science universe

Energy budgets

An energy budget describes the ways in which energy is transformed from one state to another within some defined system, including an analysis of inputs, outputs, and changes in the quantities stored. Ecological energy budgets focus on the use and transformations of energy in the biosphere or its components.

Solar electromagnetic radiation is the major input of energy to Earth. This external source of energy helps to heat the planet, evaporate water, circulate the atmosphere and oceans, and sustain ecological processes. Ultimately, all of the solar energy absorbed by Earth is re-radiated back to space, as electromagnetic radiation of a longer wavelength than what was originally absorbed. Earth maintains a virtually perfect energetic balance be- tween inputs and outputs of electromagnetic energy.

Earth’s ecosystems depend on solar radiation as an external source of diffuse energy that can be utilized by photosynthetic autotrophs, such as green plants, to synthesize simple organic molecules such as sugars from in- organic molecules such as carbon dioxide and water. Plants use the fixed energy of these simple organic com- pounds, plus inorganic nutrients, to synthesize an enormous diversity of biochemicals through various metabolic reactions. Plants utilize these biochemicals and the energy they contain to accomplish their growth and reproduction. Moreover, plant biomass is directly or indirectly utilized as food by the enormous numbers of heterotrophic organisms that are incapable of fixing their own energy. These organisms include herbivores that eat plants, carnivores that eat animals, and detritivores that feed on dead biomass.

Worldwide, the use of solar energy for this ecological purpose is relatively small, accounting for much less than 1% of the amount received at Earth’s surface. Al- though this is a quantitatively trivial part of Earth’s energy budget, it is clearly very important qualitatively, be- cause this is the absorbed and biologically fixed energy that subsidizes all ecological processes.

Forms of energy

Energy is defined as the ability, or potential ability, of a body or system to do work. Energy can be measured in various units, such as the calorie, defined as the amount of energy required to raise the temperature of one gram of pure water from 59–61°F (15–16°C). (Note that the dietician’s calorie is equivalent to one thousand of these calories, or one kilocalorie.) The Joule is another unit of energy, defined as the amount of work required to lift a weight of 1 kg by 10 cm, and equivalent to 0.24 calories.

Energy can exist in various states, all of which are interchangeable through various sorts of physical/ chemical transformations. The basic categories of energy are: electromagnetic, kinetic, and potential, but each of these can also exist in various states, as is described below:

(1) Electromagnetic energy is the energy of photons, or quanta of energy that have properties of both particles and waves, and that travel through space at a constant speed of 3 X 108 meters per second (that is, at the speed of light). The components of electromagnetic energy are characterized on the basis of wavelength ranges, which ordered from the shortest to longest wavelengths are known as: gamma, x ray, ultraviolet, light or visible, infrared, and radio. All bodies with a temperature greater than absolute zero (that is, -459°F [-273°C], or zero degrees on the kelvin scale) emit electromagnetic energy at a rate and spectral quality that is strictly determined by their surface temperature. Relatively hot bodies have much larger emission rates and their radiation is dominated by shorter wavelengths, compared with cooler bodies. The Sun has a surface temperature of about 11,000°F (6,093°C) and most of its radiation is in the wavelength range of visible light (0.4-0.7 æm or micrometers) and shorter-wave infrared (0.7-2 æm), while Earth has a surface temperature of about 77°F (25°C) and its radiation peaks in the longer-wave infrared range at about 10 æm.

(2) Kinetic energy is the energy of dynamic motion, of which there are two basic types, the energy of moving bodies, and that of vibrating atoms or molecules. The later is also known as thermal energy, and the more vigorous the vibration, the greater the heat content.

(3) Potential energy has the capacity to do work, but it must be mobilized to do so. Potential energy occurs in various forms, including the following: (a) Chemical potential energy is stored in the inter-atomic bonds of molecules. This energy can be liberated by so-called exothermic reactions, which have a net release of energy. For example, heat is released when the chemically reduced sulfur of sulfide minerals is oxidized to sulfate, and when crystalline sodium chloride is dissolved into water. All biochemicals also store potential energy, equivalent to 4.6 kilocalories per gram of carbohydrate, 4.8 Kcal/g of protein, and 6.0-9.0 Kcal/g of fat. (b) Gravitational potential energy is stored in mass that is elevated above some gravitationally attractive surface, as when water occurs above the surface of the oceans, or any object occurs above the ground surface. Unless obstructed, water spontaneously flows downhill, and objects fall downwards in response to gradients of gravitational potential energy. (c) Other types of potential energy are somewhat less important in terms of ecological energy budgets, but they include potential energies of compressed gases, electrical potential gradients associated with voltage differentials, and the potential energy of matter, which can be released by nuclear reactions.

Energy transformations and the laws of thermodynamics

As noted previously, energy can be transformed among its various states. For example: (a) electromagnetic energy can be absorbed by a dark object and converted to thermal kinetic energy, resulting in an in- creased temperature of the absorbing body; (b) gravitational potential energy of water high on a plateau is transformed into the kinetic energy of moving water and heat at a waterfall, or it can be mobilized by humans to drive a turbine and generate electrical energy; and (c) solar electromagnetic radiation can be absorbed by the chlorophyll of green plants, and some of the absorbed energy can be converted into the chemical potential energy of sugars, and the rest converted into heat.

All transformations of energy must occur according to certain physical principles, known as the laws of thermodynamics. These are universal laws, which means that they are always true, regardless of circumstances. The first law states that energy can undergo transformations among its various states, but it is never created or destroyed, so the energy content of the universe remains constant. A consequence of this law for energy budgets is that there must always be a zero balance between the energy inputs to a system, the energy outputs, and any net storage within the system.

The second law of thermodynamics states that trans- formations of energy can only occur spontaneously under conditions in which there is an increase in the entropy of the universe. (Entropy is related to randomness of the distributions of matter and energy). For example, Earth is continuously irradiated by solar radiation, mostly of visible and near-infrared wavelengths. Some of this energy is absorbed, which heats the surface of Earth. The planet cools itself in various ways, but ultimately this is done by radiating its own electromagnetic radiation back to space, as longer- wave infrared radiation. The transformation of relatively short-wave solar radiation into the longer-wave radiation emitted by Earth represents a degradation of the quality of the energy, and an increase in the entropy of the universe.

A corollary, or secondary proposition of the second law of thermodynamics is that energy transformations can never be completely efficient, because some of the initial content of energy must be converted to heat so that entropy can be increased. Ultimately, this is the reason why no more than about 30% of the energy content of gasoline can be converted into the kinetic energy of a moving automobile, and why no more than about 40% of the energy of coal can be transformed into electricity in a modern generating station. Similarly, there are upper limits to the efficiency by which green plants can photo- synthetically convert visible radiation into biochemicals, even in ecosystems in which ecological constraints related to nutrients, water, and space are optimized.

Interestingly, plants absorb visible radiation emitted by the Sun, and use this relatively dispersed energy to fix simple inorganic molecules such as carbon dioxide, water, and other nutrients into very complex and energy- dense biochemicals. The biochemicals of plant biomass are then used by heterotrophic organisms to synthesize their own complex biochemicals. Locally, these various biological syntheses represent energy transformations that substantially decrease entropy, rather than increase it. This is because relatively dispersed solar energy and simple compounds are focused into the complex bio- chemicals of living organisms. Are biological transformations not obeying the second law of thermodynamics?

This seeming physical paradox of life can be successfully rationalized, using the following logic: The localized bio-concentrating of negative entropy can occur because there is a constant input of energy into the sys- tem, in the form of solar radiation. If this external source of energy was terminated, then all of the negative entropy of organisms and organic matter would rather quickly be spontaneously degraded, producing heat and simple inorganic molecules, and thereby increasing the entropy of the universe. This is why life and ecosystems cannot survive without continual inputs of solar energy. Therefore, the biosphere can be considered to represent a localized island, in space and time, of negative entropy, fueled by an external (solar) source of energy. There are physical analogues to these ecological circumstances—if external energy is put into the system, relatively dispersed molecules of gases can be concentrated into a container, as occurs when a person blows energetically to fill a balloon with air. Eventually, however, the balloon pops, the gases re-disperse, the original energy input is converted into heat, and the entropy of the universe is increased.

Energy budgets

An energy budget describes the ways in which energy is transformed from one state to another within some defined system, including an analysis of inputs, outputs, and changes in the quantities stored. Ecological energy budgets focus on the use and transformations of energy in the biosphere or its components.

Solar electromagnetic radiation is the major input of energy to Earth. This external source of energy helps to heat the planet, evaporate water, circulate the atmosphere and oceans, and sustain ecological processes. Ultimately, all of the solar energy absorbed by Earth is re-radiated back to space, as electromagnetic radiation of a longer wavelength than what was originally absorbed. Earth maintains a virtually perfect energetic balance be- tween inputs and outputs of electromagnetic energy.

Earth’s ecosystems depend on solar radiation as an external source of diffuse energy that can be utilized by photosynthetic autotrophs, such as green plants, to synthesize simple organic molecules such as sugars from in- organic molecules such as carbon dioxide and water. Plants use the fixed energy of these simple organic com- pounds, plus inorganic nutrients, to synthesize an enormous diversity of biochemicals through various metabolic reactions. Plants utilize these biochemicals and the energy they contain to accomplish their growth and reproduction. Moreover, plant biomass is directly or indirectly utilized as food by the enormous numbers of heterotrophic organisms that are incapable of fixing their own energy. These organisms include herbivores that eat plants, carnivores that eat animals, and detritivores that feed on dead biomass.

Worldwide, the use of solar energy for this ecological purpose is relatively small, accounting for much less than 1% of the amount received at Earth’s surface. Al- though this is a quantitatively trivial part of Earth’s energy budget, it is clearly very important qualitatively, be- cause this is the absorbed and biologically fixed energy that subsidizes all ecological processes.

Forms of energy

Energy is defined as the ability, or potential ability, of a body or system to do work. Energy can be measured in various units, such as the calorie, defined as the amount of energy required to raise the temperature of one gram of pure water from 59–61°F (15–16°C). (Note that the dietician’s calorie is equivalent to one thousand of these calories, or one kilocalorie.) The Joule is another unit of energy, defined as the amount of work required to lift a weight of 1 kg by 10 cm, and equivalent to 0.24 calories.

Energy can exist in various states, all of which are interchangeable through various sorts of physical/ chemical transformations. The basic categories of energy are: electromagnetic, kinetic, and potential, but each of these can also exist in various states, as is described below:

(1) Electromagnetic energy is the energy of photons, or quanta of energy that have properties of both particles and waves, and that travel through space at a constant speed of 3 X 108 meters per second (that is, at the speed of light). The components of electromagnetic energy are characterized on the basis of wavelength ranges, which ordered from the shortest to longest wavelengths are known as: gamma, x ray, ultraviolet, light or visible, infrared, and radio. All bodies with a temperature greater than absolute zero (that is, -459°F [-273°C], or zero degrees on the kelvin scale) emit electromagnetic energy at a rate and spectral quality that is strictly determined by their surface temperature. Relatively hot bodies have much larger emission rates and their radiation is dominated by shorter wavelengths, compared with cooler bodies. The Sun has a surface temperature of about 11,000°F (6,093°C) and most of its radiation is in the wavelength range of visible light (0.4-0.7 æm or micrometers) and shorter-wave infrared (0.7-2 æm), while Earth has a surface temperature of about 77°F (25°C) and its radiation peaks in the longer-wave infrared range at about 10 æm.

(2) Kinetic energy is the energy of dynamic motion, of which there are two basic types, the energy of moving bodies, and that of vibrating atoms or molecules. The later is also known as thermal energy, and the more vigorous the vibration, the greater the heat content.

(3) Potential energy has the capacity to do work, but it must be mobilized to do so. Potential energy occurs in various forms, including the following: (a) Chemical potential energy is stored in the inter-atomic bonds of molecules. This energy can be liberated by so-called exothermic reactions, which have a net release of energy. For example, heat is released when the chemically reduced sulfur of sulfide minerals is oxidized to sulfate, and when crystalline sodium chloride is dissolved into water. All biochemicals also store potential energy, equivalent to 4.6 kilocalories per gram of carbohydrate, 4.8 Kcal/g of protein, and 6.0-9.0 Kcal/g of fat. (b) Gravitational potential energy is stored in mass that is elevated above some gravitationally attractive surface, as when water occurs above the surface of the oceans, or any object occurs above the ground surface. Unless obstructed, water spontaneously flows downhill, and objects fall downwards in response to gradients of gravitational potential energy. (c) Other types of potential energy are somewhat less important in terms of ecological energy budgets, but they include potential energies of compressed gases, electrical potential gradients associated with voltage differentials, and the potential energy of matter, which can be released by nuclear reactions.

Energy transformations and the laws of thermodynamics

As noted previously, energy can be transformed among its various states. For example: (a) electromagnetic energy can be absorbed by a dark object and converted to thermal kinetic energy, resulting in an in- creased temperature of the absorbing body; (b) gravitational potential energy of water high on a plateau is transformed into the kinetic energy of moving water and heat at a waterfall, or it can be mobilized by humans to drive a turbine and generate electrical energy; and (c) solar electromagnetic radiation can be absorbed by the chlorophyll of green plants, and some of the absorbed energy can be converted into the chemical potential energy of sugars, and the rest converted into heat.

All transformations of energy must occur according to certain physical principles, known as the laws of thermodynamics. These are universal laws, which means that they are always true, regardless of circumstances. The first law states that energy can undergo transformations among its various states, but it is never created or destroyed, so the energy content of the universe remains constant. A consequence of this law for energy budgets is that there must always be a zero balance between the energy inputs to a system, the energy outputs, and any net storage within the system.

The second law of thermodynamics states that trans- formations of energy can only occur spontaneously under conditions in which there is an increase in the entropy of the universe. (Entropy is related to randomness of the distributions of matter and energy). For example, Earth is continuously irradiated by solar radiation, mostly of visible and near-infrared wavelengths. Some of this energy is absorbed, which heats the surface of Earth. The planet cools itself in various ways, but ultimately this is done by radiating its own electromagnetic radiation back to space, as longer- wave infrared radiation. The transformation of relatively short-wave solar radiation into the longer-wave radiation emitted by Earth represents a degradation of the quality of the energy, and an increase in the entropy of the universe.

A corollary, or secondary proposition of the second law of thermodynamics is that energy transformations can never be completely efficient, because some of the initial content of energy must be converted to heat so that entropy can be increased. Ultimately, this is the reason why no more than about 30% of the energy content of gasoline can be converted into the kinetic energy of a moving automobile, and why no more than about 40% of the energy of coal can be transformed into electricity in a modern generating station. Similarly, there are upper limits to the efficiency by which green plants can photo- synthetically convert visible radiation into biochemicals, even in ecosystems in which ecological constraints related to nutrients, water, and space are optimized.

Interestingly, plants absorb visible radiation emitted by the Sun, and use this relatively dispersed energy to fix simple inorganic molecules such as carbon dioxide, water, and other nutrients into very complex and energy- dense biochemicals. The biochemicals of plant biomass are then used by heterotrophic organisms to synthesize their own complex biochemicals. Locally, these various biological syntheses represent energy transformations that substantially decrease entropy, rather than increase it. This is because relatively dispersed solar energy and simple compounds are focused into the complex bio- chemicals of living organisms. Are biological transformations not obeying the second law of thermodynamics?

This seeming physical paradox of life can be successfully rationalized, using the following logic: The localized bio-concentrating of negative entropy can occur because there is a constant input of energy into the sys- tem, in the form of solar radiation. If this external source of energy was terminated, then all of the negative entropy of organisms and organic matter would rather quickly be spontaneously degraded, producing heat and simple inorganic molecules, and thereby increasing the entropy of the universe. This is why life and ecosystems cannot survive without continual inputs of solar energy. Therefore, the biosphere can be considered to represent a localized island, in space and time, of negative entropy, fueled by an external (solar) source of energy. There are physical analogues to these ecological circumstances—if external energy is put into the system, relatively dispersed molecules of gases can be concentrated into a container, as occurs when a person blows energetically to fill a balloon with air. Eventually, however, the balloon pops, the gases re-disperse, the original energy input is converted into heat, and the entropy of the universe is increased.

Magnetic energy

A magnetic is a piece of metal that has the ability to attract iron, nickel, cobalt, or certain specific other kinds of metal. Every magnet contains two distinct regions, one known as the north pole and one, the south pole. As with electrical charges, unlike poles attract each other and like poles repel each other.

A study of magnets allows the introduction of a new concept in energy, the concept of a field. An energy field is a region in space in which a magnetic, electrical, or some other kind of force can be experienced. For example, imagine that a piece of iron is placed at a distance of 2 in (5 cm) from a bar magnet. If the magnet is strong enough, it may pull on the iron strongly enough to cause it to move. The piece of iron is said to be within the magnetic field of the bar magnet.

The concept of an energy field was, at one time, a very difficult one for scientists to understand and accept. How could one object exert a force on another object if the two were not in contact with each other? Eventually, it became clear that forces can operate at a distance from each other. Electrical charges and magnetic poles seem to exert their forces throughout a field along pathways known as lines of force.

One of the great discoveries in the history of physics was made by the English physicist James Clerk Maxwell (1831-1879) in the late nineteenth century. Maxwell found that the two major forms of energy known as electricity and magnetism are not really different from each other, but are instead closely associated with each other. That is, every electrical current has associated with it a magnetic field and every changing magnetic field creates its own electrical current.

As a result of Maxwell’s work, it is often more correct to speak of electromagnetic energy, a form of energy that has both electrical and magnetic components. Scientists now know that a number of seemingly different types of energy are all actually forms of electromagnetic energy. These include x rays, gamma rays, ultraviolet light, visible light, infrared radiation, radio waves, and microwaves. These forms of electromagnetic energy differ from each other in terms of the wavelength and frequency of the energy wave on which they travel. The waves associated with x rays, for example, have very short wavelengths and very high frequencies, while the waves associated with microwaves have much longer wavelengths and much lower frequencies.

Sound, chemical, and nuclear energy

The fact that people can hear is a simple demonstration of the fact that sound is a form of energy. Sound is actually nothing other than the movement of air. When sound is created, sound waves travel through space, creating compressions in some regions and rarefactions in other regions. When these sound waves strike the human eardrum, they cause the drum to vibrate, creating the sensation of sound in the brain. Similar kinds of sound waves are responsible for the destruction caused by explosions. The sound waves collide with building, trees, people, and other objects, causing damage to them.

Chemical energy is a form of energy that results from the forces of attraction that hold atoms and other particles together in molecules. In water, for example, hydrogen atoms are joined to oxygen atoms by means of strong forces known as chemical bonds. If those are broken, the forces are released in the form of chemical energy. When a substance is burned, chemical energy is released. Burning (combustion or oxidation) is the process by which chemical bonds in a fuel and in oxygen molecules are broken and new chemical bonds are formed. The total energy in the new chemical bonds is less than it was in the original chemical bonds, and the difference is released in the form of chemical energy.

Nuclear energy is similar to chemical energy except that the bonds involved are those that hold together the particles of a nucleus, protons and neutrons. The fact that

most atomic nuclei are stable is proof that some very strong nuclear forces exist. Protons are positively charged and one would expect that they would repel each other, blowing apart a nucleus. Since that does not happen, some kinds of force must exist to hold the nucleus together.

One such force is known as the strong force. If some- thing happens to cause a nucleus to break apart, the strong force holding two protons together is released in the form of nuclear energy. That is what happens in an atomic (fission) bomb. A uranium nucleus breaks apart into two roughly equal pieces, and some of the strong force holding protons together is released as nuclear energy.

Magnetic energy

A magnetic is a piece of metal that has the ability to attract iron, nickel, cobalt, or certain specific other kinds of metal. Every magnet contains two distinct regions, one known as the north pole and one, the south pole. As with electrical charges, unlike poles attract each other and like poles repel each other.

A study of magnets allows the introduction of a new concept in energy, the concept of a field. An energy field is a region in space in which a magnetic, electrical, or some other kind of force can be experienced. For example, imagine that a piece of iron is placed at a distance of 2 in (5 cm) from a bar magnet. If the magnet is strong enough, it may pull on the iron strongly enough to cause it to move. The piece of iron is said to be within the magnetic field of the bar magnet.

The concept of an energy field was, at one time, a very difficult one for scientists to understand and accept. How could one object exert a force on another object if the two were not in contact with each other? Eventually, it became clear that forces can operate at a distance from each other. Electrical charges and magnetic poles seem to exert their forces throughout a field along pathways known as lines of force.

One of the great discoveries in the history of physics was made by the English physicist James Clerk Maxwell (1831-1879) in the late nineteenth century. Maxwell found that the two major forms of energy known as electricity and magnetism are not really different from each other, but are instead closely associated with each other. That is, every electrical current has associated with it a magnetic field and every changing magnetic field creates its own electrical current.

As a result of Maxwell’s work, it is often more correct to speak of electromagnetic energy, a form of energy that has both electrical and magnetic components. Scientists now know that a number of seemingly different types of energy are all actually forms of electromagnetic energy. These include x rays, gamma rays, ultraviolet light, visible light, infrared radiation, radio waves, and microwaves. These forms of electromagnetic energy differ from each other in terms of the wavelength and frequency of the energy wave on which they travel. The waves associated with x rays, for example, have very short wavelengths and very high frequencies, while the waves associated with microwaves have much longer wavelengths and much lower frequencies.

Sound, chemical, and nuclear energy

The fact that people can hear is a simple demonstration of the fact that sound is a form of energy. Sound is actually nothing other than the movement of air. When sound is created, sound waves travel through space, creating compressions in some regions and rarefactions in other regions. When these sound waves strike the human eardrum, they cause the drum to vibrate, creating the sensation of sound in the brain. Similar kinds of sound waves are responsible for the destruction caused by explosions. The sound waves collide with building, trees, people, and other objects, causing damage to them.

Chemical energy is a form of energy that results from the forces of attraction that hold atoms and other particles together in molecules. In water, for example, hydrogen atoms are joined to oxygen atoms by means of strong forces known as chemical bonds. If those are broken, the forces are released in the form of chemical energy. When a substance is burned, chemical energy is released. Burning (combustion or oxidation) is the process by which chemical bonds in a fuel and in oxygen molecules are broken and new chemical bonds are formed. The total energy in the new chemical bonds is less than it was in the original chemical bonds, and the difference is released in the form of chemical energy.

Nuclear energy is similar to chemical energy except that the bonds involved are those that hold together the particles of a nucleus, protons and neutrons. The fact that

most atomic nuclei are stable is proof that some very strong nuclear forces exist. Protons are positively charged and one would expect that they would repel each other, blowing apart a nucleus. Since that does not happen, some kinds of force must exist to hold the nucleus together.

One such force is known as the strong force. If some- thing happens to cause a nucleus to break apart, the strong force holding two protons together is released in the form of nuclear energy. That is what happens in an atomic (fission) bomb. A uranium nucleus breaks apart into two roughly equal pieces, and some of the strong force holding protons together is released as nuclear energy.

Energy

Energy is a state function commonly defined as the capacity to do work. Since work is defined as the movement of an object through a distance, energy can also be described as the ability to move an object through a distance. As an example, imagine that a bar magnet is placed next to a pile of iron filings (thin slivers of iron metal). The iron filings begin to move toward the iron bar because magnetic energy pulls on the iron filings and causes them to move.

Energy can be a difficult concept to understand. Un- like matter, energy can not be taken hold of or placed on a laboratory bench for study. We know the nature and characteristics of energy best because of the effect it has on objects around it, as in the case of the bar magnet and iron filings mentioned above.

Energy is described in many forms, including mechanical, heat, electrical, magnetic, sound, chemical, and nuclear. Although these forms appear to be very different from each other, they often have much in common and can generally be transformed into one another.

Over time, a number of different units have been used to measure energy. In the British system, for example, the fundamental unit of energy is the foot-pound. One foot- pound is the amount of energy that can move a weight of one pound a distance of one foot. In the metric system, the fundamental unit of energy is the joule (abbreviation: J), named after the English scientist James Prescott Joule (1818-1889). A joule is the amount of energy that can move a weight of one newton a distance of one meter.

Potential and kinetic energy

Every object has energy as a consequence of its position in space and/or its motion. For example, a base- ball poised on a railing at the top of the observation deck on the Empire State Building has potential energy be- cause of its ability to fall off the railing and come crashing down onto the street. The potential energy of the baseball—as well as that of any other object—is dependent on two factors, its mass and its height above the ground. The formula for potential energy is p.e. = m X g X h, where m stands for mass, h for height above the ground, and g for the gravitational constant (9.8 m per second per second).

Potential energy is actually a manifestation of the gravitational attraction of two bodies for each other. The

baseball on top of the Empire State Building has potential energy because of the gravitational force that tends to bring the ball and Earth together. When the ball falls, both Earth and ball are actually moving toward each other. Since Earth is so many times more massive than the ball, however, we do not see its very minute motion.

When an object falls, at least part of its potential energy is converted to kinetic energy, the energy due to an object’s motion. The amount of kinetic energy possessed by an object is a function of two variables, its mass and its velocity. The formula for kinetic energy is k.e. = 1/2m X v2, where m is the mass of the object and v is its velocity. This formula shows that an object can have a lot of kinetic energy for two reasons. It can either be very heavy or it can be moving very fast. For that reason, a fairly light baseball falling over a very great distance and traveling at a very great speed can do as much damage as a much more massive object falling at a slower speed.

Conservation of energy

The sum total of an object’s potential and kinetic energy is known as its mechanical energy. The total amount of mechanical energy possessed by a body is a constant. The baseball described above has a maximum potential energy and minimum kinetic energy (actually a zero kinetic energy) while at rest. In the fraction of a second be- fore the ball has struck the ground, its kinetic energy has become a maximum and its potential energy has reached almost zero.

The case of the falling baseball described above is a special interest of a more general rule known as the law of conservation of energy. According to this law, energy can never be created or destroyed. In other words, the total amount of energy available in the universe remains constant and can never increase or decrease.

Although energy can never be created or destroyed, it can be transformed into new forms. In an electric iron, for example, an electrical current flows through metallic coils within the iron. As it does so, the current experiences resistance from the metallic coils and is converted into a different form, heat. A television set is another de- vice that operates by the transformation of energy. An electrical beam from the back of the television tube strikes a thin layer of chemicals on the television screen, causing them to glow. In this case, electrical energy is converted into light. Many of the modern appliances that we use in our homes, such as the electric iron and the television set, make use of the transformation of energy from one form to another.

In the early 1900s, Albert Einstein announced per- haps the most surprising energy transformation of all. Einstein showed by mathematical reasoning that energy can be converted into matter and, vice versa, matter can be transformed into energy. He expressed the equivalence of matter and energy in a now famous equation, E = m X c2, where c is a constant, the speed of light.

Forms of energy

The operation of a steam engine is an example of heat being used as a source of energy. Hot steam is pumped into a cylinder, forcing a piston to move within the cylinder. When the steam cools off and changes back to water, the piston returns to its original position. The cycle is then repeated. The up-and-down motion of the piston is used to turn a wheel or do some other kind of work. In this example, the heat of the hot steam is used to do work on the wheel or some other object.

The source of heat energy is the motion of molecules within a substance. In the example above, steam is said to be “hot” because the particles of which it is made are moving very rapidly. When those particles slow down, the steam has less energy. The total amount of energy contained within any body as a consequence of particle motion is called the body’s thermal energy.

One measure of the amount of particle motion within a body is temperature. Temperature is a measure of the average kinetic energy of the particles within the body. An object in which particles are moving very rapidly on average has a high temperature. One in which particles are moving slowly on average has a low temperature.

Temperature and thermal energy are different concepts, however, because temperature measures only the average kinetic energy of particles, while thermal energy measures the total amount of energy in an object. A thimbleful of water and a swimming pool of water might both have the same temperature, that is, the average kinetic energy of water molecules in both might be the same. But there is a great deal more water in the swimming pool, so the total thermal energy in it is much greater than the thermal energy of water in the thimble.

Electrical energy

Suppose that two ping pong balls, each carrying an electrical charge, are placed near to each other. If free to move, the two balls have a tendency either to roll toward each other or away from each other, depending on the charges. If the charges they carry are the same (both positive or both negative), the two balls will repel each other and roll away from each other. If they charges are opposite, the balls will attract each other and roll toward each other. The force of attraction or repulsion of the two balls is a manifestation of the electrical potential energy existing between the two balls.

Electrical potential energy is analogous to gravitational energy. In the case of the latter, any two bodies in the universe exert a force of attraction on each other that depends on the masses of the two bodies and the distance between them. Any two charged bodies in the universe, on the other hand, experience a force of attraction or repulsion (depending on their signs) that depends on the magnitude of their charges and the distance separating them. A lightning bolt traveling from the ground to a cloud is an example of electrical potential energy that has suddenly been converted to it “kinetic” form, an electrical current.

An electrical current is analogous to kinetic energy, that is, it is the result of moving electrical charges. An electrical current flows any time two conditions are met. First, there must be a source of electrical charges. A battery is a familiar source of electrical charges. Second, there must be a pathway through which the electric charges can flow. The pathway is known as a circuit.

An electric current is useful, however, only if a third condition is met—the presence of some kind of de- vice that can be operated by electrical energy. For example, one might insert a radio into the circuit through which electrical charges are flowing. When that happens, the electrical charges flow through the radio and make it produce sounds. That is, electrical energy is transformed into sound energy within the radio.

Energy

Energy is a state function commonly defined as the capacity to do work. Since work is defined as the movement of an object through a distance, energy can also be described as the ability to move an object through a distance. As an example, imagine that a bar magnet is placed next to a pile of iron filings (thin slivers of iron metal). The iron filings begin to move toward the iron bar because magnetic energy pulls on the iron filings and causes them to move.

Energy can be a difficult concept to understand. Un- like matter, energy can not be taken hold of or placed on a laboratory bench for study. We know the nature and characteristics of energy best because of the effect it has on objects around it, as in the case of the bar magnet and iron filings mentioned above.

Energy is described in many forms, including mechanical, heat, electrical, magnetic, sound, chemical, and nuclear. Although these forms appear to be very different from each other, they often have much in common and can generally be transformed into one another.

Over time, a number of different units have been used to measure energy. In the British system, for example, the fundamental unit of energy is the foot-pound. One foot- pound is the amount of energy that can move a weight of one pound a distance of one foot. In the metric system, the fundamental unit of energy is the joule (abbreviation: J), named after the English scientist James Prescott Joule (1818-1889). A joule is the amount of energy that can move a weight of one newton a distance of one meter.

Potential and kinetic energy

Every object has energy as a consequence of its position in space and/or its motion. For example, a base- ball poised on a railing at the top of the observation deck on the Empire State Building has potential energy be- cause of its ability to fall off the railing and come crashing down onto the street. The potential energy of the baseball—as well as that of any other object—is dependent on two factors, its mass and its height above the ground. The formula for potential energy is p.e. = m X g X h, where m stands for mass, h for height above the ground, and g for the gravitational constant (9.8 m per second per second).

Potential energy is actually a manifestation of the gravitational attraction of two bodies for each other. The

baseball on top of the Empire State Building has potential energy because of the gravitational force that tends to bring the ball and Earth together. When the ball falls, both Earth and ball are actually moving toward each other. Since Earth is so many times more massive than the ball, however, we do not see its very minute motion.

When an object falls, at least part of its potential energy is converted to kinetic energy, the energy due to an object’s motion. The amount of kinetic energy possessed by an object is a function of two variables, its mass and its velocity. The formula for kinetic energy is k.e. = 1/2m X v2, where m is the mass of the object and v is its velocity. This formula shows that an object can have a lot of kinetic energy for two reasons. It can either be very heavy or it can be moving very fast. For that reason, a fairly light baseball falling over a very great distance and traveling at a very great speed can do as much damage as a much more massive object falling at a slower speed.

Conservation of energy

The sum total of an object’s potential and kinetic energy is known as its mechanical energy. The total amount of mechanical energy possessed by a body is a constant. The baseball described above has a maximum potential energy and minimum kinetic energy (actually a zero kinetic energy) while at rest. In the fraction of a second be- fore the ball has struck the ground, its kinetic energy has become a maximum and its potential energy has reached almost zero.

The case of the falling baseball described above is a special interest of a more general rule known as the law of conservation of energy. According to this law, energy can never be created or destroyed. In other words, the total amount of energy available in the universe remains constant and can never increase or decrease.

Although energy can never be created or destroyed, it can be transformed into new forms. In an electric iron, for example, an electrical current flows through metallic coils within the iron. As it does so, the current experiences resistance from the metallic coils and is converted into a different form, heat. A television set is another de- vice that operates by the transformation of energy. An electrical beam from the back of the television tube strikes a thin layer of chemicals on the television screen, causing them to glow. In this case, electrical energy is converted into light. Many of the modern appliances that we use in our homes, such as the electric iron and the television set, make use of the transformation of energy from one form to another.

In the early 1900s, Albert Einstein announced per- haps the most surprising energy transformation of all. Einstein showed by mathematical reasoning that energy can be converted into matter and, vice versa, matter can be transformed into energy. He expressed the equivalence of matter and energy in a now famous equation, E = m X c2, where c is a constant, the speed of light.

Forms of energy

The operation of a steam engine is an example of heat being used as a source of energy. Hot steam is pumped into a cylinder, forcing a piston to move within the cylinder. When the steam cools off and changes back to water, the piston returns to its original position. The cycle is then repeated. The up-and-down motion of the piston is used to turn a wheel or do some other kind of work. In this example, the heat of the hot steam is used to do work on the wheel or some other object.

The source of heat energy is the motion of molecules within a substance. In the example above, steam is said to be “hot” because the particles of which it is made are moving very rapidly. When those particles slow down, the steam has less energy. The total amount of energy contained within any body as a consequence of particle motion is called the body’s thermal energy.

One measure of the amount of particle motion within a body is temperature. Temperature is a measure of the average kinetic energy of the particles within the body. An object in which particles are moving very rapidly on average has a high temperature. One in which particles are moving slowly on average has a low temperature.

Temperature and thermal energy are different concepts, however, because temperature measures only the average kinetic energy of particles, while thermal energy measures the total amount of energy in an object. A thimbleful of water and a swimming pool of water might both have the same temperature, that is, the average kinetic energy of water molecules in both might be the same. But there is a great deal more water in the swimming pool, so the total thermal energy in it is much greater than the thermal energy of water in the thimble.

Electrical energy

Suppose that two ping pong balls, each carrying an electrical charge, are placed near to each other. If free to move, the two balls have a tendency either to roll toward each other or away from each other, depending on the charges. If the charges they carry are the same (both positive or both negative), the two balls will repel each other and roll away from each other. If they charges are opposite, the balls will attract each other and roll toward each other. The force of attraction or repulsion of the two balls is a manifestation of the electrical potential energy existing between the two balls.

Electrical potential energy is analogous to gravitational energy. In the case of the latter, any two bodies in the universe exert a force of attraction on each other that depends on the masses of the two bodies and the distance between them. Any two charged bodies in the universe, on the other hand, experience a force of attraction or repulsion (depending on their signs) that depends on the magnitude of their charges and the distance separating them. A lightning bolt traveling from the ground to a cloud is an example of electrical potential energy that has suddenly been converted to it “kinetic” form, an electrical current.

An electrical current is analogous to kinetic energy, that is, it is the result of moving electrical charges. An electrical current flows any time two conditions are met. First, there must be a source of electrical charges. A battery is a familiar source of electrical charges. Second, there must be a pathway through which the electric charges can flow. The pathway is known as a circuit.

An electric current is useful, however, only if a third condition is met—the presence of some kind of de- vice that can be operated by electrical energy. For example, one might insert a radio into the circuit through which electrical charges are flowing. When that happens, the electrical charges flow through the radio and make it produce sounds. That is, electrical energy is transformed into sound energy within the radio.