Heat, Quantity of

the amount of energy received or given off by a system in a heat transfer process—that is, in an energy transfer process where the external parameters of the system, such as volume, are not changed. Together with work done, the quantity of heat is a measure of the change in the internal energy V of a system. When heat is transferred to or from a system, the internal energy of the system changes as a result of direct interactions (collisions) between the molecules of the system and the molecules of the surrounding systems.

The quantity of heat is only one of the components of the total change in U in a physical process. Unlike U, which is a single-valued function of the state parameters, the quantity of heat cannot be represented in the form of a difference in the values of some function of the state parameters. Consequently, an infinitesimal quantity of heat, which corresponds to an infinitesimal change in the state of a system, cannot in general be the differential of any function of the state parameters. The quantity of heat Q transferred to a system, like the work A, depends on the means by which the system passes from its initial state to its final state.

In reversible processes, according to the second law of thermodynamics, δQ = T dS, where T is the absolute temperature of the system and ¿5 is the change in the system’s entropy. Thus, the transfer of heat to a system is equivalent to the transfer to the system of a definite amount of entropy. The removal of heat from a system is equivalent to a decrease in entropy. In the general case of irreversible processes, δQ ≤ T dS.