**Pions
or ****Π
M****esons**

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^{-}, Π^{0}**.
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 1m^{2}.

**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.