Which of the following best describes the relationship between two systems in thermal equilibrium?

When two systems are in thermal equilibrium, this means they are at the same temperature, which results in no net heat exchange between them. This is the fundamental principle of thermal equilibrium, established through the zeroth law of thermodynamics, which states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

In the context of the options provided, A is incorrect because, if the systems were at different temperatures, they would be exchanging heat until equilibrium is reached. B is also not relevant, as thermal equilibrium is not determined by the forces exerted by the systems on each other; it is purely related to temperature. D, while describing a potential physical characteristic of the systems, does not capture the essence of thermal equilibrium, which specifically focuses on temperature. Thus, the most accurate description of the relationship between two systems in thermal equilibrium is that they have reached a state where temperature is uniform, which aligns with the idea represented by "None" in the choices.