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
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
Pascal - SI unit of pressure; one neutron
per square meter.
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
Piezoelectricity - Electric potential produced by
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
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
Position-Time Graph - Graph of object’s motion that
shows how its position depends on clock reading, or time.
Positron - Antiparticle equivalent of
Difference - Difference in electric potential
between two points.
Potential Energy - Energy of object due to its
position or state.
Potentiometer - Electrical device with variable
Power - Rate of doing work; rate of
Precision - Degree of exactness in a
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.