Equilibrium - The state in which the observable properties (chemical,mechanical, thermodynamic, etc.) no longer change with time.

Equilibrium of A Rigid Body - The state in which the vector sum of all the. external forces acting on the body is zero (translational equilibrium), or in which the algebraic sums of the torques about each of three mutually perpendicular axes are zero (rotational equilibrium).

Equilibrium, Radioactive - In a radioactive series: a quasi equilibrium condition in which the products decay approximately with the period of the parent. This occurs if the half-life of the parent is very much longer than that of any of the products and is known as secular equilibrium. If, however, the half-life of the parent is not much longer, the condition is known as transient equilibrium.

Equipartition of Energy - In a system consisting of a large number of particles, which is in equilibrium and can be treated classically: the principle, enunciated by. Boltzmann, that the mean kinetic energy per particle is the same for each degree of freedom and is equal to½ kT, where k is Boltzmann's constant and T the absolute temperature. The mean kinetic energy of each kind of particle is then ½kT where S is the number of degrees of freedom of one particle.

Equipotential Surface - (1) Electrical a surface on which every point is at the same potential and to which the lines of force are perpendicular. (2) Geodetic a surface on which the Earth's external potential is constant, and to which its resultant direction is perpendicular. The equipotential surface which coincides approximately with mean sea level is known as the geoid.

Erg - The CGS unit of work and energy. It is the work done (i.e. the energy transferred) when the point of application of 1 dyne is displaced by a distance of I cm in the direction of the force It is equal to 10-7 J exactly.

Erichsen Test - A cupping test to measure the ductility of metal strip or sheet.

Eriometer - A device for measuring the diameter of small particles or thin fibres by observing the diameter of the optical diffraction pattern which they produce.

Escape Probability - Of neutrons in a nuclear reactor: the proportion of neutrons produced which escape without being absorbed. Separate probabilities can be defined, for example, for thermal neutrons, or neutrons in the process of being slowed down; but the actual values assumed will depend on the particular approximation to neutron behaviour which is being used.

Escape Velocity - Of a body from a planet: the minimum initial velocity needed by the body to overcome the gravitational attraction of the planet and escape into space. For Earth it is 6.9 miles (11.1 km)/s. for the Moon 1.5 mites (2.4 km)/s and for the Sun 384 miles (618 km)/s.
Etch Pits - Depressions on the surface of a crystal which has been etched for a sufficiently only time. The shape of the pits is related to the symmetry of the crystal.

Eupathescope - A device for estimating the heat lost by an individual in an enclosure.

Evaporation - The escape from the surface of a liquid (or solid) of those molecules whose' energy of thermal agitation is greater than the work function of the surface.

Evaporimeter - A device for measuring the rate of evaporation.

Evershed Effect - Refers to the general motion of the gases in the penumbral regions of sunspots: on the basis of spectrographic evidence this motion is believed to be radially outwards from the sunspot centre.

Exchange - The postulated continuous exchange of charged particles between two similar quantum mechanical system. The interaction between two such systems gives rise to exchange energy and exchange forces.

Exchange Degeneracy - Exchange degeneracy is said to occur when the total energy of two systems in which exchange takes place is the same as if no exchange takes place is the same as if no exchange had occurred.

Exchange Integral - For two electrons one of two types of integral, of which the first (known as the Coulomb integral) represents the mutual electrostatic energy of two charge clouds $*$ and '1'*'1' where $ and 'I' are the two independent electronic wave functions. The second (the exchange integral proper) represents the mutual energy of two charge clouds $ * 'I' and $'1'*.

Excitation - The addition of energy to a system to bring it into an excited state.

Excitation Energy - For a given atomic or nuclear state the difference between the energy of the given state and that of the ground state. Also known as excitation potential.

Entrance Pupil - The image of the aperture stop or diaphragm of an optical system as seen from the object.

Entropy - A quantity which is characteristic of the thermodynamic state of a system, whose significance usually relates to changes in its value, rather than to the value itself. Various aspects may be considered: (1) In a reversible process the change in entropy, dS. is given by dQ/T, where Q is the quantity of heat taken in at absolute temperature T. For an irreversible process (e.g: a physical system which is left to itself and allowed to distribute its energy in its own way) the total entropy of the system always increases, and entropy may be regarded as a measure ,of the unavailability of energy. (2) The entropy of a system in a given state is related to the statistical probability of finding it in that state by the equation S = K. In W, where S is the entropy, k is Boltzmann's constant, and W is the probability in question.
Eotovs Torsion Balance - A torsion balance, used extensively for differential measurements in geo-gravitational surveying (e.g. for prospecting); in which two masses attached to a beam are spaced apart vertically as well as horizontally, permitting the measurement of the local distortion of the Earth's gravitational field arising from the presence of geological features such as hidden mineral deposits.

Ephemeris - A table giving the predicted astronomical coordinates of a body at specific times, based upon the theory of the body' motion,

Epicadmium Neutrons - Neutrons of kinetic energy greater than the cadmium cut-off energy.

Epitaxy - The growth of one crystal on the face of another in such a way that the orientations of the two crystals bear a definite relationship to each other.

Equalize - A reactive or dissipative network inserted in a communication system to overcome distortion.
Equation of State - An equation (sometimes known as a characteristic equation) relating such variables as pressure, volume and temperature, which describes the state of a solid liquid or gaseous system. The best known is perhaps the equation of state for an ideal gas pV = RT, where p is the pressure, V the volume per mole, R the gas constant per mole, and T the absolute temperature. Some of the well-known equations of state are given in the following entries.

Equation of State, Clausius - A modification to the vander waals equation which corrects the pressure-correction term for its variation with temperature. In this term V2 is replaced by T (V +c)2 , where V is the volume per mole, T the absolute temperature, and e a function of the constants. relating to intermolecular forces and finite molecular size.
Expansion - The increase of a given mass of substance in length, area or volume, as result of a change in physical conditions commonly a change in temperature. Explosion. A rapid increase of pressure accompanied by heat, light, sound, and mechanical shock, arising as a result of a sudden release of energy. The source of the energy may be chemical, electrical, mechanical, or nuclear.

Explosion Limits - Limits which specify the range of concentration over which a gas mixture can be caused to ignite explosively. They are usually expressed as the .volume percentage of the combustible gas. Thus, for hydrogen in air, the lower limit is 4.1 % and the upper 75%.

Exposure - (1) of X- or y-rays at a given place: a measure of the radiation that is based on its ability to produce ionization in air. The , unit is the rontgen. (2) In general the incidence of radiation on matter (living or inanimate), either by accident or intent.

Extension - Of a metal an abbreviation for extension to fracture". It is usually expressed as the percentage increase in length between gauge marks on specimen of standard dimensions when extended to fracture.

Extensive Variable - One which !s proportional to the amount of matter being considered. Thus, volume, entropy and total energy are typical extensive ,variables. An extensive variable is also known as extensive quantity or extensive parameter.