PHYSICS DICTIONARY

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Einstein

It is unit of energy used in photo chemistry. One Einstein is energy per mole of photons carried by a beam of monochromatic light.

One Einstein= 3.99 x 10^-10 ν, Joules per mole; ν is frequency

Einstein’s Mass-Energy Relation

The total energy of a moving body is equal to relativistic mass times the square of speed of light. E=mc²; ‘m’ is mass of body in motion; m= m_o/sqrt(1-v²/c²); m₀ is rest mass of body; C is velocity of light.  

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.

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.

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.

Elastic Limit

When the strain is increased slightly beyond proportionally limit in a wire, the stress is not proportional to strain. However the wire exhibits elastic property. When deforming force is removed, wire regains natural wavelength. This is called elastic limit. 

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.

Elastomer

A polymeric material that may experience reversible elastic deformation.

Electret

The word is first coined by Oliver Heaviside in 1885. It is derived from words electricity (electr) and magnet (et). It is dielectric material that has a quasi permanent electric charge or dipole polarization.        

Electric Current

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

Electric Field

There exists a region around any charge, filled with imaginary lines of force. The electric field is defined as force acting on any test charge placed in that region.

                                                (or)

It is defined as torque exerted on test dipole placed in that region. 

Electric Potential

Potential energy which a unit charge would have, if brought to a specified point in space from some reference point.

Electric Susceptibility

It is proportional constant between microscopic property “polarization” and macroscopic property “electric field”.

Electrical Discharge

It results from creation of conducting path between two points of different electrical potential in the medium, in which the points are immersed. Usually the medium is gas. It occurs when potential difference is very high.

Electricfield Intensity

Physical quantity used to measure strength of electric field in the space around a charged object. It is defined as force per unit charge placed at a point in the field.  

Electrodeposition

It is also known as electroplating. It is the process of depositing material on to conducting surface from a solution containing ionic species, by passing electric current.

Electroluminescence

The phenomenon in which electric energy can be used to generate photoemission in a solid is known as electro luminescence. 

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Ductility

It is the mechanical property which is a measure of degree of plastic deformation that has been sustained at fracture. It is expressed quantitatively as either percent elongation or percent reduction in area from a tensile test.

Dulong & Petit’s Law

It is the law which states that the atomic heat capacity for all solids is same and is nearly equal to 25 Jmol^-1K^-1 and is independent of temperature. 

Dwarf Star

It is small and low mass star that emits relatively less amount of light.  Most dwarf stars, including sun, burn their hydrogen at a much slower rate than giant and super giant stars and are consequently less luminous and have longer life spans that those main sequence stars do.

Dynamics

The branch of classical mechanics that is concerned with effects of forces on motion of a body or system of bodies, especially of forces that do not originate within the system itself is called as dynamics.

Dynamo Meter

Device used for measuring the force or torque or power available from a rotating shaft.

Dynamo

 It is an electrical device which converts mechanical energy into electrical energy utilizing the phenomenon of electromagnetic induction.

Dyne

 It is unit of force specified in CGS system of units.

1 dyne = 10^-5 Newtons 

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Dissociation

It is phenomenon of infinite separation of atoms of a molecule.

Distance of Closest Approach

It is the distance from the nucleus of an atom at which the alpha particle comes to rest and its kinetic energy is totally converted into electrostatic potential energy.

Distance

It is the total length of actual path covered by a moving body.

Distortion

The image of a plane square like object placed perpendicular to the principal axis is not of the same geometrical shape as the object. This aberration is called as distortion and arises due to variation of magnification with lateral distance of an object point from the axis of lens.

Diathermanous

Substances which are transparent for thermal radiations are called the diathermanous.

Divergence

It represents the total amount of flux escaping an infitesimal volume at a point in vector field.

D-lines of Sodium

The bright doublet (line spectrum) which is responsible for yellow light from Sodium lamp is called as D-lines of sodium. The line emission spectrum of lamp consists of two wavelengths 5890 A^o & 5896A^o. The transition which gives rise to doublet is from 3P_3/2 & 3P_1/2 to 3S_1/2 levels. The doublet is due to splitting of 3P level into state with angular momentum 3/2 & 1/2 by magnetic energy of electron spin in presence of internal magnetic field caused by orbital motion of electron.  

Domain Theory

Theory in magnetism as per which any ferromagnetic or ferri-magnetic material that is at a temperature below curie temperature, is composed of small volume regions in which there is mutual alignment in the same direction of all magnetic dipole moments. Such a region is called a domain, and each one is magnetized to its saturation magnetization. Adjacent domains are separated by domain boundaries across which the direction of magnetization gradually changes. Normally, domains are macroscopic in size and for polycrystalline specimen; each grain may consist of more than a single domain. Thus, in a macroscopic piece of material, there will be large number of domains, and all may have different magnetization orientation. The magnitude of magnetization field for entire solid is vector sum of magnetizations of all domains, each domain contribution being weighted by its volume fraction.

Donor

It is an impurity atom when added to host crystal lattice (semiconductor), contributes an electron to conduction bond after formation of covalent bonds with four valence electrons.

Doping

The intentional addition of foreign atoms (acceptors/donors) into semiconductor with controlled concentrations. 

Doppler Effect (Light)

Change in frequency of light radiation when source or observer move with respect to each other.

Doppler Effect (Sound)

Apparent change in the pitch of sound due to relative motion between source and observer is called as Doppler effect.

Dose (Radiation)

Materials other than air exposed to ionizing radiation will differ in their rate of energy absorption. It is therefore necessary to have a standard for defining energy absorption by ionization in different materials. The quantity for this purpose is called absorbed dose ‘D’ and measures energy deposited by ionizing radiation per unit mass of material. The SI unit is Gray equal to absorption of     1 joule per Kg of material.

Dose Equivalent

The effect of a particular radiation on a biological system depends on absorbed dose ‘D’ and on quality factor ‘QF’ of radiation. Dose equivalent is product of

D & QF.

Dosimeter

Device which measures absorbed dose deposited in its sensitive volume by ionizing radiation.

Dosimetry

Subject which deals with measurement of absorbed dose or dose rate resulting from interaction of ionizing radiation with matter.

Dot Product

Dot product of two vectors is the product of the magnitudes of two vectors and cosine of angle between them.

Double Refraction

Some crystals have the property that the light rays incident on them splits into two refracted rays inside the crystal. The property is called double refraction.

Doublet

Combination of two simple lenses is called as doublet. 

Drift Tube

It is used in linear accelerator in which there is arrangement of dynodes at varying potential to drift electrons to higher velocities.

Drift Velocity

Velocity attained by charge carriers under influence of electric field. The velocity is directly proportional to electric field and inversely proportional to mass.

Dry Bulb Temperature

It is the temperature which is usually referred to as simply ambient air temperature.

Dry Cell

It is a galvanic electric cell in which the chemicals in the electrolyte are made into a paste such as ammonium chloride and manganese dioxide so that they cannot easily spill from their container. A chemical reaction within the battery creates an electrical charge that flows from inside to an outer circuit that is connected to an electrical device.

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Dimension

Dimension of a space or object is informally defined as minimum number of coordinates required to specify any point with in it. Dimension of physical quantity is combination of basic physical dimensions (usually mass, length, time, temperature etc) which describe it.

Dimensional Analysis

Analysis to find relation among physical quantities by using their dimensions.

Dimensional Equation

When the dimensional formula of physical quantity is expressed in the form of an equation, then such equation is known as dimensional equation. 

Dimensional Formula

An expression showing the powers to which the fundamental units are to be raised to obtain one unit of derived quantity is called as dimensional formula.

Diode

It is an electronic device that rectifies electric current i.e. current flow in one direction only.

Dipole Moment

The product of either charge in an electric dipole with the distance separating them.

Dipole Relaxation

When external alternating field is applied to some materials it leads to formation of permanent dipole moment, the phenomenon termed as dipole relaxation. 

Dipole(Electric)

A dipole is an entity in which equal positive and negative charges are separated by a small distance.

Dirac Constant

Constant used in quantum mechanics equal to Planck’s constant divided by 2π. It has a value of  1.05 x 10^-34 joules seconds.

Direct Band Gap Semiconductor

The semi conductor in which an electron in conduction band can fall to any empty state in valence band giving off the energy difference (energy gap) as a photon of light. 

Direct Current

Continuous electric current flowing in one direction without substantial variation in magnitude is called as direct current.

Directly Ionizing Radiation

Comprises charged particles (electrons, protons, charged particles, heavy ions) that deposit energy in the absorber through a direct one-step process involving Coulomb interactions between the directly ionizing charged particle and orbital electrons of the atoms in the absorber.

Discriminator

Discrimination in radiation detection circuits refers to the process of distinguishing between different types of radiation on the basis of pulse height. A discriminator circuit selects the minimum or maximum pulse height that is to be counted.

Disintegration Constant

See decay constant.

Disintegration

Physical process by which atomic nucleus emits particles or energetic rays, which make composition of nucleus to change.

Dislocation

A linear crystalline defect around which there is atomic misalignment.

Dispersion of Light

The phenomenon of splitting of light into different colors is called as dispersion. It arises due to variation of refractive index with wavelength.

Dispersion

Whenever the speed of wave depends on its frequency, supporting medium is called dispersive. The phenomenon of changing of shape by wave having range of frequencies in a medium is called as dispersion.

Dispersive Power of Grating

The rate of variation of angle of diffraction with the wavelength of light is called dispersive power of grating.

Dispersive Power of Prism

The rate of variation of the angle of deviation with the wavelength of light is called as dispersive power of prism.

Dispersive Power

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. 

Displacement

The shortest distance between the initial and final positions of a body.

                                                  or

Distance travelled by an object in a specified direction.

Displacement Current

Changing electric field leads to a current which flows as long as electric field is changing and produces magnetic field. The current is called as displacement current.