In thermodynamics, a thermostat is a system closed of constant temperature , likely to be used to carry out heat transfers with a body placed at its contact.

Properties

That is to say (S) a thermostat of constant volume at the temperature $T\_0$. Let us place this thermostat in thermal contact with a system of test $(\backslash \; Sigma)$, and suppose that the thermostat receives the elementary heat transfer $\backslash \; Q$ delta during the infinitesimal transformation.

Energy interns U thermostat varies then:

in accordance with the first principle. The volume of the constant thermostat being supposed, one has in addition:

where $C\_V$ is the heat capacity with constant volume of the thermostat. One deduces from these two formulas that the temperature variation of the thermostat is connected to the elementary heat transfer by:

However, for a thermostat, it is necessary that this temperature variation is identically null for any finished elementary heat transfer. One from of deduced that the heat capacity with constant volume of the thermostat must be infinite:

As this heat capacity with constant volume is extensive, one deduced that the size of the thermostat must be infinite with the direction where the number of moles N is infinite:

In practice, the molar heat capacities of all the bodies being of the same order of magnitude, a system (S) could be regarded as a thermostat compared to a system of test $(\backslash \; Sigma)$ if and only if the number of moles of the system (S) is much higher than the number of moles of the system $(\backslash \; Sigma)$:

Related articles

• Thermodynamic

• Physical statistics
• Thermostat