PHYSICS DICTIONARY

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Propagation Vector

Vector indicating the direction of wave propagation and the phase delay per unit length

Propellant

A substance usually a mixture of fuel and oxide used for propelling a rocket.  

                                          or

A compressed inert gas that serves to dispense the contents of an aerosol container when the pressure is released is called as propellant.                       

Proper Time

The time recorded by a clock moving with a given system is called proper time for that system.

Proportional Counter

It is type of radiation detector which operates in proportional region. To a limited degree, the fill-gas will determine what type of radiation the proportional counter will be able to detect. Argon and helium are the most frequently used fill gases and allow for the detection of alpha, beta, and gamma radiation. When detection of neutrons is necessary, the detectors are usually filled with Boron-Tri-Fluoride gas.

Proportional Region

It is one of the regions in which gas filled detectors are operated. When voltage applied to the anode wire in gas filled detector is increased to a magnitude, the electrons reaching anode wire gets increased due to gas multiplication. There is exponential increase in the number of electrons reaching anode. The magnitude of charge getting collected is proportional to the quantity of initial charge generated during primary ionization created in the detector by incident radiation.

Proton

Sub atomic particle having positive charge of one electron unit and is a constituent of nucleus.

Proton-Proton Cycle

The proton-proton cycle is a series of nuclear reactions which convert Hydrogen nuclei (protons) into Helium nuclei (alpha-particles). All Main Sequence stars do this, in one way or another. For stars like the Sun, the process almost always begins with the collision of two protons, hence its name, the "proton-proton" cycle. 

Pseudo Force

 See fictious force

Pseudo Vector

 Any vector which commutes with parity operator is a pseudo vector.

Pulse Height Analyzer

Electronic device used to record amplitude distribution of pulses produced by a pulse mode radiation detector. It is helpful for analysis of energy resolution of the detector.

Pumping

It is a mechanism used to achieve population inversion.

Pyro-Heliometers

 Instrument used for the determination of solar constant.

Pyrometer

 Instrument used for measuring high temperatures.

There are two types of pyrometer. They are i) total radiation pyrometer ii) optical or spectral pyrometer      

i) Total radiation pyrometer: The instruments that measure total radiation emitted by the body under test are called as total radiation pyrometers. Temperature is determined by making use of Stefan’s law.

ii) Optical or Spectral Pyrometer: The optical pyrometers compare the intensity of radiation of a certain wavelength emitted by the body with that of radiation of same wavelength emitted by a standard body at known temperature. The temperature of the body is obtained by applying Wien’s displacement law or planks law. 

 

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Power

The rate at which work is done, or energy expended, per unit time.

Poynting Vector

 It is the energy per unit area per unit time transported by electromagnetic field.

Poynting’s Theorem

According to the theorem, the work done on charges by electromagnetic field is equal to decrease in energy stored in field less the energy that flowed out through the surface.

Precession of Equinoxes

The precessional motion of earth’s axis causes a change in direction of line of equinoxes which is called precession of equinoxes.

Precipitation Hardening

 Hardening and strengthening of a metal alloy by extremely small and uniformly dispersed particles that precipitate from a supersaturated solid solution is called as precipitation hardening.

Pressure Law

At constant volume, the pressure of a given mass of a gas increases (or decreases) by (1/273)^th of its pressure at 0 ^oC on increasing (or decreasing) its temperature by 1^oC.

Pressure Transducer

 It is an electronic device that converts pressure into electrical signals that can be recorded digitally.

Pressure

 Pressure on any surface is defined as normal force per unit area of surface.

Prevost’s Theory of Heat Exchange

 Each body emits thermal radiation at all temperatures, but for absolute zero; and it absorbs radiations emitted from all other bodies in its surroundings.

Primary Bonds

Inter atomic bonds that are relatively strong and for which bonding energies are relatively large. Primary bonding types are ionic, covalent, and metallic.

Primary Colors

Almost all visible colors can be obtained by the additive color mixing of three colors that are in widely spaced regions of the visible spectrum. If the three colors of light can be mixed to produce white, they are called primary colors and the standard additive primary colors are red, green and blue.

Principal Points

 The principal points are a pair of conjugate points on the principal axis for which linear magnification is unity and positive.

Principle of Calorimeter

If no heat is lost to surroundings, the heat lost by hot body must be equal to heat gained by cold body.

Principle of Calorimetry

 When hot body and cold body are joined together, they attain common temperature and heat lost by hot body is equal to heat gained by cold body provided there is no loss of heat to surroundings by conduction, convection or radiation.

Principle of Degradation of Energy

 All the available energy of universe is tending towards zero i.e., entropy is reaching its maximum value.

Principle of Equal Apriori Probability

According to this principle, the probability of finding the phase point for given system in any region of phase space is identical with that for any other region of equal extension or volume.

Principle of Equivalence

In the neighborhood of any given point it is not possible to distinguish between the gravitational field produced by the attraction of masses and the field produced by accelerating a inertial frame of reference.

Principle of Homogeneity of Dimensions

The principle states that physical quantities of same dimensions only can be added or subtracted or equated.

Principle of Invariance of Charge

 According to this principle, the total charge in an isolated system is unchanged by motion of its charge carriers.

                                                 or

 The total electric charge of an isolated system is relativistically invariant.

Principle of Least Time

Same as Fermat’s principle of extremum path.

Principle of Regenerative Cooling

Regenerative cooling is a method of cooling gases in which compressed gas is cooled by allowing it to expand and thereby taking heat from the surroundings, the cooled expanded gas then passes through a heat exchanger where it cools the incoming compressed gas.

Principle of Reversibility

If all velocities in a dynamical system are reversed, the whole previous motion has to be obtained provided there is no dissipation.

Principle Quantum Number

It is one of four quantum numbers associated with energy levels in an atom. The principle quantum number has integer values    1, 2, 3……. corresponding to order of orbit containing electrons.

Prism

It is a homogenous, transparent medium (such as glass) enclosed by two refracting plane surfaces at an angle.

Probability Density Function

The outcome of a repeatable experiment, usually referred to as a random variable, is not always discrete and usually can take any value within a continuous range. If ‘x’ is continuous then the probability that the outcome lies between x and x+dx can be written as P(x)=f(x;θ)dx. Here f(x, θ) is called the probability density function.

Process

A process is the path along which a change of state takes place. The process can occur under variety of conditions which must be defined because many things depend on nature of process. For a substance, isothermal, isochoric, adiabatic, cyclic etc are examples of process.

Progressive Wave Motion

Wave motion in which there is no transfer of medium in direction of propagation wave, but there is always transfer of energy in the direction of propagation of wave.

Progressive Wave

If we produce waves continuously in a medium, at any instant, all the particles of medium start vibrating in same manner, but different particles are in different states of their vibrations. The disturbance or wave so produced in the medium is called progressive wave.

Projectile

A body projected with certain velocity making an angle other than 90^o to the horizontal is known as projectile.

Prompt Neutron

Neutrons released in fission within 10^-14 sec or less. 

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Polarization Index

It is the ratio of the Insulation Resistance measured pertaining to an insulator for 10 minutes to the insulation resistance value measured after 1 minute. The Polarization Index should be above 2.0 to be permissible.

Polarization (Dielectric)

The total electric dipole moment per unit volume of dielectric material is called as polarization. It is also a measure of contribution to total electric displacement by a dielectric material.

or

For an atom, the displacement of center of negatively charged electron cloud relative to the positive nucleus is called as polarization. This is induced by an electric field. A phenomenon observed in dielectrics.

Polarization

The property of acquiring one sidedness by a wave is known as “Polarization”. The wave unsymmetrical about a direction is called polarized wave while that symmetrical is called unpolarized wave. The “Polarization” is phenomenon which shows that electromagnetic waves are transverse in nature.

Polaroid

It is a simple and cheap device to obtain the plane polarized light. It consists of tiny crystals of quinine idosulphate in a film of nitro cellulose with their optic axis parallel to each other. When an un polarized light is incident on it, each ray splits into two polarized refracted rays, out of which one of the refracted ray, having the vibrations perpendicular to principal section is completely absorbed, while other refracted ray having vibrations parallel to principal section, is transmitted. Each polarized film has a characteristic direction which is called as direction of polarization.

Polyatomic Molecule

It is a molecule containing more than two atoms.

Polycrystal

 It is a substance in which crystal structure is a collection of many small crystals or grains.

Polygon

 A closed plane geometrical figure (2 D) having 3 or more straight sides that meet in pairs in same number of vertices, and do not intersect other than at these vertices. The sum of interior angles is (n-2)×180^o for n-sides; sum of exterior angles is 360^o.

Polymer

 A solid, non-metallic, normally organic compound of high molecular weight; the structure of which is composed of small repeating units is called as polymer.

 

Polymorphism

The property exhibited by some crystals according to which they exist at different crystal structures at different temperatures.

Polytropic Process

Any thermodynamic process represented by PVⁿ =const is known as polytropic process.

Population Inversion

Process by which number of atoms in excited state (meta-stable) is made higher compared to atoms in ground state which increases stimulation probability high in LASER, is called population inversion.

Positive Feedback

When the feedback energy is in phase with input signal and thus aids it is called “positive feedback”. This mechanism increases gain of amplifier.

Positron

The positron is anti particle of an electron with same mass and spin as electron and charge equal in magnitude but opposite in sign to that of electron. In 1928, Paul Dirac was first to postulate Positrons existence and in 1932 Carl D. Anderson discovered it as the first evidence of anti-matter in his experimental study of cosmic rays.

Positronium

Positronium is a semistable, Hydrogen like atomic configuration consisting of a positron and electron revolving about their common center of mass before the process of annihilation occurs. Because it consists of two particles of equal mass. Positronium is sometimes considered to be the lightest atom. Its life time is of order of 10^-7 sec and its reduced mass equals 0.5Me. Croation physicist Stjepan Mohorovicic predicted the existence of Positronium in 1934 and the Austrian American physicist Martin Deutsch discovered it in 1951.

Potential Energy

Energy possessed by a body by virtue of its state or position is called as potential energy. In fact it is referred as stored energy of body. 

Powder Metallurgy

 The fabrication of metal pieces having intricate and precise shapes by the compaction of metal powders, followed by densification heat treatment is called as powder metallurgy.

Powder Method of X-rays

Physical technique used for the identification of substances, and for other types of analyses, principally for crystalline materials in the solid state. In these techniques, a monochromatic beam of x-rays is directed onto a  polycrystalline (powder) specimen, producing a diffraction pattern that is recorded on film or with a diffractometer. This x-ray pattern is a fundamental and unique property resulting from the atomic arrangement of the diffracting substance. Different substances have different atomic arrangements or crystal structures, and hence no two chemically distinct substances give identical diffraction patterns. Identification may be made by comparing the pattern of the unknown substance with patterns of known substances in a manner analogous to the identification of people by their fingerprints.

The x-ray powder method is widely used in fundamental and applied research; for instance, it is used in the analysis of raw materials and finished products, in phase-diagram investigations, in following the course of solid-state chemical reactions, and in the study of minerals, ores, rocks, metals, chemicals, and many other types of material. The use of x-ray powder diffraction methods to determine the actual atomic arrangement, which has been important in the study of chemical bonds, crystal physics, and crystal chemistry. 

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Planet

The term "Planet" is derived from the Greek word "Planetes" - meaning wanderer. Astronomers of the International Astronomical Union (IAU) voted on and passed the first scientific definition of a planet in August 2006. According to this new definition, an object must meet three criteria in order to be classified as a planet. First, it must orbit the Sun. Second, it must be big enough for gravity to squash it into a round ball. And third, it must have cleared other objects out of the way in its orbital neighborhood. To clear an orbit, a planet must be big enough to pull neighboring objects into the planet itself or sling-shot them around the planet and shoot them off into outer space. 

Planck’s Law

Planck in 1900 propounded quantum theory to explain black body radiation successfully. According to Planck, the emission and absorption of radiation is not continuous, but is in form of bundles or packets of definite energy. Each bundle or packet is called quanta. Each photon has definite energy and definite momentum whose value is proportional to the frequency of radiation.

Plancks Constant

It is universal constant, named after its Planck who proposed quantum theory. As per the theory, energy of photon is proportional to frequency of radiation. The proportionality constant is named as planks constant. The value is 6.67x10^-34 joule second.

Plasma

Phase of matter that has enough energy for electrons to get separated from nucleus. It consists of independently moving electrons and nuclei. It is conductive and reacts strongly to electric and magnetic fields.

Plastic Deformation

It is deformed state of a substance where the stress is no longer proportional to strain and permanently non recoverable. From atomic perspective, plastic deformation corresponds to breaking of bonds with original atom neighbors and then reforming bonds with new neighbors as large number of atoms more relative to each other.

Plasticity

The property of a solid body whereby it undergoes a permanent change in shape or size even after removal of deforming forces when subjected to a stress exceeding particular value, called yield value.

Plutonium

It is second Transuranic element of the actinide series to be discovered. Pu²³⁹, isotope of Plutonium has a half life of 24,100 years. It is formed in reactor pile by bombardment of U²³⁸ with neutrons. It is fissionable upon incidence of neutrons.

Pneumatics

It is concept in applied physics which deals with study of application of pressurized air or gases to do mechanical work.

Point Defect

The departure from periodic arrangement around the vicinity of a lattice point is termed as point defect. Two types of point defects normally found in crystalline ionic substances are i) Schottky defect ii) Frenkel defect

Point Group

Collection of symmetry operations which when applied about a lattice point leaves the lattice invariant. The group of such a set of symmetry operations is called a point group.

Poise

A unit of coefficient of viscosity, defined as the tangential force per unit area required to maintain unit difference in velocity between two parallel planes separated by 1cm of fluid.

Polar coordinates

Two coordinates used for locating a point in a plane by the length of its radius vector and the angle this vector makes with polar axis.

Polar dielectric

Dielectrics which have non symmetrical structure having permanent dipole moment.

Polar molecule

A molecule in which there exists a permanent electric dipole moment by virtue of the asymmetrical distribution of positively and negatively charged regions.

Polariscope

An optical device used to measure rotation of plane vibration of polarized light.

or

A instrument used for detecting polarized light or examining objects under polarized light especially for detecting stress or strain in transparent materials.      

Polarimeter

The device which measures the angle by which plane of polarization of the incident plane polarized light is rotated by a given optically active substance is called as polarimeter.

Polarization (Electromagnetic Wave)

The property of acquiring one sidedness by a wave is known as polarization. The wave unsymmetrical about a direction is called polarized wave while symmetrical is called as unpolarized wave. The polarization is the phenomenon which shows that electromagnetic waves are transverse in nature.

Polarization (Ionic)

Displacement of anions & cations in opposite direction.