Question

The zeroth law of thermodynamics for three systems A, B and C in contact demands that

• A. A and B are in thermal equilibrium
• B. B and C are in thermal equilibrium
• C. A and C are in thermal equilibrium
• D. A, B and C are in thermal equilibrium

Answer 1

Answer: (C)

A and C are in thermal equilibrium

Answer 2

Zeroth Law Statement
If system A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then A must be in thermal equilibrium with C.

• Thermal equilibrium implies equality of temperatures:

$$T_A = T_B \quad\text{and}\quad T_B = T_C$$

• By transitivity,

$$T_A = T_C$$

• Hence A and C are in thermal equilibrium.