First thoughts about temperature and energy

Temperature and energy

Here is a simple situation. Put a hot object, say a pasty, in an insulated picnic box, cut off from the outside so that it is near enough isolated. There are no great surprises when you look later, the pasty is cooler, and the air inside the box is warmer. The pasty can no longer be used to warm your hands, but neither can the air inside the box. In some way the energy is less useful. Although appearing in the same store (thermal store of energy), it is shared out more evenly after the pasty has cooled. The same quantity of energy is present, but it is now spread more widely in the energy stores associated with the pasty and surrounding air.

There is a measure of how useful the energy is and that is the temperature of the object. The higher the temperature of an object, the greater is its potential usefulness. A large mass of stuff at a low temperature is of little use in warming things, whereas a smaller mass at a higher temperature is useful (even though both may represent the same total amount of energy in a thermal store). Most useful, of course, are objects of large mass at high temperatures. One spectacular example of this is the exploitation of the hot interior of the Earth in geothermal power stations.

As the pasty cools down, energy is shifted to a wider range of stores, and we say that the energy is dissipated (see episode 02). Dissipation always involves the move to less concentrated, less useful energy stores.

Energy in thermal stores

The same quantity of energy can appear in thermal energy stores in two ways. A large block of matter can be at a low temperature, or a small block of matter can be at a high temperature. The same amount of energy is shared out amongst a few particles or amongst many particles. Temperature is a measure that does not depend on how much of the stuff is present whereas energy is a measure that does. We'll come back to this in more detail in episode 05.