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

<Prev>                   <Next>

 

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.

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

<Prev>                   <Next>

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. 

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

<Prev>                   <Next>

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.  

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

<Prev>                   <Next>

Big Bang Theory

The Big Bang theory is an effort to explain what happened at the very beginning of our universe. According to the theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something called as singularity. The Big Bang theory is the scientific theory that is most consistent with observations of the past and present states of the universe, and it is widely accepted within the scientific community. Georges Lemaître first proposed the Big Bang theory which he called as "hypothesis of the primeval atom". The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions such as homogeneity and isotropy of space. 

Bimetallic Strip

Two different metal strips of equal lengths placed on each other is called bimetallic strip. On heating, bimetal bends with material of greater linear expansion to convex side. On cooling, it bends with material of greater linear expansion on concave side.   

Binary System

The pair of stars which orbit around each other is referred to as binary system. The center of mass of the binary system lies in between the two stars. The two stars rotate about this point. 

Binding Energy per Nucleon

See average binding energy.

Binding Energy

The energy equivalent of mass defect is called binding energy. It is this energy which binds nucleons together. Hence it is the energy required for breaking a nucleus into free neutrons and protons.

Binoculars

Binoculars are a parallel combination of two telescopes for viewing an erect 3 dimensional image with both eyes. The image of same size can be viewed with both eyes which is uncomfortable for users to see with single eye. The distance between telescopes is adjustable.

Bioluminescence

It is the phenomenon of emission of light by living creatures because of chemical reaction.

Biophysics

Biophysics is an interdisciplinary science that deals with the application of physics to biological processes and phenomena.

Biot & Savart law

This law gives relation between magnetic field due to current carrying conductor and current flowing in the element. dB at a distance ‘r’ from a current element ‘dL’ carrying a current I is found to be proportional to I , to the length dL and inversely to the square of the distance r. The direction of magnetic field is perpendicular to line element dL as well as radius r.

Biprism

A triangular prism with vertex angle of nearly 180^o used to obtain images of a single source in observing the interference light.

Birefringence

Some crystals have property of splitting incident light ray into two refracted rays. It is due to optical anisotropy in the binding forces between the atoms forming a crystal. They have two indices of refraction. This property is called birefringence.

Black Body

A body which completely absorbs radiation of all wavelengths incident on its surface and doesn’t reflect any part of it is called as black body.

                                                        (or)

It is a body which emits thermal radiations of all wavelengths when heated to high temperature. 

Black Hole

A black hole is a region of space-time of extreme density with such strong gravitational attraction due to which nothing can escape, even light. When a star burns through the last of its fuel, it may find itself collapsing. For smaller stars, up to about three times the sun's mass, the new core will be a neutron star or a white dwarf. But when a larger star collapses, it continues to fall in on itself to create a stellar black hole.

Bloch Theorem 

It is a mathematical theorem which gives us the form of electron wave function in a periodic potential. As per this theorem, electron in a one dimensional lattice behaves as plane wave.

Blue Moon

When two full moons occur in a single month, the second full moon is called a "Blue Moon".