PHYSICS DICTIONARY

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Vacancy Diffusion

The diffusion mechanism where in net atomic migration is from lattice site to an adjacent vacancy.

Vacancy

 A normally occupied lattice site in crystal from which an atom or ion is missing.

Vacuum Pressure

If pressure of a fluid is below atmospheric pressure, it is designated as gauge pressure.

Vacuum

According to definition of American vacuum society, “vacuum” refers to a given space filled with gas at pressures below atmospheric i.e. having a density of molecules less than about 2.5 x 10¹⁹ molecules /cm³. 

Valence Band

 For solids, the electron energy levels band that contains valence electrons. These electrons lie in outermost orbits of atoms at absolute zero temperature.

 

Valence Electrons

 The electrons occupying outermost orbits which usually participate in interatomic bonding are called as valence electrons.

Van De Graff Generator

 It was invented by American physicist Robert J. VandeGraff in 1929. It is a machine capable of building up potential difference of few million volts, and fields close to breakdown field of air which is about 3x10⁶ V/m. The potential is build up with help of static change by friction. 

Vanderwaal’s Gas

 See real gas

Vanderwalls Bond

A secondary interatomic bond between adjacent molecular dipoles, which may be permanent or induced is called Vanderwall’s bond. They are formed as a result of dipole attractions, the dipoles being formed as a result of unbalanced distribution of electrons in asymmetrical molecules.

Vapor

 Gaseous state below the critical temperature of substance is known as vapor.

Vapor Degreasing

The process in which solvent is heated to boiling point, and parts to be cleaned are hung in the chamber exposing to hot vapors. The vapors get condensed on metal surfaces during which dissolves oil & grease and flows back to solvent condition.

Variance

The variance gives us a sense of how closely concentrated a set of values is around its average value, and is calculated in the following manner: Subtract the mean of the distribution from each of the values. Square each result. Add all of the squared results. Divide the result by the number of values minus 1.

Vector Product

 See cross product.

Vector

 It is a physical quantity having both magnitude and direction and which obey laws of vector addition.

Velocity Addition Theorem

The addition of any velocity to the velocity of light merely reproduces again the velocity of light. Thus velocity of light is maximum range of velocity attainable in nature.

Velocity Gradient

 It is rate of change of velocity of liquid layers with distance, the distance being taken normal to the direction of flow.

Vibration

 It is the to & fro motion of a particle from one extreme position to the other and back again. It is motion of a particle from mean position to one extreme position, then to other extreme position and finally back to mean position.

Vibration-Rotational Spectrum

 Molecular spectra obtained in near infrared region.

Virtual Image

A virtual mage is formed at a position where the paths of principal rays cross when projected backward from their paths beyond the lens. Although a virtual image does not form a visible projection a screen, it is no sense “imaginary” i.e. it has a definite position and size and can be “seen” or imaged by the eye, camera or other optical instrument.

or

When a beam of light diverging from a point after reflection (or refraction) appears to diverge from second point, then the second point is called as virtual image of first. The virtual image cannot be formed on screen. 

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Ultra Thin Films

Films having thickness below 100 A^o.

Ultrasonic Waves

Sound waves of high frequency greater than 20,000 Hz are called as ultrasonic waves. These frequencies are beyond audible limit.

Umbra

(Latin for “shadow”) is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body.

Uniaxial Crystal

It is a double refracting crystal which has only one axis along which both the refracted rays have same speed. Ex: Calcite, Quartz, etc.

Uniform Circular Motion

 The motion of an object with constant speed in a circle.

Unit Cell

Basic structural unit or building block of crystal structure, which   defines the crystal structure by virtue of its geometry and the atom positions within it. A unit cell is chosen to represent the symmetry of crystal structure, where in all the atom positions in the crystal may be generated by translation of unit cell integral distances along each of its edges.  

Universal Gravitational Constant

 It is the force between two unit masses separated by unit distance is called universal gravitational constant. The value of ‘G’ is 6.67 x 10^-11 Nm²Kg^-2. 

Unsaturated Vapor

 If a liquid is in open container, the vapor molecules rapidly diffuse away from liquid, and in general produce unsaturated vapor.          

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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.