The laws list
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particlewave duality to pseudoforce.


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P.

 parsec

The unit of distance defined as the distance indicated by an
Earthorbit parallax of 1 arcsec. It equals about 206 264 au, or
about 3.086 x 10^{16} m.
 particlewave duality

See waveparticle duality.
 pascal; Pa

The derived SI unit of pressure defined as 1 N acting over an area
of 1 m^{2}; it thus has units of N/m^{2}.
 Pascal's principle

Pressure applied to an enclosed imcompressible static fluid is
transmitted undiminished to all parts of the fluid.
 Paschen series

The series which describes the emission spectrum of hydrogen when
the electron is jumping to the third orbital. All of the lines
are in the infrared portion of the spectrum.
 Pauli exclusion principle (W. Pauli; 1925)

No two identical fermions in a system, such as electrons in an
atom, can have an identical set of quantum numbers.
 Peltier effect (J.C.A. Peltier; 1834)

The change in temperature produced at a junction between two
dissimilar metals or semiconductors when an electric current
passes through the junction.
 permeability of free space; magnetic constant; mu_0

The ratio of the magnetic flux density in a substance to the
external field strength for vacuum. It is equal to 4 pi x 10^{7}
H/m.
 permittivity of free space; electric constant; epsilon_0

The ratio of the electric displacement to the intensity of the
electric field producing it in vacuum. It is equal to 8.854 x
10^{12} F/m.
 Pfund series

The series which describes the emission spectrum of hydrogen when
the electron is jumping to the fifth orbital. All of the lines
are in the infrared portion of the spectrum.
 photoelectric effect

An effect explained by A. Einstein that demonstrate that light
seems to be made up of particles, or photons. Light can excite
electrons (called photoelectrons in this context) to be ejected
from a metal. Light with a frequency below a certain threshold,
at any intensity, will not cause any photoelectrons to be emitted
from the metal. Above that frequency, photoelectrons are emitted
in proportion to the intensity of incident light.
The reason is that a photon has energy in proportion to its
wavelength, and the constant of proportionality is the Planck
constant. Below a certain frequency  and thus below a certain
energy  the incident photons do not have enough energy to knock
the photoelectrons out of the metal. Above that threshold energy,
called the workfunction, photons will knock the photoelectrons out
of the metal, in proportion to the number of photons (the
intensity of the light). At higher frequencies and energies, the
photoelectrons ejected obtain a kinetic energy corresponding to
the difference between the photon's energy and the workfunction.
 Planck constant; h

The fundamental constant equal to the ratio of the energy of a
quantum of energy to its frequency. It is the quantum of action.
It has the value 6.626 196 x 10^{34} J s.
 Planck constant, reduced; hbar

See Dirac constant.
 Planck equation

The quantum mechanical equation relating the energy of a photon
E to its frequency nu:
E = h nu.
 Planck radiation law

A law which described blackbody radiation better than its
predecessor, thus resolving the ultraviolet catastrophe.
It is based on the assumption that electromagnetic radiation is quantized.
For a blackbody at thermodynamic temperature T, the radiancy
R over a range of frequencies between nu and nu + dnu
is given by
R = 2 pi h nu^{3}/[c^{3} [exp (h nu/k T)  1]].
Compare RayleighJeans law.
 Poisson equation (S.D. Poisson)

The differential form of Gauss' law, namely,
div E = rho,
 Poisson spot (S.D. Poisson)

Poisson originally predicted the existence of
such a spot, and used the prediction to demonstrate how the wave
theory of light must be in error to produce such a
counterintuitive result. Subsequent observation of the Arago spot
provided a decisive confirmation of the wave nature of light.
 pseudoforce

A "force" which arises because an observer is naively treating
an accelerating frame as an inertial one.
Examples Coriolis pseudoforce,
centrifugal pseudoforce.

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