Zeroth Law: If A is in thermal equilibrium with C, and B is in thermal equilibrium with C, what can be said about A and C?

• A. A and C are in thermal equilibrium with each other.
• B. A and C are not in thermal equilibrium.
• C. A has a higher temperature than C.
• D. B mediates the exchange only.

Answer: (A)

Thermal equilibrium means two systems in contact exchange no net heat and have the same temperature. If A is in thermal equilibrium with C, they share the same temperature and there is no net heat flow between them. That is exactly what it means for A and C to be in thermal equilibrium with each other. The Zeroth Law also tells us that if A is in equilibrium with C and B is in equilibrium with C, then A and B are in equilibrium with each other, reinforcing that equal temperature is a common property among all in contact. The other possibilities would require a temperature difference or an intermediary role that isn’t present here, so they don’t fit.