The Direction of Heat Interaction

In stating a unit of heat, we explained the standard procedure as including a temperature rise of 1 kg of water. It is found experimentally that whenever, as outcome of a heat interaction, the temperature of 1 kg of water falls from 15.5oC to 14.5oC, the magnitude of the heat communication is again 1 kcal. In order to differentiate among these two situations, a directional language is employed with an equivalent algebraic sign convention.

 When, in a heat interaction among two systems H and L, system H is at the higher temperature, we state that a “heat transfer” has occur from H to L. It should be observed that the words “transfer”, “to” and “from” are figurative. They are universally used since of their convenience, though there is actually no substance “hotness” that is being transferred in the logic in which water can be poured from a vessel into a glass.

The “transfer” metaphor obtains historically from the long-discredited caloric hypothesis. The nature of heat as an interaction identical to work might perhaps is more logically conveyed when we use phrases like “heat done on” a system or “heat done by” the other. Unluckily, such phrases conflict too much with general usage.