The laws list: P

particle-wave duality to pseudoforce.

parsec

 

The unit of distance defined as the distance indicated by an Earth-orbit parallax of 1 arcsec. It equals about 206 264 au, or about 3.086 x 10¹⁶ m.

 

pascal; Pa

 

The derived SI unit of pressure defined as 1 N acting over an area of 1 m²; it thus has units of N/m².

 

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 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 nu³/[c³ [exp (h nu/k T) - 1]].

Compare Rayleigh-Jeans 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.