Thermal Conduction - Zeroth Law of Thermodynamics

Zeroth Law of Thermodynamics

One statement of the so-called zeroth law of thermodynamics is directly focused on the idea of conduction of heat. Bailyn (1994) writes that "... the zeroth law may be stated:

All diathermal walls are equivalent."

A diathermal wall is a connection of contiguity between two bodies that allows the passage of heat by conduction between them.

This statement of the 'zeroth law' belongs to an idealized theoretical discourse, and actual physical walls do not match its grandiloquent generality.

But with suitable restrictions, the statement has physical import. For example, the material of the wall must not suffer a phase transition, such as evaporation or fusion, at the temperature at which it has to conduct heat. But when only thermal equilibrium is being considered, and time is not urgent, so that the conductivity of the material does not matter too much, one suitable conductor of heat is as good as another. Conversely, another aspect of the zeroth law is that, subject again to suitable restrictions, a given diathermal wall is indifferent to the nature of the heat bath to which it is connected. For example the glass bulb of a thermometer will act as a diathermal wall whether exposed to a gas or to a liquid, provided they do not corrode it or melt it.

These indifferences are amongst the defining characteristics of heat transfer. In a sense they are symmetries of heat transfer.