# equilibrium

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Related to Equillibrium: equilibrium constant

## 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 leverlever,
simple machine consisting of a bar supported at some stationary point along its length and used to overcome resistance at a second point by application of force at a third point. The stationary point of a lever is known as its fulcrum.
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 masscenter of mass,
the point at which all the mass of a body may be considered to be concentrated in analyzing its motion. The center of mass of a sphere of uniform density coincides with the center of the sphere.
). 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 thermodynamicsthermodynamics,
branch of science concerned with the nature of heat and its conversion to mechanical, electric, and chemical energy. Historically, it grew out of efforts to construct more efficient heat engines—devices for extracting useful work from expanding hot gases.
, 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 equilibriumchemical equilibrium,
state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system. For example, when hydrogen gas, H2, and iodine gas, I2
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

The state of a body in which the forces acting on it are equally balanced.

## equilibrium

see SOCIAL EQUILIBRIUM.
Collins Dictionary of Sociology, 3rd ed. © HarperCollins Publishers 2000
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Equilibrium

in thermodynamics, such a slow transition of a thermodynamic system from one equilibrium state to another that all intermediate states may be regarded as equilibrium states. It is characterized by a very slow (infinitely slow at the limit) variation of the thermodynamic parameters of state. Any equilibrium process is a reversible process, and, conversely, any reversible process is an equilibrium process.

## equilibrium

[‚ē·kwə′lib·rē·əm]
(chemistry)
(mechanics)
Condition in which a particle, or all the constituent particles of a body, are at rest or in unaccelerated motion in an inertial reference frame. Also known as static equilibrium.
(physics)
Condition in which no change occurs in the state of a system as long as its surroundings are unaltered.
(statistical mechanics)
Condition in which the distribution function of a system is time-independent.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## equilibrium

The state of being equally balanced; a state of a body in which the forces acting on it are equally balanced.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

## equilibrium

1. any unchanging condition or state of a body, system, etc., resulting from the balance or cancelling out of the influences or processes to which it is subjected
2. Physics a state of rest or uniform motion in which there is no resultant force on a body
3. Chem the condition existing when a chemical reaction and its reverse reaction take place at equal rates
4. Physics the condition of a system that has its total energy distributed among its component parts in the statistically most probable manner
5. Physiol a state of bodily balance, maintained primarily by special receptors in the inner ear
6. the economic condition in which there is neither excess demand nor excess supply in a market
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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