PHYSICS DICTIONARY

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Capacitance

It is the ability of dielectric material between conductors to store charge, when a difference of potential exists between the conductors.

Capacitor

Passive device designed to store electrical charge. It is an arrangement of two conductors separated by dielectric.

Capillarity

The phenomenon of rise or fall of liquid in a capillary tube is known as capillarity.

Capillary Tube

It is a very thin tube made of rigid material such as plastic or glass. The tube is used to collect samples of liquids which flow into tubes against effects of gravity in a physical process called capillary action. The process uses two physical forces; “Surface Tension” and “adhesion”.

Carat

It is a unit used for measure of proportion of gold in an alloy, expressed as number of parts of gold in 24 parts of alloy. At present, 1 carat =200 mgm.

Carbon-Nitrogen Cycle:

A chain of thermonuclear reactions in which Nitrogen isotopes are formed in intermediate stages and Carbon acts essentially as a catalyst to convert four Hydrogen atoms into one Helium atom with the emission of two positrons is called as Carbon-Nitrogen cycle. The entire sequence is thought to generate significant amount of energy in the sun and certain other stars.

Carburizing

It is a process by which surface Carbon concentration of a ferrous alloy is increased by diffusion from surrounding environment.

Cardinal points

Points of an optical system, if known, the image would be determined easily without knowing the details of structure of system. Cardinal points constitute two principal points, two focal points and two nodal points.

Carnot’s Engine

Theoretical ideal engine, the concept of which is proposed by Sadi Carnot in 1824. As per this concept, there is no loss of heat due to friction etc and working substance is perfect gas. The engine is reversible and its efficiency depends only on temperature of source and sink, between which it works. No engine in practice can have efficiency more than it.

Carnot’s Cycle

Heat engine cycle concept proposed by Nicolas Sadi Carnot in 1823. It is the most efficient heat engine cycle consisting of two isothermal and two adiabatic processes. It is a reversible cycle involving no change in entropy i.e. it is an ideal concept in which total heat supplied is converted to work.

Carnot’s Theorem

Carnot’s theorem states that “working between the two given temperatures, no engine can be more efficient than a reversible Carnot engine. In other words, efficiency of a reversible engine is greater for two given temperatures than any other engine. 

Carrier Wave

Wave which carries information from transmission station and receiving station.

Cartesian Coordinates

Cartesian coordinates are rectilinear two dimensional or three dimensional, which are also called rectangular coordinates. The 3 axes of 3 dimensional Cartesian coordinates conventionally denoted by X, Y & Z axes are chosen to be linear and mutually perpendicular. In 3 dimensions, the coordinates X, Y & Z may lie anywhere in interval (-∞ & +∞ ).

Cathetometer

It is an instrument for the accurate measurement of small difference of height; especially of the differences in the height of upper surfaces of two columns of mercury or other fluid, or of the same column at different times. It consists of a telescope leveling apparatus which sides up or down a perpendicular metallic stand very finely graduated. The telescope is raised or depressed in order to sight the objects or surfaces, and the differences in vertical height are known.   

Cathode Ray Oscilloscope

It is an electronic display unit, first developed by German Physicist Ferdinand Braun in 1897. It contains a cathode ray tube that generates an electron beam that is used to produce visible patterns, or graphs, on a phosphorescent screen. The graphs plot the relationships between two or more variables, with horizontal axis normally being a function of time and vertical axis usually function of voltage generated by input signal to the oscilloscope. It is widely used test instrument for commercial engineering & scientific applications comprising acoustic research, television production engineering and electronics design.   

Cathode Ray Tube

An arrangement of vacuum tube equipped with electron gun which emits electrons. The electrons are accelerated and made to pass through electromagnetic field which gets deflected and falls on to fluorescent screen to form images.

Cathode Rays

Electron beam in vacuum tubes is referred to as cathode rays. They were first observed by German physicist Johann Hittorf and were named in 1876 by Goldstein.

Cathode

It is the electrode in an electrochemical or galvanic couple at which a reduction reaction occurs; thus the electrode that receives electrons from an external circuit is called as cathode. 

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British Thermal Unit(BTU)

It is a standard unit of energy that is used in the United States and sometimes in UK. It represents the amount of thermal energy necessary to raise the temperature of one pound of pure water by one Fahrenheit from the temperature at which water has its greatest density i.e. 39 ^oF. The Btu is a measure in so called English system of units.

1Btu = 1055 Joules, a unit of energy in SI units.

Brittle Substances

Substances which break as soon as the stress is increased beyond elastic limit are called brittle substances.

Bronze

A copper rich copper tin alloy.

Brownian Motion

Irregular, random and zigzag motion of suspended particles in liquid is called Brownian motion.

Bubble Chamber

It was designed by D M Glaser in 1952. It is device used to detect ionizing radiation. The principle underlying this chamber is as follows:

The chamber uses super heated liquid (a liquid that has been heated above its normal boiling point by increasing the pressure over it. When the pressure is suddenly reduced to atmospheric pressure, the liquid remains in unstable super heated state for some time.  If during this interval, an ionizing particle traverses the liquid within few milli seconds after the pressure is released, ions are formed which act as centers for formation of vapor bubbles. The vapor bubbles grow at rapid rate and attain a visible size in a time of order of

10^-100 μsec. 

Bulk Modulus

The ratio of bulk stress to bulk strain within the elastic limit is called bulk modulus or coefficient of volume elasticity.

Bulk Stress

If a body is subjected to the same force normal to all its faces such that there is a change in its volume then normal force per unit area is called bulk stress.

Buoyancy

The property of fluid as per which, a net upward force is exerted on the object by the fluid in which object is immersed partially or completely. 

Buoyant Force

As per Archimedes (a Greek mathematician), a body which is completely or partially immersed in a fluid will experience an upward force called buoyant force, which is equal to weight of fluid displaced by object. The buoyant force is due to difference between the pressure at the bottom of the object pushing up on it, and the pressure at the top pushing down.

Burgers Vector

The magnitude and direction of the lattice distortion associated with a dislocation is expressed in terms of a vector called burger vector.

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Bosons

A group of elementary particles named after well known physicist Satyendranath Bose. They are of two types i) Mass less Bosons created by fields; Photons & Gravitons fall under this category ii) Bosons which are formed in strong interactions; Pions & Kaons fall under this category.

  

Boundary

Anything which separates system and surroundings is called boundary (envelope or wall). The envelope may be imaginary or real; it may be rigid or non-rigid; it may be a conductor of heat (diathermic wall) or a non-conductor of heat (adiabatic wall).

Boyle’s Law

Law as per which at constant temperature, the volume of a given mass of a gas is inversely proportional to its pressure.

Brackett Series

It is the spectrum of wavelengths falling in Infrared region formed due to electron transitions from higher energy states to fourth orbit.  

Bragg Curve

It is plot between residual energy of charged particle passing through matter vs stopping power. Variation of stopping power with respect to residual energy is called as Bragg curve. By residual energy, we mean the instantaneous energy of the particle retained by it as it travels through the material.

Bragg’s Law

Bragg’s law was introduced by Sir W H Bragg and his son  Sir W L Bragg. The law states that when X-rays are incident on crystal lattice with angle of incidence equal to angle of scattering; peaks of scattered intensity are observed (constructive interference) when difference in path length is an integral number of wavelength.

Branching Ratio

Branching ratio is a term that is used to characterize the probability of decay through a mode with respect to all other modes. For instance if a nuclide decays through α and γ modes with branching ratios of 0.8 and 0.2, it would imply that α-particle is emitted in 80% of decays while photons are emitted in 20% of decays.

Brass

A copper rich copper zinc alloy. Alloy consists of Copper and Zinc in variable proportions. 

Brazing

American welding society defines it as metal coalescence above 800^oF. Brazing involves bonding of two surfaces by using molten filler material along with flux to eat through oxide layer.

Breakdown Voltage

It is the minimum voltage which when applied to an insulator loses its insulating property and becomes highly electrically conductive.

Breeder Reactor

Type of nuclear reactor capable of producing fissionable material simultaneously at the expense of burning of fissile material, is called as Breeder reactor. Upon neutron absorption by U-238, it gets converted to Pu-239 which is fissile. The coolant used is liquid metal, usually Sodium as it acts as good heat transfer medium without slowing down of neutrons which is essential for breeding ratio. The reactor core uses high enriched U-235 initiate production of fast neutrons. The core is surrounded by blanket of U-238. Upon neutron absorption by U-238, it gets converted to Pu-239 which is fissile. This makes the chain reaction to sustain for longer periods.      

Breeding

Nuclear process used in breeder reactor as per which non fissionable isotopes yield fissionable isotopes upon capture of fast neutrons.

Bremsstrahlung X-rays

The Bremsstrahlung radiation incident on a suitable target, knock out electrons leading to X-rays from target material called as Bremstrahlung X-rays. 

Bremstrahlung

It is a German word which means braking radiation. Radiation emitted in the form of photons when an energetic electron gets decelerated in the field of atomic nucleus in the matter.  Generally speaking, if a charged particle has energy much greater than its rest energy emits Bremsstrahlung radiation if it encounters resistance while moving through a medium.  

Brewster’s Angle of Incidence

It is the angle of incidence of electromagnetic wave at an interface which yields reflected beam which is totally polarized parallel to the interface and a transmitted beam that is partially polarized perpendicular to it.

                                            or

It is the angle at which incident electromagnetic wave crosses interface between two medium without suffering reflection.

Brewster’s Law

According to this law, the light reflected from a surface is completely polarized if the reflected beam and the beam refracted into material form a right angle. If the incident beam is polarized in plane of incidence there will be no reflection at all. Only if incident beam is polarized normal to plane of incidence, it will get reflected.

Brillouin Zones

Electrons moving in a periodic potential lattice can have energy values only between allowed regions or zones called as Brillouin zones. 

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Body Centered Cubic

Crystal structure having cubic unit cell with atoms located at all eight corners and a single atom at cubic center.

Body Waves

They are type of seismic waves which pass through the deep layers of earth.

Bohr Magneton  

The most fundamental magnetic moment is Bohr Magneton, and it is spin magnetic moment for each electron in an atom. Its value is 9.27 x 10^-24 Am²

Bohr’s Atomic Theory

Neil Bohr proposed atomic theory as per which the atom is treated as a shell with positively charged nucleus at center and the electrons revolve round the nucleus in different circular orbits which are quantized in energy. The electrons do spin while revolving in the orbits.  

Bohr’s Complimentary Principle

Principle enunciated by Neil Bohr in 1928, as per which, wave and particle nature cannot be attributed to any particle or object simultaneously. It is impossible to design measuring device that demonstrates both phenomena simultaneously.

Boiling Point

The temperature at which a liquid starts changing its phase into vapor. 

Boiling Water Reactor

It is a type of nuclear reactor in which reactor core is in direct contact with pressurized de-mineralized light water pool. This water boils and turns into steam at the surface. The light water serves as both coolant and moderator. The steam generated at the surface will drive the turbine to generate electricity. The fuel generally used is natural Uranium.

 

Boiling

Phenomenon of rapid vaporization of a liquid, which typically occurs when a liquid is heated to a temperature such that its vapor pressure is above that of surroundings, such as air pressure is called boiling.

Bolometer

Device used to detect thermal radiation which works on principle of change of resistance with temperature.

Boltzmann Constant

It is a thermal energy constant which appears as proportional constant between energy of the particle in a medium and temperature. It has a value of 1.38 x 10^-23 J/atom-Kelvin (8.62 x 10^-5 eV/atom-K) .

Boltzmann’s Canonical Distribution Law

According to this law, the probability for a system to be found in a particular microstate of energy E, in thermal equilibrium at an absolute temperature ‘T’ with a heat source is proportional to  e^-E/KT.

Bonding Energy

The energy required to separate two atoms that are chemically bonded to each other. It may be expressed on a per atom basis or per mole of atoms.

Bonding

Pairing of two or more atoms involving valence electrons is called bonding. The type of bond depends upon electron structure of constituent atoms.

Boron-10:

It is an isotope of Boron having an abundance of 19.9% in natural Boron (¹¹B). The isotope has good cross-section for interaction with thermal neutron and value is about 3837± 9 barns.

Bose Einstein Condensation

Phenomenon of rapid increase in number of particles in ground state at temperatures below critical temperature is called as Bose Einstein Condensation.

Bose Einstein Statistics

Statistics given by Bose & Einstein, as per which

i) The particles which are dealt are identical, indistinguishable particles with spin zero or an integer, such as Photon, Phonon, and Helium atom at low temperature (which are called Bosons).

ii) The number of phase cells cannot be increased as per our desire because there is minimum volume of a phase cell.

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

iv) There can be any number of particles in a cell.