PHYSICS DICTIONARY

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Physics

Physics is the science of matter and its motion—the science that deals with concepts such as force, energy, mass, and charge. As an experimental science, its goal is to understand the natural world. In one form or another, physics is one of the oldest academic disciplines; through its modern subfield of astronomy, it may be the oldest of all.

Sometimes synonymous with philosophy, chemistry and even certain branches of mathematics and biology during the last two millennia, physics emerged as a modern science in the 17th century and these disciplines are now generally distinct, although the boundaries remain difficult to define. Advances in physics often translate to the technological sector, and sometimes influence the other sciences, as well as mathematics and philosophy.

Physiorption

It is an exothermic, reversible surface phenomenon, which occurs at low temperatures as per which adsorbate molecules are attracted by weak Vanderwaal’s forces towards adsorbent molecules.   

Pickling

Process of chemical removal of oxides and other surface layers, leaving the cleaned part with a metallic appearance is called as pickling.

Piezoelectric Crystal

Crystal subjected to mechanical stress, results in polarization and there occurs emission of electrons. It acts as a mechanical tranducer.

Piezoelectric Effect

Phenomenon exhibited by certain symmetric crystals. In this effect, if one pair of opposite faces of a crystal is subjected to pressure, the other pair of opposite faces develops opposite electric charges. The sign of the charges changes when the faces are subjected to tension instead of pressure.

Piezoelectric Oscillator

 When two opposite faces of a crystal like quartz, tourmaline etc, are being cut perpendicular to optic axis, are subjected to alternating voltage, the other pair of opposite faces experiences stresses and strains. The quartz crystal will continuously contract and expand. Elastic vibrations are setup in crystal. When frequency of alternating voltage is equal to natural frequency of vibration of crystal or its simple higher multiples, the crystal is thrown into resonant vibrations and the amplitude will be large. These vibrations are longitudinal in nature.

PIN Diode

Diode which consists of PN junction with a wide intrinsic region sandwiched between N and P region. The intrinsic region is very lightly doped either ‘P’ type or ‘N’ type. They are used in switches etc.

 

PIN photo diode

 It is a junction photo diode operated in reverse bias in which an undoped intrinsic region is inserted between P+ & N+ regions. Upon photons incidence on a PIN diode, photons are absorbed mainly in depletion region. The absorbed photons create electron-hole pairs. The photo generated carriers in the depletion region are accelerated in opposite directions by reverse bias and give rise to a photo current.   

Pirani Gauge

It is named after its constructor Pirani. It consists of glass or metal envelope containing a heated filament of a metal with a high temperature coefficient of resistance, such as Platinum or Tungsten. As the pressure in gauge tube increases, the temperature of the filament and therefore its electrical resistance tend to decrease. The usual control circuit for a Pirani gauge is Wheatstone bridge, in which one leg of the bridge is the filament of the gauge tube and the other three legs have resistances nearly equal to that of the gauge tube.

Pitch

It is defined as sensation of ear that depends upon frequency. A shrill sound is produced by a source of high frequency. The pitch of sound changes due to Doppler principle.

Pitch of Sound

 Sensation which depends upon the frequency.

Planck’s Constant

 Constant introduced by Max Planck for explaining quantum nature of light. Energy of light radiation is defined to have value of product of Planck’s constant and its frequency. The value of Planck’s constant is 6.674 x 10^-34 joule seconds.

Planck’s Law

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

Plane of Polarization

Plane which is perpendicular to plane of vibration and pass through the direction of propagation of light is called plane of polarization.

Plane of Vibration

The plane which contains the direction of vibrations of electric vector in the polarized light and the direction of travel of light is called plane of vibration.

Plane Polarized Light

In plane polarized light, there are vibrations only along a line in particular plane perpendicular to the direction of propagation of light. 

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Photo Conductor

It is an optoelectronic device that absorbs optical energy and converts into electrical energy. The operation of device is based on increase of conductivity of a specific region with photo excitation. The photo generated electrons and holes are collected by opposite contacts and result in a photo current.

Photo Detector

 An optoelectronic device that absorbs optical energy and converts it into electrical energy, which usually manifests as photo current is called as photo detector.

Photo Diode

It is a reverse biased PN junction diode which gives current when light falls on it. If the energy of incident light is greater than forbidden gap energy than the valence electrons absorb this energy and gas excited to conduction band. In this way they lead to photo current.

Photo Fission

Fisssion induced in some nuclei by absorption of high energetic photons.

Photo Multiplier Tube

 It is a highly sensitive photo cell, converting light energy into electrical energy. Scintillations from luminescence material are made to incident on photo cathode which generates electrons due to electric effect.

Photo Transistor

A transistor that regulates current or switches it ‘on’ and ‘off’ based on the intensity of the light it is exposed to rather than an external electric signal.

Photo cell

A device in which the photoelectric or photovoltaic effect or photoconductivity is used to produce a current or voltage when exposed to light or other electromagnetic radiation. They are used in exposure meters, burglar alarms, etc.

Photoelectric Effect

This absorption process refers to an interaction of the incident photon of energy greater than binding energy of a bound electron of absorber atom, as a result of which entire photon energy is absorbed and electron is ejected. 

Photoelectron

Terminology attributed to the electron ejected from a work substance by photoelectric effect.

Photographic Film

Photographic film may be utilized in x-ray work and dosimetry. The film tends to darken when exposed to radiation. This general darkening of the film is used to determine overall radiation exposure. Neutron scattering produces individual proton recoil tracks. Counting the tracks yields the film’s exposure to fast neutrons. Filters are used to determine the energy and type of radiation. Some typical filters used are aluminum, copper, cadmium, or lead. These filters provide varying amounts of shielding for the attenuation of different energies. By comparing the exposure under the different filters, an approximate spectrum is determined.

Photoluminescence

Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One of the ways this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties.

Photon

Term attributed to particle nature of light. The concept evolved in quantum theory proposed by Max Planck. A photon represents one quantum of electromagnetic energy and is treated as a fundamental particle in the Standard Model of particle physics. It has rest mass zero. 

Photonics

Branch of physics which deals with technology or application of electromagnetic energy in terms of its basic unit photon, incorporating Optics, LASER technology, electrical engineering, information storage and processing.

Photovoltaic Effect

Photovoltaic mechanism is the direct conversion of light into electricity at the atomic level. The principle behind this effect is photoelectric effect. This effect is used in solar cells.

PHWR

It is type of nuclear reactor in which pressurized heavy water is used as both coolant and moderator. Natural Uranium is used as fuel in the reactor core. The heat transport system consists of two coolant channels. The primary coolant channel consists of pressurized heavy water under flow. The secondary coolant channel consists of light water. The heat transfer from primary to secondary channel makes light water get boiled and the steam generated is used to run turbines for conversion of mechanical energy into electrical energy.

Physical Quantity

A physical quantity is defined as quantity that can be used in mathematical equations of science and technology. It is characterized by its numerical value (magnitude) and associated unit.

Physical Sputtering

Cathodic sputtering which doesn’t involve chemical reaction between bombarding gas ions and cathode is known as physical sputtering. 

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Perpetual Machine of First Kind

The perpetual machine is a hypothetical concept which provides mechanical work more than the heat absorbed by it. Perpetual machine will continue forever without external energy.

Perpetual Machine of Second Kind

It is an imaginary concept as per which,  a machine which can absorb heat from source and completely convert it into work without converting it into internal energy.

Perturbation Theory

A set of mathematical methods for obtaining approximate solutions to complex equations for which no exact solution is possible or known, generally involving an iterative algorithm in which each new term contributing to the solution has less significance than the last. In quantum physics, Feynman diagrams are used to calculate the terms for the perturbation theory solution of interactions between particles.

Peta

Prefix in metric system denoting 10¹⁵ of physical quantity

Pfund Series

The spectrum of wavelengths formed due to electronic transitions from higher states to fifth orbit of atom. The wavelengths fall in far infrared region. 

Phase (SHM)

 Fraction of time interval that has elapsed since the particle crossed the mean position of rest in the positive direction.

                                                  (or)

It is the ratio of displacement of vibrating particle at any instant to the amplitude of vibrating particle.

Phase Constant

 It is the initial phase of the particle in SHM.

Phase Plate

Device used to produce phase difference or path difference in ordinary and extra ordinary waves produced by plane polarized light.

Phase Point

The instantaneous state of a particle in phase space is represented by a point known as phase point.

Phase Space

In Heisenberg’s point of view, the state of a single particle is determined by position coordinates x, y, z and momentum coordinates Px, Py, Pz totally comprising six dimensions. This space is called as phase space.

Phase Velocity

The phase velocity of a wave is the rate at which the phase of wave propagates in space. This is velocity at which phase of any one frequency component of wave travels. For such a component, any given phase of wave will appear to travel at phase velocity.

or

When a progressive wave of a definite wavelength propagates in a medium, the velocity of propagation of its wave front is called phase velocity or wave velocity.

Phase (Wave Motion)

Ratio of displacement of vibrating particle at any instant to the amplitude of vibrating particle

or

It is defined as fraction of time interval that has elapsed since the particle crossed the mean position of rest in the possible direction.

Phase

A homogeneous portion of a system that has uniform physical and chemical characteristics is called as phase.

or

It is a Greek word “appearance” and is defined as a physically distinct but homogeneous part of a system which is separated from other parts of the system by finite bounding surface. Thus, each different, physically homogeneous part of a system is called as phase. For instance in water system consisting ice, water and its vapor, ice is the solid phase, water is liquid phase and vapor is gaseous phase.

Phon

A unit of apparent loudness, equal in number to the intensity in decibels of 1000 Hertz have perceived to be as loud as sound being measured.

Phonon

Hypothetical particle introduced for defining quantum of vibrational energy associated with atoms in crystal lattice.

Phosphorescence

Type of luminescence in which emission of light occurs with removal of excitation source with decay time of few seconds to hours or days.

Phot

A phot is a unit of illuminance, or luminous flux through an area. It is given as one lumen per cm².

Photo Conductivity

 Electrical conductivity that results from photon induced electron excitations in which light is absorbed. The phenomenon which is usually observed in photo conductors and other semi conductor devices.

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Paschen Series

The spectrum of wavelengths formed due to electronic transitions from higher energy states to third orbit of atom. The spectrum falls in infrared region.

Passive Component

 A passive device is one that contributes no power gain (amplification) to a circuit or system. It has no control action and doesn’t require any input than a signal to perform its function. In other words, they are components with no brains. Resistors, Capacitors, Inductors, etc fall under this class of components.

Pauli’s Exclusion Principle

The principle is postulated by an Austrian physicist Wolfgang Pauli in 1925. Pauli’s exclusion principle says that two similar particles cannot exist in same state, i.e. they cannot have both   position and velocity same within the limits given by uncertainty principle. The exclusion principle is crucial because it explains why matter particles do not collapse to a state of light density under influence of forces produced by particles of spin-0, 1& 2. If matter particles have nearly same positions, they must have different velocities, which mean that they will not stay in same position for long.

Peak Inverse Voltage

 It is maximum reverse voltage that a diode can with stand without destroying the junction.

Peltier Effect

When an electric current is passed through a junction of two dissimilar conductors , heat is either absorbed or released at the junction , depending on direction of current.

Penetration Depth

 It is the depth from surface of metal at which magnetic field falls to 1/e of its original value.

Penning Effect

Mechanism of adding small quantities of other gas having lower ionization potential into main gas, to reduce overall work function of gas composition is called Penning effect. If the main gas atoms having meta-stable excited states collide with additive gas atoms, then energy is transferred to additive gas by ionizing its atoms. In this fashion more charge is produced per unit energy of incident energy.

Penumbra

 (“from the Latin words Paene which means “nearly” and Umbra means “shadow” ) is the region in which only a portion of the light source is obscured by occluding body.  

                                        or     

Penumbra is the region where some or the entire light source is obscured. 

 

Perihelion

 It is the point in the orbit of a planet, where it is nearest to the sun.

Perigee

The point in the orbit of an object orbiting the earth, nearest to center of earth is called as Perigee. At its perigee, the objects travels faster than at any point in the orbit.

Periodic Motion

 Any motion that repeats itself at regular intervals of time is known as periodic motion.

Periodic Table

Table used to classify all the elements according to electron configuration. The elements are arranged in table with increasing atomic number in horizontal rows. The arrangement is such that all elements arrayed in a column or group has similar valence electron structures as well as chemical and physical properties. These properties change gradually and symmetrically, moving horizontally across each period.   

Periscope

An arrangement of number of optical instruments like mirrors, lenses that enable user to view objects that are not in direct line of vision, such as one in a submarine for looking above surface of water.

Permanent Magnets

 They are materials which retains property of magnetism unless heated to a high temperature. They are made of hard steel or special alloys.

Permanent Set

 If wire is loaded beyond elastic limit, it will not regain its original length even after deforming force is removed. When the external load is completely removed, length of wire increased permanently called permanent set.

Permeance

The reciprocal of reluctance is called as Permeance.

Permeation

 Passage of a gas into, through, and out of a solid barrier; having no holes large enough to permit more than a small fraction of gas to pass through any hole, is known as permeation.

Permittivity

 It is the characteristic of dielectric material which determines that how much the electric field influences the material.

Perpendicular Axes Theorem

Sum of the moments of inertia of a uniform plane lamina about any two perpendicular axes in its plane is equal to the moment of inertia of it about an axis perpendicular to its plane which passes through the point of intersection of first two axes.