charged.

3. A small conveyor belt, not shown in Figure 3–8, pours a mixture of positively charged glass beads and negatively charged black powder over the previously illuminated surface of the drum. The black particles are attracted to the positively charged areas left on the drum in Step 2. The black particles are not attracted to the neutralized areas of the polished selenium surface. Any excess black-powder toner and glass beads fall away.

4. The piece of paper that is to receive the finished print now passes under the drum.

The paper has been positively charged by a wire so that it attracts the negatively charged black powder from the surface of the drum.

5. Finally, the paper carrying the black-powder image passes either under a radiant heater or over a heated roller, where the black particles are melted into the paper to form a permanent copy. A brush (not shown) removes any particles still on the drum, and the selenium surface is ready for recharging.

Electrostatic Generators

Many types of static generators constructed in the past produced tiny currents at high voltage. A much more efficient device is the Van de Graaff generator, shown in Figure 3–9, developed by the American physicist Robert J. Van de Graaff (1901–1967). This device is used to create high voltages for speeding up charged particles in atom-smashing experiments. This generator can also be used to test lightning protection equipment. Small models of the Van de Graaff generator are available for small-scale laboratory work.

In Figure 3–9, the sphere at the top of the device charges to a few hundred thousand volts, but the number of electrons accumulated there is small enough so that the spark is harmless.

3–7 POTENTIAL ENERGY OF ELECTRONS

An electron forcibly taken away from one neutral object and put on another neutral object has gained potential energy; see Figure 3–10. Force is needed to pull the electron away from Object A because the electron is attracted back to A by the positive charge that remains on A. If several electrons are transferred to Object B, they repel additional electrons from A. These electrons possess potential energy. Given the opportunity, the electrons will return to A, producing heat as they return. If these electrons are permitted to return to A through an electric motor, their potential energy is converted to mechanical energy.

This concept may be compared to another example of potential energy, as illustrated in Figure 3–11. Water can be taken from the pond and put into the tank, but it cannot get there by itself. In other words, energy must be used to carry the water up to the tank. The water, however, then has potential energy. Given the opportunity, the water in the tank will run back down to the ground, releasing energy on whatever it hits; that is, it can do useful work if it passes over a water wheel or drives a water turbine.

The energy used to carry the pond water up to the tank is not wasted. Each gallon of water in the tank has more potential energy than a gallon of water in the pond. There is a potential energy difference between the water in the tank and the water in the pond.

In a similar manner, there is a potential energy difference between the electrons in Object B in Figure 3–10 and the electrons in Object A. In other words, there is a potential difference between B and A. It is precisely this difference in potential energy that is expressed by the word voltage, a very common term in our electrical vocabulary that is more fully explained in Section 4–2.

3–8 ELECTROSTATIC LINES OF FORCE

Many years ago, people were puzzled as to just how two objects could exert force on each other when they were some distance apart with no material connection between them. How do two oppositely charged objects attract each other in a vacuum in the absence of light and heat? Even today, this question is still puzzling. We have no evidence that any small particles pass back and forth between them to pull them together or push them apart. There is no evidence that any sort of wave passes from one object to another.

Years ago, a solution to this puzzle was proposed by picturing invisible lines of force, like ropes or rubber bands, pulling two opposite charges together, as shown in Figure 3–12. The pattern of these lines may actually be seen by scattering splinters or dust of some non- conducting material between two strongly charged objects, as illustrated by Figure 3–13. Shredded wheat, short fibers, grass seed, or wood splinters can be used. The short fibers become charged by induction and tend to become aligned in patterns like those shown in Figure 3–12. These patterns, referred to as electrostatic lines of force, show the existence of a very real force between two electrically charged objects. Later, as you proceed with your studies, you will learn that similar force fields exist between magnetic poles as well

(see Chapter 15). The graphic representation of such a magnetic force field may look similar, but it is a distinctly different subject. Do not confuse one with the other.

SUMMARY

• A negatively charged object has a surplus of electrons; a positively charged object lacks some electrons.

• A charged electroscope identifies unknown charges.

• Objects may become charged by being near a charged object, without contact. This is known as electrostatic induction.

• Potential difference is measured in volts. It is the energy difference between electrons in different locations.

• Charged objects are surrounded by an electric field that consists of electrostatic lines of force extending from the charged object.

Achievement Review

1. What is static electricity?

2. Is static electricity more often noticed on conductors or insulators? Why?

3. Can conductors be charged?

4. Describe what happens when a positively charged object comes near but does not touch a positively charged electroscope.

5. What happens when a negatively charged object comes near a positively charged electroscope?

6. What is electrostatic induction?

7. When a negatively charged object approaches a neutral object, what sort of charge is induced on the neutral object?

8. Charged objects are said to possess energy. Explain.

9. What are electrostatic lines of force?

10. How does a lightning arrester stop lightning?

11. State a few industrial uses of static charge.

12. What industries try to avoid static charges?