What is Energy?
1. Introduction.Energy. We all use it. We all talk about it. We all pay for it. Governments formulate energy policy and have huge departments to administer energy policy. We worry about our natural sources of energy "running out". Some days we have low energy, and take pills to boost it. We observe children at play and say "I wish you could bottle all that energy."
Yet if asked "Just what is energy?", most persons are unable to give a coherent answer. A freshman physics student may answer by giving various "kinds" or "forms" of energy: kinetic energy, potential energy, heat energy, chemical energy, nuclear energy, and electrical energy. But that's not an answer; it's a shopping list.
We speak of energy in language that suggests energy is "something contained in material things". We speak of "extracting" energy from coal and oil. We say energy can be "converted from one form to another". Such language suggests that we are thinking of energy as a "substance". But the notion of bottling pure energy is absurd. Has anyone ever seen pure energy?
Science education has convinced most people that the energy available from earth's natural resources is finite, but realizing the finiteness of a resource doesn't stop us from wasting it carelessly and unnecessarily. Still, if the average person were asked whether it's possible that we might someday build machines that produce more energy than they consume, they would dismiss the idea as absurd, saying "You can't get something from nothing," or "There's no free lunch."
In the early history of technology even this superficial understanding of energy was not known. The very idea of energy is a recently developed concept. The history of physics can be divided into several periods: (1) Antiquity to the 17th century, when the concept of force had not yet been precisely formulated. (2) 17th century to 18th century, when force and torque were quite well understood and classical Newtonian mechanics evolved into a very successful model. (3) 19th century, when work, energy and momentum became quite well-understood (at least by physicists and engineers), and the laws of thermodynamics were formulated. (4) 1900 to the present, when atomic theory and quantum mechanics achieved tremendous success.
But for all of these advances, the physics student today must still progress through the physics concepts of each of these periods, but at an accelerated pace. This process generally begins with Aristotelian concepts, which must be replaced: first by understanding acceleration, then appreciating the concept of force as a vector quantity, learning the work-energy theorem, struggling with the simultaneous application of the laws of conservation of momentum and conservation of energy, and then on to the subtleties of the laws of thermodynamics. For those who do not fall by the wayside, the holy grails of quantum mechanics and relativity loom menacingly as the final hurdle toward becoming a physicist.
Mathematics teachers say that many students "lose it" in math instruction when fractions are introduced, usually in the 4th grade. Physics teachers know that many students "lose it" when vectors and forces are introduced. Those who progress beyond these hurdles often succeed (barely) by cramming, cribbing, and a "plug-and-chug" approach to problem solving, but without genuine understanding. They achieve only a fragile and superficial grasp of the subject, often just enough for low-level functionality in a technical field. They may not fully realize what they have missed.
2. A long history of failures.
This section is an aside, and may be omitted.Early civilizations had little motivation to seek perpetual motion, but they had no reason to doubt that such a thing might be possible in nature. They observed the heavens, and kept records of positions of stars and planets, which demonstrated clearly that these motions reliably repeated, and that this majestic dance of the planets never noticably "slowed down" and was likely eternal. The Greeks even supposed that things in the heavenly realm were not made of ordinary matter, but a perfected substance, the "quintessence". In the heavenly realm everything was perfect, so motions of celestial objects could continue forever. But in the earthly realm, that of the four elements earth, water, air and fire, things naturally seek, but never achieve, perfection. This would suggest, philosophically, that perpetual motion was not possible on Earth.
To the ancients, the notion of perpetual motion was quite distinct from the possibility of an inexhaustable source of energy in the form of useful work. These civilizations had little need to seek such machines for doing work. Natural energy resources such as wind and water power were sufficient for their needs, and animals and slaves provided the work required for commerce.
Still, some were intrigued by the possibility of achieving perpetual motion by mechanical means. The first documented proposals for perpetual motion machines originated in India, in about the 8th century. One, described by the Indian author Bhaskara (c. 1159), was a wheel with containers of mercury around its rim. As the wheel turned, the mercury moved within the containers in such a way that the wheel would always be heavier on one side of the axle. [Graphic by Hans-Peter Gramatke, used with permission.]
One can imagine that many persons built such devices, filling the containers with mercury, sand, water, or rolling balls. No doubt it seemed plausible that with a little nudge the wheel would turn, then turn faster and faster, driven by continual imbalance of weight.
But real wheels of this sort would only turn a bit, then stop. Given a faster initial speed they would turn longer, but always come to rest. Perhaps a little tinkering with the angle of the vials would help. Perhaps a larger number of vials would be better, or a larger wheel. No modifications helped. The perversity of the wheel must have been discouraging and disheartening, but for some, hope never dies. Perhaps it's all the fault of that pesky friction, they may have thought.
These ideas re-appear in Europe in the year 1235. The French architect Villard de Honnecort described an overbalanced wheel with hinged hammers equally spaced around its rim. This idea had also originated in India. [The picture displays ambiguous perspective. The wheel is actually supposed to be perpendicular to the frame and to the horizontal axle.] Villard's description (translated) is:
Many a time have skillful workmen tried to contrive a wheel that should turn of itself; here is a way to make such a one, by means of an uneven number of mallets, or by quicksilver (mercury).
The reference to quicksilver indicates that Villard was familiar with the Bhaskara device. Villard claimed his machine should be useful for sawing wood and raising weights. His diagram shows seven hammers, and he insisted on an odd (uneven) number of hammers, explaining
...there will always be four on the downward side of the wheel and only three on the upward side; thus the mallet or bag will always fall over to the left as it reaches the top, ad infinitum.One may speculate that part of the rationale for the number of hammers on this device is the simple observation that an odd number of weights can't be divided into two equal numbers, so one side of the wheel must always have one more weight than the other. But the extra weight is equally often on one side of the wheel as on the other, so this argument fails. But, whether the number of hammers is odd or even, such a wheel comes to rest very soon.
This "overbalanced wheel" idea reappeared in an astounding variety of forms over the centuries. We show a better diagram from a later time. A system of pegs or stops was required to hold the hammers at a large distance from the axle after they flipped over the top and allow them to hang freely as they came around the other side.
The laws of levers had been well known since the time of Archimedes. Perhaps the rationale was that the balls had larger lever arms on one side of the axle. However, this argument actually shows the futility of this device. Even though there are fewer balls on one side of the axle at any given position, these have larger lever arms. When one sums the products of mass and lever arm for all hammers, the result is zero. The system is balanced at seven positions if it has seven hammers.
Perhaps a different process tempted people to think the wheel would maintain motion. As a hammer swings and falls near the top of the wheel, the wheel might gain some speed when the hammer hits its peg. But the hammer slows the wheel during its fall, so the net gain in speed is zero.
We have mostly second-hand accounts of the understanding of the principles of this machine. However, I do not think that the folks who were fascinated with this idea were unaware of the static balance condition of the wheel. The wheel would not initiate motion from rest. You must give it a push. The dynamics of motion were not well understood. Perhaps the wheel could maintain motion once it was manually set in motion, with the hammers giving it extra boost as they rapidly flipped across the top, perhaps this was due to some "advantage" obtained from the motion of each weight flipping to a position with a larger lever arm. Things poorly understood seem mysterious. They tantalize restless minds, who imagine that wonderful secrets lurk in those murky realms, awaiting discovery by diligent seekers.
This flipping action is much like that of a sling that gives a person the ability to throw a rock a greater distance, or the sling siege engine catapult known as the Trebuchet. Honnecort wrote about these machines of war, describing one with an 8x12x12 foot box of sand as counterweight (which could weigh 80 tons). Some had arms 50 feet long and were capable of slinging a 300 pound stone 300 yards. This connection between the swinging hammers of his perpetual wheel and the swinging arm of a Trebuchet is my speculation, unsupported by historical evidence.
Even though the sling action of a Trebuchet allows a greater efficiency of energy conversion compared to the rigid-arm catapult, the machine still puts out no more energy than that of the falling weight that drives it. Modern Trebuchets (built by hobbyists) have achieved energy conversion efficiencies of greater than 65%.
Why is the analogy to the Trebuchet not valid? The Trebuchet completes only one swing, and that's the end of it. Then the machine must be reset, by doing a great deal of work to "cock" it. The Villard wheel is expected to complete many cycles without energy input.
In Villard's wheel, there's no net gain in speed, and there's irreversible energy loss when hammers hit pegs. If given a push, the wheel will turn jerkily for a while. If it were given a very forceful initial push to cause it to rotate at high speed, the hammers would assume positions at maximum radius and the wheel would turn much more smoothly and efficiently. It would then be essentially a flywheel and the hammers would be non-functional. But it would gradually lose speed and rotational energy because of air drag and bearing friction, just as any spinning wheel would.
The overbalanced wheel idea was re-invented many times over the centuries, sometimes in fantastically elaborate variations. None ever worked as their inventors intended. But hope never dies. I've seen examples made by country blacksmiths and basement tinkerers. The classical mechanics necessary to analyze mechanical systems is now well known, and when one takes the trouble to do this there's no mystery at all why they don't turn forever, and no reason why they should.
A reverse look at classical physics.
1. Energy.Modern textbooks sweep aside the tortuous history of our struggle to understand energy, momentum and force, and present these subjects in a neatly organized logical exposition of formulae and procedures for solving physics problems. Usually Newton's laws and the concept of force are introduced first, then kinetic and potential energy, work, and momentum. Students, however, often fail to grasp the underlying concepts and logical connections. Let's approach the subject a bit differently—not that there's anything wrong with the usual presentation. Consider this just a "review from a different perspective" for the person who has gone through the usual presentation and still has lingering questions. I will avoid mathematical derivations, for these can be found in standard textbooks. But, make no mistake, the math is essential for understanding. Words alone can only give a suggestion of the strength of the logical connections.
How many kinds of energy are there? It depends on how you define "kinds". My answer would be "Two—kinetic and potential." Kinetic energy is energy due to a body's motion, expressed as mv2/2. Potential energy is energy due to a system's configuration in space in situations where the components of the system can exert force on one another.
Already I hear some readers object. "What about heat? Nuclear energy? Psychic energy?" Let's take these one by one. Psychic energy is nothing but pseudoscientific moonshine, unworthy of further comment here. There's no evidence of its existence. "Heat" is a colloquial term for what physicists call "thermal energy". It is nothing more than the total disordered kinetic energy of the particles in a body, of molecules and atoms. Nuclear energy is a form of potential energy of the elementary particles that are scrunched into nuclei. And so it goes. Any energy you can name that has physical existence is either kinetic or potential energy, or a mixture of both.
We have long studied how bodies exchange energy one with another. Already we must be careful with language. The word "exchange" suggests transfering something material from one body to the other, like merchants exchanging money. That's misleading, for energy isn't a material substance. What we are talking about are situations where two bodies interact with one another in some way and one gains the same amount of energy as the other loses. We are avoiding being explicit about how that happens. The conventional view is that the intermediary of the process is force exerted by one body on the other. That force is responsible for an exchange of energy we call work. But are forces "real"? Is energy "real"? What we observe is that the behavior of each body is changed by the transaction. One or both bodies may be caused to change its velocity (a change in kinetic energy), or one or both bodies may be moved to somewhere else where their potential energy is different than it was before.
Observe the common feature of changes in energy. Whether you change the kinetic or potential energies of a body, in either case you move the body or change its motion. And when you use the language of force, you conclude that in order to change the energy of a body, the force that acts to make that change must move the body.
Kinetic energy is a good starting point, for the kinetic energy of a body can be easily measured by measuring the mass and speed of a body. Early on in ths history of machines the spring-scale was invented, which is a convenient way to measure force. When we crunch the numbers by calculating the amount the body's energy changes due to a force acting on it, we find that the change is the size of the force multiplied by the distance the body has been moved. We call this "the work done by the force on the body". There's a refinement that must be made in this when the direction of the force and the distance the body moves aren't parallel. The work must then be defined as the product of the force and distance multiplied by the cosine of the angle between the force and distance vectors. When the direction of a distance is important physically, we call it a displacement and treat it by vector mathematics.
Now that we have defined work in a useful way, we conclude that if the force and the body's displacement are at right angles to each other, that force does no work on the body. If the force acting on a body is zero, no work is done on or by it. And if the forces acting on a body produce no displacement, then no work has been done by them. Finally, if no work is done on a body, its energy does not change. These are confirmed by experiment.
Let's return to some everyday examples that everyone can experience.
Finally, we have noted that interacting bodies can result in changes in the energy of both bodies due to the forces they exert on each other. But material bodies can also interact in other ways, and one of the most important ways is by thermal interaction. When two bodies are in contact, those chaotically moving particles in each body interact where the bodies are in contact. These are force interactions, but this is a microscopic version of what we described above, so small that it has no observable effect on the motion of the bodies as a whole. But in this way one body can gain thermal energy and the other loses the same amount. The amount of energy exchanged is called "heat". So, in physics we recognize two ways bodies exchange energy, through work or through heat. We avoid using the term "heat" as something "in" or "posessed by" a body. Likewise we never speak of "work in a body". Both "heat" and "work" are measures of energy changes that result from force interactions.
For the sake of discussion we should distinguish two realms: microscopic and macroscopic phenomena. Microscopic physical phenomena are those at the size of molecules and smaller. Thermal energy is a microscopic phenomenon. Bulk motion of solid objects like machinery or planets is macroscopic.
There is a fundamental assymetry in all this. Because thermal energy is chaotic (disorganized) kinetic energy, it cannot be completely converted to macroscopic work. Some work must be done to convert some amount of heat to more work, and there are limits to how much can be converted. However, in the other direction, work may be entirely converted to thermal energy. This is quite reasonable when you consider that to convert thermal energy of a body at rest into motion of the body as a whole we must do work to redirect the chaotic motion of many particles with velocities in all possible directions so they move in the same direction.
How can energy be stored? The question is misleading, since energy isn't a material substance. Of course, we can collect and store materials that have energy, such as storing gasoline, which has chemical potential energy in its molecular structure. We can store thermal energy, say of heated water, part of which can be extracated by heat engines (subject to the limitations of thermodynamic laws). Matter itself represents stored energy, and in some cases some of that can be transfered to other forms through nuclear reactions.
But it is a mistake to assume, as some do, that anything that can exert a force represents a "source" of energy. We were cautious above to avoid treating "force" as anything more than a convenient way to express and quantify energy interactions between material bodies. I get questions about the possibility of "extracting energy from gravity". To a physicist, this is an absurd idea, but it's not easy to explain why to someone who doesn't deal with physics everyday.
Re-examine the examples above. In the book and table example, the potential energy of the book on the table was due to the fact that we did work on the book to increase its distance from the earth. The book's weight was due to the earth's gravitational force on the book. So when the book fell from the table, its kinetic energy increased and was finally converted to thermal energy. Where did that energy come from? Not from gravity, it came from the work someone did when lifting the book from the floor. The whole sequence of processes did not extract anything from gravity, and did not diminish the gravitational force due to the earth. It came from the person or machine doing the lifting.
No machine ever made, nor any natural process, has ever extracted any energy from gravity.
I have had people tell me that there's infinite energy in a magnetic field. As "proof" they cite the refrigerator magnet clinging to the vertical wall of the refrigerator and supporting its own weight against the pull of gravity, presumably forever. Some add that it will only fall when the "stored energy" in the magnet is exhausted. So, they say, the magnet must have an unlimmited amount of energy stored in it. These people are slow learners. The magnet, at rest on the refrigerator wall, is doing no work, and is expending no energy, for its force of attraction causes no motion of the magnet or the refrigerator. The magnet does have some stored energy (from the work required to magnetize it), but it isn't using up any of that. Nor is it stealing any energy from gravity. It simply isn't expending energy in any form. Consider a similar situation. Use glue to fasten a block of wood to the the refrigerator wall. No magnetism is involved. The block will stay there, supporting its own weight, indefinitely. Would the person who used the magnet example now claim that the glue has unlimited energy stored in it? What about a nail driven in the wall to hang a picture? Or a closet coat-hook? Folks who still use the refrigerator magnet argument as an argument for extracting unlimited energy from magnets are committing not only an error of physics, but an error of critical thought and a singular lack of common-sense.
No machine ever made, nor any natural process, has ever extracted more energy from a magnet than the small amount used to magnetise the magnet.
One reason we hear such outrageous misrepresentations of physics is that many people do not do the mathematics, not even the simple calculations of the amounts of work done in processes and the amounts of energy exchanged. A magnet in a permanent magnet motor can last for years of constant operation, and the slight reduction in its magnetic properties represents a tiny change in energy, miniscule compared to the amount of work done by the motor during its operation.
2. Potential energy.Let's look more carefully at the potential energy we mentioned. How do we measure it. We have, so far, assumed that we can measure mass (with any of several ancient measuring instruments) and force (with spring-scales). Velocity is measured with measuring sticks and stop-watches. (It was the difficulty of measuring short time intervals that delayed our understanding of motion in the early history of physics.) So we have the tools for measuring kinetic energies, and also for measuring work. With these we can do experiments to study how bodies exchange energies through force interactions.
The potential energy of a system is equal to the work required to assemble the system's componenets, working against the interactive forces of its components and any external forces acting on them. There are complications to measuring that, due to energy wasted in dissipative processes, producing thermal energy, sound, etc. but we needn't let that distract us from the important issues.
Suppose we stretch or compress the spring of a spring-scale. We must do work to accomplish that. After the string is compressed, we latch it in the compressed position. Where is the energy equal to the work we did? We say it is "potential energy, stored in the spring". We can verify that by unlatching the spring and letting it do work on something else as it expands. The potential energy of the compressed spring is just equal to the work we did against the elastic force of the spring as we compressed it.
When we lifted the book from floor to table, we say the book on the table had been given potential energy, and we can calculate that energy to be mgh where m is its mass, g is a constant and h is the vertical distance it was lifted. We can verify that by letting it fall and measuring its kinetic energy just before it hits the floor. The potential energy of the book on the table is just equal to the work we did against the gravitational force as we lifted the book from floor to table.
3. Gravity.We call this "gravitational potential energy". But the name can miselead us if we don't examine what it represents. So let's look more carefully at gravity.
Isaac Newton puzzled about the question "Why do bodies fall." The story has been told many times, but the bottom line is that he postulated that the earth exerts a force on bodies near it, and not only near, but this gravitational force extends even to the moon and beyond, though it diminishes in strength with distance from the earth, varying as 1/R2 where R is the distance from the earth's center. This was an idea that had been suspected by a number of other people at the time, but it was Newton who did the necessary mathematics to confirm that it did indeed agree with what we knew about falling bodies, and with the orbit of the moon. Yet the idea shocked many, for it proposed that there's a force "influence" that can act on bodies even at a distance, with no other evidence than the fact that it is affecting their observed motion. Gravity is not "observed", it is "inferred" from observations of motion of bodies. And gravity is not just a result of the earth, but, Newton said that any body with mass influences every other body with mass, in amount proportional to the product of their masses. The final equation stated that the force each body exerts on the other is of size F = GmM/R2 where m and M are the masses, G is the universal gravitational constant and R is the distance of the separation of the bodies.
We are saying that gravity is responsible for altering the observed motion of bodies in the same manner as when a two bodies are affected by forces due to bodies being in contact. Contact isn't necessary with gravity. While we easily accepted contact forces, being something we "feel" when we push or lift an object, we cannot feel the force that the earth and moon exert on each other. Yet in both cases we do not directly measure the force. The force is assumed, inferred, from the change of motion we observe. In one sense niether force is "real". In another sense, both are equally "real". We leave discussion of "what is really real" to another document. Real or not, we can measure the motion and calculate the sizes and directions of forces, and it all forms a consistent picture.
4. Force.One experimental fact emerges when we start measuring things precisely and accurately. When two bodies interact, and the process results in one of them gaining energy, the other one loses the same amount of energy. Such facts led physicists to the law of conservation of energy. Now this law applies whether the interaction is one where one body does work on the other, and also when one body heats the other. Both are exchanges due to force(s) acting through distance, whether at the macroscopic level, or the microscopic level (heat).
When a force and displacement are in the same direction, we say that force does positive work on the body it acts upon. When a force and displacement are in opposite directions, we say that force does negative work on the body it acts upon. In other words, if body A exerts a force F on B, and the force and displacement are in the same direction, then A does positive work on B. If body A exerts a force F on B, and the force and displacement are in the opposite direction, then A does negative work on B. This is the same as saying that if body A does work on body B then B does an equal size and opposite signed work on A. From this one can work back to Newton's third law: "If body A exerts a force on body B then body B exerts an equal and oppositely directed force on A."
So, by working backwards from the concept of energy, and energy conservation, we arrive at Newton's laws. Historically the order of discovery of these concepts was the opposite sequence. But whichever direction you follow the logic, the conservation of energy and Newton's laws are inextricably linked. If one is true, so is the other. And if either were untrue, both would be untrue. We have absolutely no evidence that either of these are faulty at the macroscopic level of classical physics.
Latest revision Dec, 2012.