Physics Glossary – P

Pauli Exclusion Principle

The principle that no two particles in the same quantum state may exist in the same place at the same time. Particles that obey this principle are called fermions; particles that do not are called bosons.

Photoelectric Effect

Is the flow of electric current in a material or through a vacuum (as in a photocell) when the material is exposed to light. Although the effect itself had been known for some time, Albert Einstein first described how it was caused by absorption of photons, or quanta of light, in the interaction of light with the electrons in the substance.


The layer of the Sun from which all visible light reaches us. The Sun is too hot to have a solid surface and the photosphere consists of a plasma at about 6000 degrees centigrade.


A Pion is a shortened form of the name Pi meson; this subatomic particle comes in three forms: the pi-zero (p0), pi-plus (p+) and pi-minus (p-). Pi mesons are the lightest mesons.


A gas containing free ions and electrons, and therefore capable of conducting electric currents. A “partially ionized plasma” such as the Earth’s ionosphere is one that also contains neutral atoms. The word plasma has a Greek root which means to be formed or molded.

Polar Orbit

A satellite orbit passing over both poles of the Earth. During a 12-hour day, a satellite in such an orbit can observe all points on Earth. Polar orbits are useful for viewing the planet’s surface. As a satellite orbits in a north-south direction, Earth spins beneath it in an east-west direction. As a result, a satellite in polar orbit can eventually scan the the entire surface.

Plank’s Constant

Planck’s constant, denoted h, named after the physicist Max Planck, is a physical constant which appears in all quantum mechanical equations. Its value is approximately h = 6.6261 × 10-34 Js.

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