Heat exchange after thermal equilibrium

[vcsharp2003]

TL;DR Summary

Can there be any heat exchange between two systems that are in thermal equilibrium?

In screenshot below, systems A and B are separated by an adiabatic wall initially while each of them exchanges energy with system C via a diathermic wall. Once A and B reach thermal equilibrium with C, then A,B are allowed energy exchange via a diathermic wall, and energy exchange between A and C as well as B and C is prevented by using an adiabatic wall.

To my knowledge systems that are in thermal equilibrium should not exchange heat energy. Is this true or they could exchange heat depending on circumstances? May be expandable ideal gas systems at different pressures and joined by a common diathermic movable piston could exchange heat as the piston moves from high pressure side moves towards the lower pressure side.

 

[DrClaude]

The 0th law of thermodynamics states that if A is in thermal equilibrium with C and B is in thermal equilibrium with C, then A is in thermal equilibrium with B. My guess is that the figure is used to illustrate this principle.

Note that while there is no net exchange of heat energy between two systems in equilibrium (2nd law), there are energy fluctuations (A and B will exchange energy back and forth, and this averages to 0). For a large enough system (or in the thermodynamic limit), these fluctuations are too small to be measured.

 

[vcsharp2003]

My guess is that the figure is used to illustrate this principle.

Yes, this was a diagram used to explain Zeroth Law of Thermodynamics in the textbook.

Thankyou for the detailed answer. It's clear to me now.