Heat reservoir

Not to be confused with Thermal bath (thermodynamics).

In thermodynamics, a heat reservoir, thermal reservoir, or heat bath is a system whose heat capacity is so large that when it is in thermal contact with some other system of interest its temperature remains effectively constant. The heat bath is effectively an infinite reservoir of energy and accessible quantum states at a given temperature. It is considered to be a constant temperature source. The temperature of the reservoir does not change irrespective of whether heat is added or extracted to or from it.

A reservoir is that part of the environment which can exchange heat with a system. It has sufficiently large heat capacity that its temperature is not affected by the quantity of heat transferred to or from it. A reservoir is thus characterized by a quasi-constant temperature. The changes that do take place in the thermal reservoir as heat enters or leaves are so slow and so small that processes within it are quasi-static. A reservoir which is at higher temperature and supplies heat is known as heat source. A reservoir at a lower temperature is known as a sink.

Under certain conditions (including low rates of heat transfer and localised systems) the atmosphere may be regarded as a heat bath.