Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

All Energy is Potential or Kinetic

In the same way that we found that a change in temperature is really only a change in kinetic energy at the atomic level, we now find that every other form of energy turns out to be a form of potential energy. Boiling, for instance, means knocking some of the atoms (or molecules) out of the liquid and into the space above, where they constitute a gas. There is a net attractive force between essentially any two atoms that are next to each other, which is why matter always prefers to be packed tightly in the solid or liquid state unless we supply enough potential energy to pull it apart into a gas. This explains why water stops getting hotter when it reaches the boiling point: the power being pumped into the water by your stove begins going into potential energy rather than kinetic energy.

All these energy transformations turn out at the atomic level to be changes in potential energy resulting from changes in the distances between atoms.

As shown in the figure above, every stored form of energy that we encounter in everyday life turns out to be a form of potential energy at the atomic level. The forces between atoms are electrical and magnetic in nature, so these are actually electrical and magnetic potential energies.

This figure looks similar to the previous ones, but the scale is a million times smaller. The little balls are the neutrons and protons that make up the tiny nucleus at the center of the uranium atom. When the nucleus splits (fissions), the potential energy change is partly electrical and partly a change in the potential energy derived from the force that holds atomic nuclei together (known as the strong nuclear force).

Even if we wish to include nuclear reactions in the picture, there still turn out to be only four fundamental types of energy:

  • kinetic energy (including heat)
  • gravitational potential energy
  • electrical and magnetic potential energy
  • nuclear potential energy

Astute students often ask me how light fits into this picture. This is a very good question, and in fact it could be argued that it is the basic question that led to Einstein's theory of relativity as well as the modern quantum picture of nature. Since these are topics for books 4, 5, and 6 of this series, we will have to be content with half an answer at this point. Essentially we may think of light energy as a form of kinetic energy, but one for which kinetic energy is not given by (1/2)mv2 but rather by some other equation. (We know that (1/2)mv2 would not make sense, because light has no mass, and furthermore, high-energy beams of light do not differ in speed from low-energy ones.)

Discussion Question

A A Referring back to the pictures at the beginning of the chapter, how do all these forms of energy fit into the shortened list of categories given above?

Last Update: 2009-06-21