Exactly What is Energy?


In elementary school many of us learned that “energy is the ability to do work,” and that it cannot be created or destroyed” (conservation of energy). But these memorized and parroted phrases are not always easy to apply to real situations. For example, suppose you had a hand-cranked or pedal-driven electric generation that was connected to a light bulb. Do you think it would be just as hard to turn the generator if the light bulb were unscrewed from its socket or replaced by one of lower wattage?. Most people (even engineering students) asked this question answer “yes”, and are often surprised to find on doing the experiment that the answer is no-the generator is easier to turn with the bulb removed or replaced by one of lower wattage. This of course must be the case by conservation of energy, since it is the mechanical energy of your turning the crack that is being converted into electrical energy, which is absent when the light bulb is unscrewed. Were the handle on the generator just as easy to turn regardless of whether a bulb is being lit or how brightly it glows, then it would be just as easy for generator to supply electric power to a city of a million people as one having only a thousand! Incidentally, you can probably forget about supplying your own power using a pedal-powered generator, since even an Avid Cyclist would only be able to supply at most a few percent of what the average American consumes.

Aside from misunderstanding what the law of energy conservation implies about specific situations, there are also some interesting and subtle complexities to the law itself. Richard Feynman was one of the great physicists of the twentieth century who made many important discoveries including the field of quantum electrodynamics, which he co-invented with Julian Schwinger. Feynman was both a very colorful person and a gifted teacher, who came up with novel ways to look at the world. He understood that the concept of energy and its conservation was more complex and abstract than many other physical quantities such as electric charge where the conservation law involves a single number-the net amount of charge. With energy, however, we have the problem that it comes in a wide variety of forms, including kinetic, potential, heat, light, electrical, magnetic, and nuclear, which can be converted into one another. To keep track of the net amount of energy and to recognize that it is conserved involves some more complicated “book keeping”, for example, knowing how many units of heat energy (calories) are equivalent to how many units of mechanical energy (Joules).


In presenting the concept of energy and the law of its conservation. Feynman made up a story of a little boy playing with 28 indestructible blocks (Feynman, 1985). Each day, the boy’s mother returns home and sees that there are in fact 28 blocks until 1 day she notices that only 27 are present. The observant mother notices one block lying in the backyard, and realizes that her son must have thrown it out the window. Clearly the number of blocks (like energy) is only “conserved” in a closed system, in which no blocks or energy enters or leaves. In the future she is more careful not to leave the window open. Another day when the mother returns, she finds only 25 blocks are present, and she concludes the missing three blocks must be hidden somewhere–but where ?.
The boy seeking to make his mother’s task harder does not allow her to open a box in which blocks might be hidden. However, the clever mother finds when she weight of one block, and she draws the obvious conclusion. The game between mother and child continues day after day, with the child finding more ingenious places to hide the blocks. One day, for example, he hides several under the dirty water in the sink, but the mother notices that the level of the water has risen by an amount equivalent to the volume of two blocks. Notice that the mother never sees any hidden blocks, but can infer how many are hidden in different places by making careful observations, and now that the windows are closed she always finds the total number to be conserved. If the mother is so inclined she might write her finding in terms of the equation for the “conservation of blocks”.

Number of visible blocks + Number hidden in box + Number hidden in sink + … = 28

Where each of the numbers of hidden blocks had to be inferred from careful measurements, and the three dots suggest any number of other possible hiding places.

Energy conservation is similar to the story with the blocks in that when you take into account all the forms of energy (all the block hiding places) the total amount works out to be a constant. But remember that in order to conclude that the number of blocks was conserved the mother needed to know exactly how much excess weight in the box, and how much rise in dishwater level, etc. Corresponded to one block. Exactly the same applies to energy conservation. If we want to see if energy is conserved in some process involving motion and heat we need to know exactly how many unit of heat (calories) are equivalent to each unit of mechanical energy (Joules). In fact, this was how the physicist James Prescott Joule proved that heat was a form of energy. Should we ever find a physical situation in which energy appears not to be conserved, there are only four possible conclusions.

Source: Ehrlich, R. 2013. Renewable Energy. London, New York: CRC Press.

*an article from Ms. Asnin, STEM Faculty Member