According to the zeroth law of thermodynamics, what happens when two systems of the same temperature are in thermal contact?

The zeroth law of thermodynamics establishes a foundational principle regarding thermal equilibrium. According to this law, if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. When two systems are at the same temperature and come into thermal contact, there will be no net heat flow between them. This means that if both systems are at the same temperature, they are already in a state of balance, and as a result, the energy transfer that typically occurs due to temperature differences will not happen. Thus, heat will not flow from one system to another since there is no temperature gradient to drive that flow. The concept of thermal equilibrium is crucial, as it underpins many principles in thermodynamics and helps define how different systems interact with one another in terms of heat transfer.