Thermal equilibrium is a state in which two or more objects or systems are in contact with each other, and there is no net flow of heat between them. In other words, the temperatures of the objects or systems in thermal equilibrium are equal, and there is no temperature gradient between them.
Thermal equilibrium can be achieved through the process of heat transfer. Heat transfer occurs when there is a temperature difference between two objects, and it can take place through conduction, convection, or radiation. When two objects are brought into contact and heat transfer occurs between them, the temperature of the hotter object decreases while the temperature of the cooler object increases until they reach the same temperature. At this point, thermal equilibrium is established.
Thermal equilibrium is a reversible process. In a reversible process, the system can be returned to its initial state by reversing each step of the process without causing any changes in the surroundings. When two objects are in thermal equilibrium, there is no net transfer of heat between them, and if the contact is maintained, they will remain in thermal equilibrium indefinitely.
The reversibility of thermal equilibrium is based on the fact that heat naturally flows from a higher temperature object to a lower temperature object until they reach the same temperature. This process can be reversed by applying an external influence, such as heating or cooling, to restore the initial temperature difference and cause heat transfer in the opposite direction.
In contrast, an irreversible process would involve some form of irretrievable change, such as a net transfer of heat or energy without the possibility of returning to the initial state. An example of an irreversible process related to heat transfer is the flow of heat from a hot object to a cooler object until they reach thermal equilibrium. Once equilibrium is reached, it is not possible to reverse the process without external intervention.
In summary, thermal equilibrium is a state of equal temperature between two objects or systems in contact. It is a reversible process because it can be undone by reversing the steps of heat transfer, allowing the system to return to its initial state without any changes in the surroundings.