At this point it will be advantageous to say a word about the meaning of the term heat. In everyday life we sometimes loosely refer to the “heat energy in a body”, instead of using the term: “internal energy”. In physics, heat is defined as energy which is transferred from one place to another owing to a temperature difference between them. There are three main processes of heat transfer, namely, conduction, convection, and radiation, which we shall study in detail in chapter 17. In thermodynamics (the branch of physics concerned with the relation between heat and work), we have to deal with such things as engines in which internal energy changes take place in a substance both as a result of heat flow and also as a result of work being done by or on the substance.
To avoid confusion, special symbols U (internal energy), Q (heat), and W (work) are used. Thus, the internal energy of some steam, for example, is represented by U in joules. This internal energy can be changed by adding or subtracting Q in joules of heat. The internal energy of steam can be increased by compressing it, i.e., doing W in joules of work on it, or the steam can be allowed to expand and transfer some of its internal energy into W in joules of useful work in, say, driving a turbine. In a case such as this it will be realized that confusion will certainly follow if we talk of the “heat energy” in a substance being transferred to work when actually we mean the transfer of internal energy to work.