CDS 2016 PYQ with Answer — Two systems are said to be in thermal equilibrium if and only if:

2016 · Q14

Two systems are said to be in thermal equilibrium if and only if:

General KnowledgePhysics

Athere can be a heat flow between them even if they are at different temperatures
Bthere cannot be a heat flow between them even if they are at different temperatures
Cthere is no heat flow between them
Dtheir temperatures are slightly different

Explanation

Under the zeroth law of thermodynamics, two systems are in thermal equilibrium if and only if there is no net flow of heat between them when placed in thermal contact, which is equivalent to their being at the same temperature. Option (C) states exactly this condition. Options (A) and (B) wrongly allow or deny heat flow at different temperatures, and (D) is vague and incorrect. Hence the correct statement is option (C).