Heat and temperature
One important aspect of students’ growing understanding of energy ideas involves sorting out the ideas of heat and temperature (hotness or coldness).
Kinetic theory describes the thermal energy of an object as being due to the random motion of its molecules. If you give more energy to be shared out amongst the atoms and molecules of some piece of matter, it usually gets hotter. But ‘hotness’ is not energy. Something hot (like the surface of the Sun, or a flame in a gas cooker, rather easily gives up energy to cooler things (energy goes without help from hotter to cooler).
What counts is the average energy per particle, not the total energy stored. So hot objects have, as it were, very concentrated energy that easily spreads out and dilutes, warming other things. This is what lies behind talk about “heat is a form of energy”. It is best to refer, as soon as possible, to the sharing out of movement energy amongst all the particles.
Students will learn that all energy transfers involve some losses through energy dissipation. They deserve (and will understand) an explanation of how this happens, and not simply a dramatic conclusion about the Universe warming in some mysterious way.
In describing ways in which energy goes from one place to another, physicists distinguish between ‘heating’ and 'working’. Heating is the process whereby energy moves from one object to another, which is in contact with it, as a result of their temperature difference.
Compressing and expanding gases
If a gas is compressed by pushing a piston quickly into a cylinder, the gas grows hotter - all the energy transferred to the gas goes into thermal energy of molecular motion. If the gas then cools back to the original temperature, it transfers thermal energy to the surroundings until they reach the same temperature. This will make its pressure fall slightly too, but still the pressure will remain higher than it was before compression.
The compressed gas, back at room temperature, can still transfer energy to other things by pushing the piston out. But the energy which it now supplies will be taken from the gas by cooling it down below room temperature.