For a system to be in equilibrium, it must satisfy two conditions:
Mechanical Equilibrium: In a state of mechanical equilibrium, the system's external forces are balanced or canceled out. This means that there is no net force acting on the system, or the sum of all the forces acting on the system is zero. In other words, the system is not experiencing any acceleration or movement. This condition ensures that the system is stable and not undergoing any changes in its position or motion.
Thermal Equilibrium: Thermal equilibrium refers to a state where there is no net flow of heat between the system and its surroundings. In thermal equilibrium, the temperatures of the system and its surroundings are equal. This condition ensures that there is no temperature difference that could drive the transfer of heat from one part of the system to another or between the system and its surroundings.
These two conditions, mechanical equilibrium and thermal equilibrium, are essential for a system to be in a state of equilibrium. It is important to note that equilibrium can exist at different scales, such as macroscopic equilibrium for large systems and microscopic equilibrium for individual particles within the system.