PHYSICS DICTIONARY

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Ice Point

The temperature, equal to 0 ^oC, at which pure water and ice are in equilibrium in a mixture at 1 atmosphere of pressure is called as ice point. Ice point is most important fixed point for defining temperature scales and for calibrating thermometers.

Ideal Fluid

Fluid is called as ideal fluid if it has zero viscosity, zero compressibility and its flow should be irrotational and steady.

Ideal Gas

It is the gas which perfectly obeys Boyle’s law and Charles law.

Identical Particles

If two particles are identical, no observable effects whatever can arise from interchanging them.  More precisely, all observable quantities must remain unaltered if the position, momentum and other dynamical variables such as spin of first particle are interchanged with those of second particle. 

Illumination

Process of focusing light on to something to make it visible or bright is called as illumination.

Image

It is an optically formed duplicate counterpart or other representative reproduction of an object, especially an optical reproduction formed by lens or mirror.

                                                 or

Optical appearance or counterpart produced by light from an object reflected in mirror refracted through lens.   

Imaginary Number

It is mathematical quantity of the form iX, where ‘X’ is real number. ‘I’ is unit imaginary number and   i² = -1.

Impedance

Characteristic of alternating current circuit made of two components: “resistance” and “reactance”,  which is a measure of total opposition to current flow. It is usually represented as Z= R+iX, where ‘R’ is ohmic resistance and ‘X’ is reactance.

Impulse

It is defined as change in momentum when large force acts for short time interval.

Impurity Diffusion

Process, where by atoms of one metal diffuse into another is termed as impurity diffusion.

 

Incandescence

Property of hot body which emits visible electromagnetic radiation by virtue of its temperature.

Independent Variable

Variable who values are independent of changes in the values of other variables. 

                                                          Or

Variable whose value determines the value of other variables.

Indirect Band Semiconductor

The semiconductor in which an electron in  conduction band (minimum) cannot fall to valence band maximum but must undergo change in momentum, which may be caused by traps in energy gap. Generally, the energy difference is given up as heat to lattice.

Indirectly Ionizing Radiation

It Comprises neutral particles (such as photons, X-rays, neutrons) that deposit energy in the absorber through a two-step process as follows: In the first step a charged particle is released in the absorber (photons release either electrons or electron/positron pairs; neutrons release protons or heavier ions). In the second step, the released charged particles deposit energy to the absorber through direct Coulomb interactions with orbital electrons of the atoms in the absorber.

Induced Charge

When a charged object is put close to a neutral object, charges are induced on the surface of the neutral object and are called induced charges.

Inductance

The energy density is proportional to the square of the magnetic field strength, which is in turn proportional to the current flowing through the coiled wire, so the energy stored in the inductor must be proportional to square of the current. The constant of proportionality is called as inductance. The unit is joules per ampere squared, abbreviated as Henry.

Induction

Concept discovered by Faraday in 1831. Any electric field that changes over time will produce magnetic field in space around it                                              

                                                               or

any magnetic field that changes over time will produce an electric field in the space around it.

Induction Coil

The induction coil was invented in the late nineteenth century as a source of high voltage for laboratory experiments. Induction coil is a device for converting low-voltage direct current (DC) into high-voltage alternating current (AC). It is a single coil of conductive material, often surrounding a metallic core, designed to establish a strong magnetic field around the coil. Changes in the current flow through the coil cause fluctuations in the magnetic field that induce a voltage across the coil. Induction coils are used for many purposes, especially as spark coils for firing spark plugs in automobile engines and starting oil burners.

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Galaxy

A collection of stars, gas and dust bound together by gravity. The Milky Way galaxy contains our solar system.

Galilean Telescope

Galileo in 1906, constructed a telescope which provides an erect image of an object only with help of two lenses. It consists of converging lens serving as objective and diverging lens as eyepiece. The eye piece is situated in front of focal point of objective, at a distance from focal point equal to focal length of eyepiece. It has disadvantage of narrow field of view.

Galilean Transformation

The consequences of research work of Galileo on the motion of projectile led him to formulate Galilean transformations. These are used to relate the motions which are observed by two observers in two different observers in two different inertial frames. As per Galilean transformation, the motion of a particle projected at any angle may be derived from the motion of particle thrown vertically upward.

Galvanic Cell

Device used to convert chemical energy into electrical energy. Galvanic cell can be used as a source of electric current due to spontaneous reaction occurring within the cell.

Galvanometer

Instrument used to indicate presence, direction or strength of small electrical current. Current flowing through the galvanometer passes through a coil near a magnetized needle on a pivot; the strength of current in coil regulates the strength of magnetic field that displaces the needle. Galvanometers can be used directly as ammeters and are core elements of many ohmmeters.

Gamma Ray Spectrometer

Instrument used for quantitative study of energy spectra of gamma ray sources. It reads out distribution of intensity of gamma rays with energy thus revealing nature of radioactive nuclides, amount of activity etc.  

Gamma Rays

Electromagnetic radiation emitted from nucleus as a result of transition of nucleons between energy states inside nucleus.

                                                      or

Electromagnetic radiation emitted in annihilation reaction between matter and antimatter.

Gamma Spectroscopy

It is a radio chemistry measurement method which determines the energy and count rate of gamma rays emitted by radioactive substances.

Gas Amplification

Multiplication of charge carriers in gas filled detectors produced by primary ionization in high E/P value, by secondary ionization. The gas amplification is characterized by first Townsend coefficient. It represents the average number of electron – ion pairs formed per unit length due to motion of an electron along the electric field direction.

Gas Constant

See universal gas constant.

Gas Degeneracy

The deviations from perfect gas behavior exhibited by Bose Einstein gas is termed as gas degeneracy.

Gas Multiplication

Term related to gas filled radiation detectors. Phenomenon of multiplying charge carriers created due to primary ionization of gas by incident radiation in the gas. The charge carriers i.e. electrons drift towards the anode at positive potential. The electrons gain momentum and get accelerated to high energies which can transfer to other atoms resulting in emission of further electrons called secondary electrons. The phenomenon of multiplication of charge in gas under influence of external high voltage is called gas multiplication.    

Gas

The gaseous state above critical temperature of substance is called as gas.

Gauge Pressure

When pressure is measured either above or below atmospheric pressure as an arbitrary datum then it is called as gauge pressure.

Gauss Eye Piece

It is similar to Ramsden’s eyepiece with the only difference that there is a thin glass plate inclined at 45^o in between the two lenses. Hence cardinal points of gauss eye piece are located at some positions as that of Ramsden’s eye piece.

Gauss’s Theorem of Electrostatics

The net outward electric flux through any closed surface of any shape in an electric field is equal to 1/ε_o times the total charge contained within that surface. 

Gauss’s Theorem of Magnetism

The total magnetic flux through any closed surface is always zero.

Gaussian Distribution

If the number of events is very large, then the Gaussian distribution function may be used to describe physical events. The Gaussian distribution is a continuous function which approximates the exact binomial distribution of events. The Gaussian distribution is also commonly called the "normal distribution" and is often described as a "bell-shaped curve".

It is a distribution of the values of a variable that when plotted, produces a symmetrical, bell-shaped curve that rises smoothly from a small number of cases at each extreme to a large number of cases in the middle.

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Fabry Perot Interferometer

An interferometer made of a transparent plate with two reflecting surfaces or two parallel highly reflecting mirrors. It is named after Charles Fabry & Alfred Perot. It makes use of multiple reflections between two closely spaced partially silvered surfaces. Part of light is transmitted each time the light reaches second surface, resulting in multiple offset beams which can interfere with each other. The large number of interfering rays produces an interferometer with extremely high resolution, somewhat like multiple slits of all diffraction grating increase its resolution.

Face Centered Cubic structure(FCC)

The crystal structure found for many metals has a unit cell of cubic geometry, with atoms located at each of the corners and the centers of all cubic faces.

Fahrenheit Scale

Temperature scale introduced by German physicist Daniel Gabriel Fahrenheit in 1724. On this scale, the freezing point of water is 32 degree Fahrenheit and boiling point 212 degree Fahrenheit at standard atmospheric pressure. This puts boiling and freezing points of water exactly 180^o apart. Therefore a degree on Fahrenheit scale is 1/180 of interval between freezing and boiling point. A temperature interval of 1^oF is equal to an interval of 5/9 ^oC.  

Farad

It is SI unit for capacitance. It is named after English Physicist Michael Faraday. A capacitor is said to have one Farad value when one coulomb of stored charge causes a potential difference of 1 Volt across its terminals.

1Farad = 1 Coulomb / Volt.

Faraday Effect

The rotation of plane of polarization of a light beam when plane polarized light is passed through a substance in a magnetic field; the direction of travelling of light is parallel to magnetic lines of force. For a given substance, the rotation is proportional to thickness traversed by light and to the magnetic field strength.

Faraday’s Constant

The total charge carried by 1 mole of electrons is given by   96487 Cmol^-1. The quantity is known as Faraday constant.

Faraday’s Law of Electrolysis

The quantitative relationships between electricity and chemical change where first described by Michael faraday in 1830. These are

i) The mass of a chemical substance involved at an electrode is directly proportional to quantity of electricity passed through the cell.

ii)The mass of different substances produced by a given quantity of electricity are proportional to equivalent mass of substances.    

Mathematical representation

a) m α Q     b)  m α M/v_e

where ‘m’ represents mass of substance deposited at cathode; ‘M’ represents molar mass of substance; v_e denotes stoichometric number of electrons in reaction involving reduction/oxidation of chemical species at respective electrode of electrolytic cell.

Faraday’s law of Electromagnetic Induction

 It states that induced emf in a closed circuit is equal to negative rate at which magnetic flux through it changes.

Fast Neutrons

Neutrons possessing high kinetic energies in the million electron volt range are called fast neutrons.

Fathom(ftm)

It is a unit of length used especially for measuring depth of water. 1 ftm = 1.8288 meters. 

Fault

Places where rock structures have broken apart and can slide past each other are called faults.

Feedback

The process of injecting a portion of output energy of some device to the input is known as feedback.

Femto

It is a prefix in the metric system denoting a factor of 10^-15.

Fermat’s Principle of Extremum Path

Fermat, in 1658, postulated that “a ray of light in passing from one point to another by any number of reflections or refractions chooses a path along which the time taken is least or extremum.                                                                                                      

Fermi Dirac Statistics

Statistics proposed by Fermi & Dirac. The postulates are as follows:   

i) Applicable to identical & indistinguishable particles with spin odd multiple of half, such as electrons, protons, neutrons etc, which are called the Fermions;

ii) The minimum volume of a phase cell is h³, so the number of phase cells cannot be increased.

iii) No new microstate is obtained by the interchange of position of particles in a cell.

iv)There cannot be more than one particle in a cell i.e. Pauli’s exclusion principle is obeyed.  

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Earth

One of the nine planets revolving around sun and is third planet orbiting from sun. The distance of orbit from sun is 149,600,000Km and has diameter of 12,756.3Kms. It is the fifth largest of the planets. The Mass is about 5.97x10²⁴ Kg

Echo

Repetition of sound produced due to reflection by a distant extended surface like cliff, hill, well, building etc., is called an echo. The effect of sound on human ear remains 1/10^th of second. If sound reflects back in a time less than 1/10^th of second, no echo is heard.

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 current circulates throughout the body.  They are called eddy currents.

Edge Dislocation

A linear crystalline defect associated with the lattice distortion produced in the vicinity of the end of an extra half plane of atoms within a crystal. The burgers vector is perpendicular to the dislocation.

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.

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.

Effusion

Effusion is the process in which individual molecules flow through a hole without collisions between molecules. This occurs if diameter of the hole is considerably smaller than the mean path of molecules. According to Graham’s law, rate at which gas effuse is dependent on their molecular weight; gases with lower molecular weight effuse more quickly.

Eigen Function

It is wave function for a given physical system which contains measurable information about system. 

Eigen Value

The word is derived from German word “Eigenwert” which means characteristic value. A scalar associated with a given linear transformation of vector space and having the property that there is some non zero vector which when multiplied by scalar is equal to vector obtained by letting the transformation operate on the vector.

 

Einstein Shift

In astrophysics, gravitational red shift or Einstein shift is the process by which electromagnetic radiation originating from a source that is in gravitational field is reduced in frequency, or red shifted, when observed in a region of a weaker gravitational field. This is a direct result of Gravitational time dilation - as one moves away from a source of gravitational field, the rate at which time passes is increased relative to the case when one is near the source. As frequency is inverse of time (specifically, time required for completing one wave oscillation), frequency of the electromagnetic radiation is reduced in an area of a higher gravitational potential (i.e., equivalently, of lower gravitational field). There is a corresponding reduction in energy when electromagnetic radiation is red-shifted, as given by Planck's relation, due to the electromagnetic radiation propagating in opposition to the gravitational gradient.

Einstein Theory of Specific Heats

A theory of the specific heat capacity of solids put forward by Albert Einstein in 1906, in which it was assumed that the specific heat capacity is a consequence of the vibrations of the atoms of the lattice of the solid. Einstein assumed that each atom has the same frequency. The theory leads to the correct conclusion that the specific heat of solids tends to zero as the temperature goes to absolute zero, but does not give a correct quantitative description of the low temperature behavior of the specific heat capacity.