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# equilibrium

[ee-kwuh-lib-ree-uhm, ek-wuh-]
equilibrium, state of balance. When a body or a system is in equilibrium, there is no net tendency to change. In mechanics, equilibrium has to do with the forces acting on a body. When no force is acting to make a body move in a line, the body is in translational equilibrium; when no force is acting to make the body turn, the body is in rotational equilibrium. A body in equilibrium at rest is said to be in static equilibrium. However, a state of equilibrium does not mean that no forces act on the body, but only that the forces are balanced. For example, when a lever is being used to hold up a raised object, forces are being exerted downward on each end of the lever and upward on its fulcrum, but the upward and downward forces balance to maintain translational equilibrium, and the clockwise and counterclockwise moments of the forces on either end balance to maintain rotational equilibrium. The stability of a body is a measure of its ability to return to a position of equilibrium after being disturbed. It depends on the shape of the body and the location of its center of gravity (see center of mass). A body with a large flat base and a low center of gravity will be very stable, returning quickly to its position of equilibrium after being tipped. However, a body with a small base and high center of gravity will tend to topple if tipped and is thus less stable than the first body. A body balanced precariously on a point is in unstable equilibrium. Some bodies, such as a ball or a cone lying on its side, do not return to their original position of equilibrium when pushed, assuming instead a new position of equilibrium; these are said to be in neutral equilibrium. In thermodynamics, two bodies placed in contact with each other are said to be in thermal equilibrium when, after a sufficient length of time, their temperatures are equal. Chemical equilibrium refers to reversible chemical reactions in which the reactions involved are occurring in opposite directions at equal rates, so that no net change is observed.
equilibrium, chemical: see chemical equilibrium.

Condition in the course of a reversible chemical reaction in which no net change in the amounts of reactants and products occurs: Products are reverting to reactants at the same rate as reactants are forming products. For practical purposes, the reaction under those conditions is completed. Expressed in terms of the law of mass action, the reaction rate to form products is equal to the reaction rate to re-form reactants. The ratio of the reaction rate constants (i.e., of the amounts of reactants and products, each raised to the proper power), defines the equilibrium constant. Changing the conditions of temperature or pressure changes the reaction's equilibrium; a high temperature or pressure may be used to “push” a reaction that at ordinary conditions makes little product. See also H.-L. Le Châtelier.

Condition in which the net force acting on a particle is zero. A body in equilibrium experiences no acceleration and, unless disturbed by an outside force, will remain in equilibrium indefinitely. A stable equilibrium is one in which small, externally induced displacements from that state produce forces that tend to oppose the displacement and return the body to equilibrium. An unstable equilibrium is one in which the least departures produce forces tending to increase the displacement. A brick lying on the floor is in stable equilibrium, while a ball bearing balanced on a knife-edge is in unstable equilibrium.

Equilibrium is the condition of a system in which competing influences are balanced and it may refer to:

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