What is Heat?

We define heat as a particular kind of nonwork interaction that involves only energy and
entropy transfers, and that is entirely distinguishable from work. The existence of heat
interactions is a consequence of the first and second laws of thermodynamics. The
requirement that heat be entirely distinguishable from work implies strict conditions on
the end states of the interacting systems, and guarantees a definite relation between such
states and the energy and entropy transfers. We illustrate these conditions by using
energy versus entropy graphs. Many experiences can be represented as heat interactions,
including the exchanges between two black bodies at temperatures that differ infinitesimally. We discuss the latter point in a companion paper at this conference.