4.2 The First Law

There are many ways to write the First Law of Thermodynamics. All the correct ones are equivalent, although that isn't always obvious. One well-known statement of the First Law of Thermodynamics is:

Energy can neither be created nor destroyed.

The energy referred to is the internal energy of the system. This is what we'd normally think of as the ordinary energy of the system. But it is called the internal energy to exclude several other types of energy. For example, think of the system as a box. If the box is thrown into the air, the entire box and the material in it have some kinetic energy because of the motion of the entire box. This is quite separate from the energy of the system itself.

Another example is carrying a system up to the top of a high cliff. Up there the system has a great deal of potential energy up there because of its position. We don't want to include such energies in thermodynamics because they really have nothing to do with what goes on in the system. Rather, they deal with things external to the system. So we restrict ourselves to the inside, hence the name internal energy.

Internal energy is designated by the symbol U.