Question

What is ment by the expression "useful energy" and how does energy change as it is used?

Answer 1

here you go for some light reading:
http://en.wikipedia.org/wiki/Carnot_cycle
http://en.wikipedia.org/wiki/Carnot_heat_engine
http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/carnot.htm
http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/CarnotEngine.htm

Answer 2

Useful energy is energy that wants to go somewhere else. Because it wants to go somewhere else, you can provide a path for it to get there, and it will go, doing whatever work you want done along the path.

For example, if you have water in a pond at high altitude, the energy in the water would rather be elsewhere, as heat in the water or atmosphere. If you provide a path for that to happen, the energy will flow, doing work for you. For example, you can dig a ditch to a lower altitude and put a paddle wheel in it. The water will flow, and the potential energy in the water will turn into heat in the water at lower altitude, the surrounding air, and the paddle wheel. Meanwhile the paddle wheel will turn, doing something useful, like irrigate your crops or grind your grain.

What is an example of energy that DOESN'T want to go anywhere else? The most common example is heat energy -- the energy in the chaotic, crashing around and vibrating back and forth microscopic motions of atoms and molecules. This energy often has no desire to go anywhere else, so it is very difficult to make it do useful work. (You can only do so, typically, by connecting it to some material at a lower temperature -- with less heat energy -- so that the heat flows from the hot to the cold object, equalizing their temperatures.)

We make the distinction because it is not always the case that energy can be made to do work. Some energy is essentially useless, because it just sits there, and won't turn any paddlewheels and generators for us. A key idea is that it is the FLOW of energy that produces useful work, not the existence of energy itself. For example, the gravitational potential energy of the entire Earth is enormous. But it's completely useless to us, because it doesn't want to go anywhere. However, if we raise a rock above the surface of the Earth (or some water, et cetera), THIS gravitational energy is useful, because it wants to rejoin the Earth, and will flow to do so if we let it.

These ideas are made formal, with equations and things, in the field of thermodynamics.