Q.What is meant by 'thermal equilibrium'?

Thermodynamics is the branch of physics describing the laws governing the interconversion of work and heat. A thermodynamic system is any finite part of the universe chosen for study, described by macroscopic parameters pressure $P$, volume $V$ and temperature $T$; everything outside it is the surroundings, separated from the system by a boundary. Systems fall into three types: an open system exchanges both matter and energy with its surroundings (e.g. a bucket of water open to the atmosphere); a closed system exchanges only energy, not matter (e.g. air molecules sealed in a room); an isolated system exchanges neither matter nor energy at all. A thermodynamic system can itself be a liquid, solid, gas or even radiation. Two bodies are …