Thermal Equilibrium

Thermal equilibrium is a state of balance in which the temperatures of two objects or systems are equal. The word “thermal” refers to heat, and “equilibrium” means balance. When an object or system is in thermal equilibrium, it has no net heat flow between itself and its surroundings. In other words, the object or system is not gaining or losing heat. It’s important to note that thermal equilibrium is different from mechanical equilibrium. In mechanical equilibrium, an object or system is not moving; in thermal equilibrium, it may be moving but there is no net exchange of heat between the object or system and its surroundings.

The most common type of thermal equilibrium is between a hot body and a cold body. When you put a hot cup of coffee on a table, for example, the cup of coffee will eventually reach thermal equilibrium with the surrounding air (assuming there’s no draft). The molecules in the coffee will transfer some of their kinetic energy (heat) to the molecules in the air until both sets of molecules have the same average kinetic energy. Once this happens, we say that the cup of coffee and the air have reached thermal equilibrium with each other.

There are three ways that two objects can achieve thermal equilibrium with each other: by conduction, by convection, and by radiation. Conduction occurs when there’s direct contact between two objects; heat always flows from the hotter object to the colder one until they reach thermal equality. Convection occurs when fluids (liquids or gases) move past each other; again, heat always flows from hotter areas to cooler ones until they reach equality. Radiation occurs when waves (including light waves) carry energy from one place to another; as with conduction and convection, heat always flows from warmer areas to cooler ones until they’re at equal temperature.

It’s also worth noting that thermal equilibrium isn’t just about temperature; it can also apply to pressure-related quantities like volume and density. For example, if you have two containers full of gas at different temperatures but identical volumes, then those gases are not in thermodynamic equilibrium with each other even though they’re at equal temperature. In order for them to be in thermodynamic equilibrium, they would need to have identical temperatures AND densities (or alternatively equal pressures).