## Pages

### PHYSICS DICTIONARY

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

Effective mass of electron
The mass of an electron inside crystal appears, in general, different from free electron mass and is referred to as effective mass. The effective mass can be greater or lesser than electron mass.

Electron Volt
The energy gained by electron when put under potential difference of 1 volt. It is equal to
1.6 x 10-19 J.

Elastic collision
It is the type of collision in which both the momentum & kinetic energy are conserved and colliding bodies do not undergo any deformation.

Electric current
It is the amount of charge flowing through a particular area in unit time

(or)

The rate of flow of electric charge.

Electromotive force
It is the work done on unit positive charge by source in taking it through low potential end to high potential end.

Entropy
Thermal property which remains constant during an adiabatic process

(or)

Measure of randomness or disorderliness of molecules.

Eddy current
When a metal body is moved in a magnetic field in such a way that the magnetic flux through it changes or is placed in a changing magnetic field, induced currents circulate throughout the body. They are called eddy currents.

Equilibrium of a rigid body
If a rigid body is subjected to a number of forces acting on it and has neither translatory motion nor rotatory motion, then it is said to be in equilibrium. Following conditions are satisfied in such a case:
a) The algebraic sum of forces acting on it is zero ( translational equilibrium)
b) The algebraic sum of moments of all the forces about any point is zero. (rotational equilibrium)

The ratio of angular dispersion between two colors to deviation of mean ray produced by the prism is called dispersion power of material of prism for those colors.

Elasticity
It is the property of a body by virtue of which it tends to regain its original size and shape after removal of deforming forces.

Efficiency of engine
The ratio of useful work obtained by the engine in one cycle to the heat absorbed from the source is called efficiency of engine.

Emissivity
The total amount of radiant energy emitted per second per unit area by a surface is called total emissivity of that surface.

Escape velocity
It is the minimum velocity with which a body should be projected from the surface of the planet so as to reach infinity.

Elastic fatigue
The state of temporary loss of elastic nature of a body due to repeated stress over a long time interval is called as elastic fatigue.

Equation of continuity
For any incompressible & non-viscous fluid flowing steadily, the product of its velocity and area of crossection at all points during its flow through a tube remain constant. Velocity of fluid is inversely proportional to area of crossection.

Ensemble
An ensemble is defined as a collection of large number of microscopically identical but essentially independent systems.

Electron gun
Arrangement to produce a fine beam of electrons accelerated to desired velocity.

Equal Apriori probability
A fundamental postulate of statistical mechanics is that a macroscopic system in equilibrium is equally likely to be in any of its accessible microscopic states satisfying macroscopic conditions of system.

(or)

The probability of finding the phase point for given system in any region of phase space is identical with that for any other region of equal extension or volume.

Ergodic hypothesis
The time average of some property of a system in equilibrium is same as instantaneous ensemble average.

Electromagnetic interactions
The type of interaction between two charged particles by exchange of photons is called as electromagnetic interaction. The strength of electromagnetic interaction is given by dimensionless fine structure constant α = 1/137 and is due to photon exchanges.

Elastic Deformation
It is type of deformation of substance in which stress and strain are proportional.

Equation of motion of rigid body
Relationship between torque applied to rigid body and angular acceleration of body is known as equation of motion of rigid body.

Exposure
It is defined as by ICRU (1980), as “the quotient of dQ by dm, where the value of dQ is absolute value of total charge of the ions of one sign produced in air when all the electrons liberated by photons in air of mass ‘dm’ are completely stopped in air.

Elastic deformation
The deformation in which stress and strain are proportional is called elastic. In such a deformation, when applied load is released, the substance returns to original shape.

External work
When the force is exerted by the system on its surroundings leading to displacement is called external work.