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Online Physics Glossary / Terms / Dictionary

Physics Definitions Start from P

Phase transition - A process by which a systems changes from one state to another state with different properties, as a result of small changes in temperature or pressure.

Photon - A discrete quantity of electromagnetic energy. Short wavelength (high frequency) photons carry more energy than long wavelength (low frequency) photons. See Electromagnetic Radiation.

Photosphere - The visible surface of the Sun. It consists of a zone in which the gaseous layers change from being completely opaque to radiation to being transparent. It is the layer from which the light we actually see (with the human eye) is emitted.

Pion - A subatomic particle composed of a quark and an anti-quark. Not seen under everyday conditions, but produced at high temperatures or in reactions between atoms.

Plasma - Plasma consists of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of the gas are controlled by electromagnetic forces among constituent ions and electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state.

Poloidal Radius - The radius of the actual loop structure. For a doughnut, it is measured from the center to the edge of the pastry (not from the center of the hole)

Proton - A positively charged elementary particle. A proton is 1836 times heavier than an electron.

Pulsar - A neutron star (burnt-out star) that emits radio waves which pulse on and off.

P-Type Semiconductor - Semiconductor in which conduction is the result of motion of holes.

Pair Production - Formation of particle and antiparticle from gamma rays.

Parabolic Mirror - Mirror the shape of a paraboloid of revolution that has no spherical aberration.

Parallel Circuit - Circuit in which there are two or more paths for current flow.

Parallel Connection - Connection of two or more electrical devices between two points to provide more than one current path.

Pascal - SI unit of pressure; one neutron per square meter.

Pascal’s Principle - Pressure applied to a fluid is transmitted undiminished throughout it.

Period - Time needed to repeat one complete cycle of motion.

Periodic Motion - Motion that repeats itself at regular intervals of time.

Photoelectric Effect - Election of electrons from surface of metal exposed to electromagnetic radiation.

Photon - Quantum of electromagnetic waves; particle aspect of these waves.

Photovoltaic Cell - Device that converts electromagnetic radiation into electrical energy.

Physics - Study of matter and energy and their relationship.

Piezoelectricity - Electric potential produced by deforming material.

Pigment - Colored material that absorbs certain colors and transmits or reflects others.

Pitch - Perceived sound characteristics equivalent to frequency.

Planck’s Constant - Ratio of energy of photon to its frequency.

Plane Mirror - Flat, smooth surface that reflects light regularly.

Plasma - State of matter in which atoms are separated into electrons and positive ions or bare nuclei.

Point Object - Object idealized as so small to be located at only one position.

Polarized Light - Light in which electric fields are all in same plane.

Position - Separation between object and a reference point.

Position-Time Graph - Graph of object’s motion that shows how its position depends on clock reading, or time.

Positron - Antiparticle equivalent of electron.

Potential Difference - Difference in electric potential between two points.

Potential Energy - Energy of object due to its position or state.

Potentiometer - Electrical device with variable resistance; rheostat.

Power - Rate of doing work; rate of energy conversion.

Precision - Degree of exactness in a measurement.

Pressure - Force per unit area.

Primary Coil - Transformer coil that, when connected to voltage source, creates varying magnetic flux.

Primary Light Colors - Red, green, or blue light.

Primary Pigment - Yellow, Green or Magenta Light.

Principal Axis - Line connecting center of curvature of spherical mirror with its geometric vertex. Line perpendicular to plane of lens passing through its center.

Principle of Superposition - Displacement due to two or more forces is equal to vector sum of forces.

Projectiles - Motion of objects given initial velocity that then move only under force of gravity.

Proton - Subatomic particle with positive charge that is nucleus of hydrogen atom.

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