We talk of heat as energy in the process of being transferred. Note, heat is not stored within matter, but rather heat is either “done on” or “done by” matter. Heat is a way of transferring energy across the boundaries of a system. It should be noted that heat is not a conserved substance, as was thought in the past (a remnant of the caloric theory of heat). Also, it is not a fluid as one might envision when the phrase heat flow is used, nor is heat a property of matter. Thus terms such as “heat of a substance” are meaningless. Since heat is neither a property of a system nor contained in a system, we speak of heat as a mode of energy transfer accompanied by a net amount of entropy transfer uniquely specified by the energy transfer as well as the temperature at which it occurs.

We may transfer heat by three different modes: conduction, convection, and radiation. Since each mode is subject to different laws, experiments such as those contained herein are necessary in order to understand the physical aspects involved in a heat-transfer problem.