Work is force applied over distance. Is it also reasonable to say that work is (the same thing as) the transfer of energy?

When work is done, the equivalent energy is transferred. But if energy is transferred, such as by heating something, is that (or could that be) called work?

Also, if a black body is subjected to a stream of energy, which it absorbs and radiates away, is any work done? The black body, let's assume, remains at a constant temperature, but energy is transferred to it and then radiated away. Is any work done?

So what I'm really asking about is how the term "work" should be used.


Good question! Yes, work is a transfer of energy, but it is not the only way to transfer energy. There are actually two kinds (in common use):

  • Work ($W$) is the transfer of energy by a mechanical process. This is the application of a force over a distance.
  • Heat ($Q$) is the transfer of energy by a thermal process. This is a statistical redistribution of energy.

The first law of thermodynamics expresses the fact that these are the two ways energy can be transferred:

$$\Delta U = Q - W$$

which says that the change in internal energy of a system is equal to the heat added to the system minus the work that the system does on its surroundings.

  • $\begingroup$ I think that the realization that heat energy and mechanical work were the same "kind of thing" was one of the most important unifications that physics has ever achieved ! $\endgroup$ – twistor59 Jan 27 '12 at 7:49
  • $\begingroup$ So it is not correct to say that when energy is transferred by heating that work has been done on the object receiving the energy? At the molecular level, though, heat is mechanical, so perhaps it is correct usage to call the transfer of energy via heating work? $\endgroup$ – RussAbbott Jan 28 '12 at 8:40

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