Equilibrium

Equilibrium, the state of a system whose configuration or large-scale properties do not change over time. For example, if a hot penny is dropped into a cup of cold water, the system of the water and penny will reach equilibrium when both are at the same temperature. At that point, the large-scale properties of the system, namely the temperature of the water and the penny, will not change over time. As another example, in mechanics a system is at equilibrium if the net force acting on a body is equal to zero. In the case of a stationary body, the large-scale property of the position of the body will remain unchanged over time. Mechanical equilibrium can be of three kinds: stable, neutral, or unstable. If the forces are such that a body returns to its original position when displaced, as in the case of a stationary pendulum, the body is in stable equilibrium. If the forces are such that a body remains in its new position when displaced, as in the case of a sphere resting on a level surface, the body is in neutral equilibrium. If the forces are such that a body continues moving until it reaches a different position when displaced, as in the case of a stick balanced on end, the body is in unstable equilibrium (see Stability).

Although the configuration or large-scale properties of a system in equilibrium do not change over time, the small-scale, microscopic configuration of a system in equilibrium is not necessarily static. For example, suppose there is a reversible chemical reaction—in the forward reaction, molecules A and B react to form molecules C and D, and in the reverse reaction, molecules C and D react to form molecules A and B. Such a system is in chemical equilibrium when the rate of the forward reaction is the same as the rate of the reverse reaction; in that case, the large-scale properties of the system, namely the amount of molecules A, B, C, and D, will not change over time. However, on the microscopic level, the configuration of the system is not static, because forward and reverse reactions are always occurring. In general, the microscopic activities of a system in equilibrium balance so that the large-scale properties of the system appear constant.