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### PHYSICS DICTIONARY

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Pions or Π Mesons

They are mesons produced both in nucleon – nucleon interaction and in interaction of high energy photons with nucleus. There are 3 kinds of Π mesons, they are Π+, Π-, Π0. Both charged Pions possess a mass of 139.6Mev and neutral one has 135MeV Pions have zero spin.

P- Type Semiconductor

A semiconductor which is doped with acceptor impurity atoms leading to holes as predominant charge carriers responsible for electrical conduction.

Pachimeter

It is an instrument for measuring the limit beyond which shear of a solid ceases to be elastic.

Pair Annihilation

Process in which positron and electron coalesce to produce two photons. A positron moving through matter usually forms ion pairs giving up energy in the process. There is about 2% chance that a positron will hit an electron and annihilate.

Pair Production

When the incident photon energy exceeds twice the rest mass energy of an electron, this interaction is more probable. In this interaction, known as pair production, the incident photon is completely absorbed, and in its place, appears an electron-positron pair.

Parallel Axes Theorem

The moment of inertia of a body about any axis is equal to sum of the moment of inertia of the same body about an axis parallel to the first axis which passes through the center of gravity and product of mass of the body and square of the perpendicular distance between the axes.

Parallelogram Law of Vectors

If two vectors acting at a point are represented both in magnitude and direction by the two adjacent sides of a parallelogram drawn through that point, then the diagonal passing through that point represents the resultant of those two vectors both in magnitude and direction.

Paramagnetic Substances

Paramagnetic substances are those in which each atom ( or ion or molecule) has a net non-zero magnetic moment on its own. When such a material is placed in an external magnetic field the latter tends to align the individual atomic moments in its own direction. However, thermal agitation tends to knock the dipoles into random directions. For strong enough magnetic field or low enough temperature, there is a net average magnetic dipole moment density in the same direction as external field. so the field with in the sample gets enhanced, and we have a pulling in of field lines. If a paramagnetic sample is placed in a non uniform field, it tends to move from the low to high field region.

Parameter

Any of a set of physical properties whose values determine the characteristics or behavior of something is called as parameter.

Paraxial Rays

The rays of light incident close to the principal axis of a spherical reflecting or refracting surface are called paraxial rays.

Parent Nuclide

Nuclide, which by undergoing radioactive decay forms a new nuclide then it is said to be parent nuclide for new one. The newly formed nucleus is referred to as daughter nuclide.

Parity

It is a symmetry property of wave function. The effect associated with parity operator results in space inversion. If the parity operation on a physical quantity leaves it invariant then parity is said to be conserved.

Parking Orbit

The orbit in which a geo-stationary satellite moves is called parking orbit.

Parsec

Unit of length used in astronomy. The name parsec stands for “parallax” of one second of arc”. One parsec is defined to be the distance from the earth to a star that has parallax of 1 arc second. The actual length of parsec is approximately 3.262 light years.

Particle Physics

Particle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them.

It is also called "high energy physics", because many elementary particles do not occur under normal circumstances in nature, but can be created and detected during energetic collisions of other particles, as is done in particle accelerators.

Modern particle physics research is focused on subatomic particles, which have less structure than atoms. These include atomic constituents such as electrons, protons, and neutrons (protons and neutrons are actually composite particles, made up of quarks), particles produced by radiative and scattering processes, such as photons, neutrinos, and muons, as well as a wide range of exotic particles. Strictly speaking, the term particle is a misnomer because the dynamics of particle physics are governed by quantum mechanics.

As such, they exhibit wave-particle duality, displaying particle-like behavior under certain experimental conditions and wave-like behavior in others (more technically they are described by state vectors in a Hilbert space).

Pascal

It is unit of pressure; one Pascal is a force of 1N spread over on area of 1m2.

Pascal’s Principle:

Pressure applied to an enclosed fluid is transmitted to every portion of fluid and to walls of containing vessel.

Paschenback Effect

An effect on spectral lines obtained when the light source is placed in a very strong magnetic field, first explained by F. Paschen and E. Back in 1921. In such a field, the anomalous Zeeman Effect which is obtained with weaker fields, changes over to what is, in a first approximation, the normal Zeeman Effect. The term “very strong field” is a relative one, since the field strength required depends on the particular lines being investigated. It must be strong enough to produce a magnetic splitting that is large compared to the separation of the components of the spin-orbit multiplet.