#Thermal equilibrium

If two bodies at two different temperatures are kept in thermal contact, flow of heat start from a body at higher temperature to another body at lower temperature and it continues till their temperature becomes equal and the net flow of heat from one body to another body is equal. Thus, a state of equality of temperature is reached and this state is called thermal equilibrium state.

Thermal equilibrium is the condition of the body in which the heat transfers from one to another and attain the sams temperature.

It is obtained when touching objects with a system to reach the same temperature.

#Zeroth law of thermodynamics

Zeroth law states that if two system separately in thermal equilibrium with a third system, then they must be in thermal equilibrium with each other.

Explanation

Let us consider three thermodynamical system A, B, and C in an isulated system in which A and B are separated from each other by an adiablatic wall (insulating wall) and system A is connected to C and C is connected to B.

Now, the system A and C should be at same temperature due to flow of heat between them. Similarly, system B will be in thermal equilibrium with system C. If the insulating wall is replaced by conducting wall then there will be no change in temperature. They are also in thermal equilibrium. Finally, it is concluded that if two bodies are in equilibrium with the same third body then the first two bodies must be in thermal equilibrium despite there is no change in temperature between them.