Question

Which of the following is an implication of the zeroth law of thermodynamics?
A) Energy in the universe can neither be created nor be destroyed.
B) A pure crystal kept at $ - 273.15^\circ C$ has very low entropy.
C) If systems A and B are in thermal equilibrium with a system C, then systems A, B and C are in thermal equilibrium with each other.
D) Entropy of the universe is always increasing.

Answer 1

Hint: The zeroth law of thermodynamics states that systems in thermal equilibrium are at the same temperature. It implies that a temperature of a system is a certain quantity worth measuring. This is because it indicates whether heat will transfer between two systems.

Complete step by step answer:
Energy in the universe can neither be created nor be destroyed is the essence of the law of conservation of energy. The law of conservation of energy states that energy of a system only changes from one form to another. Thus we can conclusively say that (A) is incorrect as it is not an implication of the zeroth law of thermodynamics but rather an implication of the law of conservation of energy.

Similarly options (B) and (D) will also be incorrect as they are implications of the second law of thermodynamics but not the zeroth law of thermodynamics.

We consider two systems named A and B which are separated by walls such that no transfer of heat takes place between them. We assume that both the systems are separately in thermal equilibrium with a third system C. So, according to the zeroth law of thermodynamics all the three systems A, B and C will be in thermal equilibrium with each other.
Thus option (C) is correct.

Note: Two or more systems are said to be in thermal equilibrium if they do not transfer heat energy among themselves even though they are in a position to do so, based on other factors.