PHYSICS DICTIONARY

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Tangential Stress

When a tangential force is applied on a body such that there is a change in shape of the body only, then the tangential force per unit area is called tangential stress.

Telescope

An arrangement of lenses and mirrors that collects visible light, allowing direct observation or photographic recording of distant objects is called as telescope. A refracting telescope uses lenses to focus light to produce a magnified image. Compound lenses are used to avoid distortions such as spherical and chromatic aberrations. A reflecting telescope uses mirrors to view celestial objects at high levels of magnification. Most large optical telescopes are reflecting telescopes because very large mirrors, which are necessary to maximize amount of light received by telescope, are easier to build than very large lenses. 

Temperature of Inversion (Gases)

The temperature of a gas is the temperature below which a gas is cooled in Joule Thomson expansion and above which gas is heated. In other words, the temperature at which sign of Joule Thomson effect gets reversed.

Temperature

 Degree of hotness of a body

Temporal Coherence

The coherence with time in the waves from coherent source travelling in one path is called temporal coherence.

Tensile Strain

 If there is increase in length of object due to application of longitudinal stress then it is called tensile strain.

Tensile Strength

 The maximum stress, in tension, that may be sustained without fracture.

Tensor Analysis

The study of the fact that “the fundamental laws of physics should possess the same form in all coordinate systems” known as invariant formulation of physical laws is embodied in “Tensor analysis”. Tensor analysis is generalization of vector analysis.

Tephigram

 It is the name given for temperature-Entropy diagram. It is useful for applications of meteorology, work value of fuel etc.

Terminal Velocity

The maximum velocity which a body attains when its falls through air under gravitational force.

Terrestrial Radiation Sources

This type of radiation is present in small quantities all around us and is more or less in escapable. The main source of terrestrial radiation is the element uranium and its decay products such as thorium, radium, and radon. Although the overall natural concentration of these radioactive materials is within the tolerable range of humans, some parts of the world have been identified where higher levels of uranium and thorium in surface soil have increased the radiation to dangerous levels. The two isotopes of radon, ²²²Rn and ²²⁰Rn, and their daughter products are the most commonly found hazardous radioactive elements in our surroundings. The main cause of concern with respect to these α-emitting isotopes is their inhalation or digestion, in which case the short range α-particles continue to cause damage to internal organs that can lead to cancer.

Tesla

 The SI derived unit of magnetic flux density, equal to the magnitude of magnetic field vector necessary to produce a force of one Newton on a charge of one Coulomb moving perpendicular to direction of magnetic field vector with a velocity of one meter per second. It is equivalent to one Weber per square meter.                    

Thermal Conductivity (Insulator)

When one part of crystal is heated, a temperature gradient is set up. In the presence of temperature gradient, heat will flow from the hotter to cooler region. The ratio of this heat current density to the magnitude of temperature gradient is called thermal conductivity. 

Thermostat

Thermostat is a device used to maintain steady temperature. It is equipped with bimetallic strip.

Theory of Everything

 Hypothetical theory aimed at single equation comprising four types of interactions.

Thermal Capacity

The quantity of heat energy required raising the temperature of a given mass of a substance by 1^oC or 1^oK is called thermal capacity.

Thermal Equilibrium

When the temperature in all parts of system is same as that of surroundings, the system is said to be in thermal equilibrium.

or

Whenever there is no exchange of heat between the bodies, the bodies are said to be in thermal equilibrium.

Thermal Evaporation Method

It is a thin film growing technique. The principle involves heating the substance more than its melting point in order to produce vapors of it to get deposited on substrate.

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Satellite

These are natural or artificial bodies revolving around a planet under its gravitational attraction.

Saturation Vapor Pressure

The maximum amount of humidity which air can hold, at a particular temperature is defined by so called saturation water vapor pressure.

Sawtooth Wave

A sawtooth wave, however, has ramps that drop or begin sharply, becoming similar to the teeth of a saw. It can be found in some music and graphics applications.

Scalar Product

 See Dot product.

Scalar

 Physical quantity having only magnitude.

Scanning Electron Microscopy

It is a technique in which surface of a specimen to be examined is scanned with an electron beam and the reflected (or back scattered) beam of electrons is collected, and then displayed at same scanning rate on a cathode ray tube. The image on the screen, which may be photographed, represents the surface features of specimen. The surface may or may not be polished and etched, but it must be electrically conductive. A very thin metallic surface coating must be applied to non-conductive materials. Magnification ranging from 10X to in excess of 50,000X is possible.  

Scanning Probe Microscope

It is an instrument which generates a topographical map, on an atomic scale, that is representation of surface features and characteristics of specimen being examined. Scanning probe microscopes employ a tiny probe with a very sharp tip that is brought into very close proximity (i.e. within an order of a nano meter) of specimen surface. This probe is then raster scanned across the plane of the surface. During scanning, the probe experiences deflections perpendicular to this plane, in response to electronic or other interactions between the probe and specimen surface. The in – surface – plane and out of plane motions of probe are controlled by piezoelectric ceramic components that have nanometer resolutions. The probe movements are monitored electronically, and transferred to and stored in a computer, which then generates 3D surface image.   

Schottky Defect

The absence of cation or anion from the position which it is expected to occupy in the periodic arrangement of ions is known as Schottky defect.   

or

The formation of vacancies in ionic crystals for charge neutrally is called Schottky defect.

Schottky Effect (Thermionic Emission)

 An accelerating electric field when applied to the thermionic emitter, the ejected electrons gets accelerated towards collector, where they constitute thermionic current. The electric field also lowers the height of potential barrier at the surface and hence increases the emission. This increase of emission and its dependence on electrical field is called as schottky effect.

Scintillation Detector

The scintillation counter is a solid state radiation detector which uses a scintillation crystal (phosphor) to detect radiation and produce light pulses. As radiation interacts in the scintillation crystal, energy is transferred to bound electrons of the crystal’s atoms. If the energy that is transferred is greater than the ionization energy, the electron enters the conduction band and is free from the binding forces of the parent atom. This leaves a vacancy in the valence band and is termed a hole. If the energy transferred is less than the binding energy, the electron remains attached, but exists in an excited energy state. Once again, a hole is created in the valence band. By adding impurities during the growth of the scintillation crystal, the manufacturer is able to produce activator centers with energy levels located within the forbidden energy gap. The activator center can trap a mobile electron, which raises the activator center from its ground state, G, to an excited state, E. When the center de-excites, a photon is emitted. The activator centers in a scintillation crystal are referred to as luminescence centers. The emitted photons are in the visible region of the electromagnetic spectrum.

Scintillation Efficiency

It is a term used to characterize the efficiency by which energy lost by incident particle is converted into scintillation photons. Generally it is described as ratio of total energy of scintillation photons and total energy deposited by incident radiation.

Scintillator

Solid state crystal which converts ionizing radiation into scintillations i.e. photons, a phenomenon of luminescence. 

Screw Dislocation

 A linear crystalline defect associated with the lattice dislocation created when normally parallel planes are joined together to form a helical ramp.

Screw Gauge

A screw gauge is a device used to measure the thickness of thin plates and the diameter of a thin wire. 

Secant

It is a straight line drawn from the center of a circle through one end of a circular arc to a tangent drawn from the other end of the arc.

or

A trigonometric function, that in a right-angled triangle is the ratio of the length of the hypotenuse to that of the adjacent side.

Second Sound

Phenomenon in which normal fluid and super fluid oscillate out of phase giving rise to thermal waves for smoothing out temperature difference is called second sound.

Second

The second is the duration of 9192631770 periods of the radiation corresponding to transition between two hyperfine levels of ground state of cesium 133 atom.

Secondary Bonds

Inter atomic and inter molecular bonds that are relatively weak and for which bonding energies are relatively small. Normally atomic or molecular dipoles are involved.

Secular Equilibrium

If the activity of parent becomes equal to that of the daughter, the two nuclides are said to be in secular equilibrium. This happens if the half life of parent is much greater than that of the daughter.

Siderial Day

A sidereal day is the time it takes for the Earth to rotate about its axis so that the distant stars appear in the same position in the sky. A measurement of the sidereal day is made by noting the time at which a particular star passes the celestial meridian (i.e. directly overhead) on two successive nights. On Earth, a sidereal day lasts for 23 hours 56 minutes 4.091 seconds.

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RAD (Radiation Absorbed Dose)

It is the unit for measure of energy deposited by ionizing radiation per unit mass of material. One RAD is equal to energy absorption of 100 ergs per gram of material.

RADAR

Radar is acronym for "Radio Detection And Ranging." A radar system usually operates in the ultra-high-frequency (UHF) or microwave part of the radio-frequency (RF) spectrum, and is used to detect the position and/or movement of objects. Radar can track storm systems, because precipitation reflects electromagnetic fields at certain frequencies. Radar can also render precise maps. Radar systems are widely used in air-traffic control, aircraft navigation, and marine navigation.

Radian

It is the angle subtended at the centre of a circle by an arc whose length is equal to the radius of that circle.

1radian = 57^o 17’44”; 2π radian=360^o

Radiant Energy (Heat)

The heat energy transferred by the mode of radiation is called radiant energy.

Radiant Energy

It is defined as energy of particles (excluding rest energy) emitted, transferred or received. (As per ICRU, 1980).

Radiation Absorbed Dose

It is the unit for measure of energy deposited by ionizing radiation per unit mass of material. One RAD is equal to energy absorption of 100 ergs per gram of material. 

Radiation Exposure

 It is defined by ICRU(1980) as “the quotient of dQ/dm, where the value of dQ is absolute value of total charge of the ions of one sign produced in air when all the electrons liberated by photons in air of mass ‘dm’ are completely stopped in air”. 

Radiation Length

Radiation length is a physical quantity in nuclear physics which describes overall attenuation of electrons and photons passing through matter. It is defined as thickness of a material that an electron travels such that it loses about 63% of its energy in Bremstrahlung.

Radiative Capture

A particle may combine with a nucleus to produce a new nucleus or a compound nucleus which is in an excited state. The excess energy is emitted in the form of  γ-ray photons. This type of process is known as radiative capture.

Radiator (Thermal)

If the temperature of the body is higher than the temperature of its surroundings, the body emits thermal radiations all around from its surface.

Radio Frequency

Frequency of radio waves or signals in the range of about 3KHz to 300GHz.

Radio Waves

They are type of electromagnetic waves with frequencies ranging from 300GHz to as low as 3 KHz.

Radioactive Decay

When nuclei emit subatomic particles, their configuration, state, and even identity may change. Except for gamma decay, in which nucleus retains its identity, all other decays transform the nucleus into a totally different one. This process is called radioactive decay.

Radioactive Disintegration

 Spontaneous breaking up of nucleus is known as radioactive disintegration.

Radioactive Transformation

The disintegration of an unstable nucleus resulting in another nucleus is called as radioactive transformation.

Radio Waves

Waves having wave length in range of 10⁶ to 10^-3 meters are called as radio waves. Sources of radio waves are electrons vibrated by electronic circuits, radio, stars, and galaxies including pulsars and quasars.

Radium

Radium was discovered by Marie Curie and her husband Pierre in 1898. Radium is an alkaline earth metal. Radium has a melting point of 700°C, boiling point of 1140°C, specific gravity estimated to be 5, and valence of 2. Pure radium metal is bright white when freshly prepared, although it blackens upon exposure to air. The element decomposes in water. Radium emits alpha, beta, and gamma rays. It produces neutrons when mixed with beryllium. A single gram of Ra-226 decays at the rate of 3.7x1010 disintegrations per second. A gram of radium produces around 0.0001 ml (STP) of radon gas (emanation) per day and about 1000 calories per year. Radium loses about 1% of its activity over 25 years, with lead as its final disintegration product. Radium is a radiological hazard. Stored radium requires ventilation to prevent the build-up of radon gas. 

Radius of Gyration

It is that distance from the axis of rotation, where the whole mass of body can be placed to get same moment of inertia which the body actually does possess.

Radius Vector

When a particle describes circular path, the line joining centre of circle and position of that particle at any instant is called as radius vector.

Rainbow

It is a phenomenon due to combination of refraction of sunlight by spherical wavelet droplets behaving like prism (dispersion) and of “Total internal reflection”.

                                                     or

The spectrum formed due to dispersion of sunlight by water drops settling down after rain.

Raman Effect

When a monochromatic beam of light is passed through a gas, liquid or transparent solid body, a small fraction of light is scattered in all directions. The scattered light consists of frequencies greater and smaller than incident beam frequency in additional to original, the phenomenon is known as Raman Effect.

Ramsden Eye Piece

It consists of two plano-convex lenses of same focal lengths with a separation of two third the focal length of either lens. The two lenses are placed with their surfaces facing each other.

Random Errors

Random errors refer to the errors that are statistical in nature.

Rank of a Tensor

The rank of a tensor only indicates the number of indices attached to its per component. The rank of Tensor gives the number of mode of changes of a physical quantity when passing from one system to other which is in rotation relative to the first.

Rayleigh Scattering

When a scattering occurs with molecules which are smaller than the wavelength of incident radiation is called Rayleigh scattering.

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Quality Factor (Radiation)

It is a dimensionless variable weighing factor to be applied to the absorbed dose to provide an estimate of relative human hazard of different types and energies of ionizing radiations. Values of quality factor are selected from experimental values of relative biological effectiveness (RBE), which is ratio of X-ray or Gamma ray dose to that of radiation in question giving same kind and degree of biological effect.  

Quality Factor (Sound)

 Physical quantity used to determine sharpness of resonance which refers to fall in amplitude with change in frequency on each side of maximum amplitude.

Quanta

Concept introduced by Planck to attribute particle nature to radiation /light. Quanta/quantum describes basic unit of energy.

Quantum Chromodynamics

The theory of quantum chromodynamics was developed through the 1950s and 1960s, resulting in a 1969 Nobel Prize in Physics for Murray Gell-Mann (although asymptotic freedom was discovered later by David Politzer, Frank Wilczek, and David Gross, who split the 2004 Nobel Prize in Physics for this work). Experimental research since then has strongly confirmed the theory, and the six flavors of quarks predicted by the theory have all been observed experimentally in laboratory settings.

The name quantum chromodynamics comes from the naming convention that the quarks have a "color," which acts sort of like a "charge" in electrodynamics (but has nothing to do, except in name, with visible colors). "Chromo" is Greek for "color," so the name comes from the way these "colors" interact with each other. Quantum chromodynamics is the theory that describes the strong nuclear interaction, one of the fundamental forces of physics. The theory of quantum chromodynamics explains the interactions of quarks and gluons, which together make up hadrons, such as protons, neutrons, and mesons.

Quantum Electro Dynamics (QED)

The relativistic theory of electromagnetic interactions based on picture and exchange of photons is called quantum electro dynamics.

Quantum Mechanics

Quantum Physics is a branch of science that deals with discrete, indivisible units of energy called quanta as described by the Quantum Theory. The quantum mechanics is concerned with explanation of behavior of physical systems at microscopic level. The main ideas of quantum mechanics are:

i)      Energy is not continuous, but comes in small but discrete units. 

ii)    Elementary particles behave both like particles and like waves.

iii)   The movement of these particles is inherently random. 

iv)   It is physically impossible to know both the position and the momentum of a particle at the same time. The more precisely one is known, the less precise the measurement of the other is.

Quantum Numbers

 They are set of four numbers, the values of which are used to label possible electron states. Three of the quantum numbers are integers, which also specify the size, shape, and special orientation of an electrons probability density; the fourth number designates spin orientation. 

Quantum Physics

"Quantum" comes from the Latin meaning "how much." It refers to the discrete units of matter and energy that are predicted by and observed in quantum physics. Even space and time, which appear to be extremely continuous, have smallest possible values. Quantum physics is the study of the behavior of matter and energy at the molecular, atomic, nuclear, and even smaller microscopic levels. There are five main ideas represented in Quantum Theory:

i) Energy is not continuous, but comes in small but discrete units.

ii) The elementary particles behave both like particles and like waves.

iii) The movement of these particles is inherently random. 

iv) It is physically impossible to know both the position and the momentum of a particle at the same time. The more precisely one is known, the less precise the measurement of the other is.

v) The atomic world is nothing like the world we live in.   

Quark

A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons. The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks.

Quarter Wave Plate

 Plate of uniaxial double refracting crystals, with optic axis parallel to their refracting surface is called as quarter wave plate. The thickness of quarter wave plate is such that it produces a path difference of X/Y or a phase difference of π/2 in between ordinary and extraordinary waves.

Quenching

Process of adding suitable gases in small quantities to principal gas in counters to negate the effects of positive ions induced Geiger discharges, by method of charge transfer collisions between ions and quench gas molecular.

Quotient Law

Law in tensor analysis as per which “an entity whose inner product with an arbitrary tensor (contravariant or covariant) is a tensor, is itself a tensor.