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Standard physics glossary
Textbook physics terms. Not a BFUT reinterpretation. 371 terms. Textbook meanings only.
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25 terms. Page 1 of 1.
particle · intermediate
Pair Production
A process in which a high-energy photon converts into a particle and its corresponding antiparticle, most commonly an electron and a positron, typically occurring near an atomic nucleus that absorbs some of the photon's momentum. Pair production requires the photon to carry at least enough energy to account for the combined rest mass of the two created particles, following Einstein's mass-energy equivalence. It is the reverse process of particle-antiparticle annihilation.
astro · intro
Parallax
The apparent shift in the position of a nearby object against a distant background, caused by observing it from two different vantage points. Astronomers use parallax to measure distances to relatively nearby stars, by observing a star's apparent position from opposite sides of Earth's orbit around the Sun six months apart and using the known baseline distance to calculate the star's distance through simple trigonometry.
em · intermediate
Paramagnetism
A weak form of magnetism in which a material becomes weakly magnetized in the same direction as an applied external magnetic field, due to unpaired electrons whose magnetic moments partially align with the field. Unlike ferromagnetic materials, paramagnetic materials lose their magnetization almost immediately once the external field is removed, since thermal motion randomizes the alignment of the electron moments. Aluminum and oxygen are common examples of paramagnetic substances.
particle · intro
Particle Accelerator
A machine that uses electric and magnetic fields to accelerate charged particles, such as protons or electrons, to very high speeds and energies, then often collides them together or with a stationary target. Particle accelerators, including large facilities like the Large Hadron Collider, allow physicists to study the fundamental structure of matter and recreate conditions similar to those shortly after the early hot phase of the universe.
quantum · intermediate
Pauli Exclusion Principle
A fundamental rule of quantum mechanics, formulated by Wolfgang Pauli, stating that no two identical fermions, such as electrons within the same atom, can occupy exactly the same quantum state simultaneously. This principle explains the structure of the periodic table, since electrons must fill successively higher energy levels rather than all crowding into the lowest one, and it underlies the degeneracy pressure that supports white dwarf and neutron stars.
mechanics · intro
Pendulum
A weight suspended from a fixed point that swings freely back and forth under the influence of gravity, exhibiting periodic motion that, for small swing angles, closely approximates simple harmonic motion. The period of a simple pendulum depends only on its length and the local gravitational acceleration, not on the mass of the weight or, for small angles, the amplitude of the swing, a property first studied systematically by Galileo.
waves · intro
Period (Wave)
The time it takes for one complete cycle of a repeating wave or oscillation to occur, measured in seconds, and equal to the inverse of the wave's frequency. A wave with a longer period completes fewer cycles per second and therefore has a lower frequency. Period is used interchangeably across many types of periodic motion, from pendulums and orbits to alternating current and vibrating strings.
thermo · intro
Phase Change
A transition of a substance between different states of matter, such as solid, liquid, and gas, occurring at specific temperatures and pressures and typically involving the absorption or release of latent heat without a change in temperature during the transition itself. Common phase changes include melting, freezing, boiling, condensation, sublimation, and deposition, each governed by the balance between thermal energy and the intermolecular forces holding the substance together.
waves · intermediate
Phase Velocity
The speed at which a single point of constant phase, such as a wave crest, moves through space, calculated as the wave's frequency multiplied by its wavelength. For simple waves traveling through a uniform medium, phase velocity is constant, but in dispersive media, where different frequencies travel at different speeds, phase velocity can differ from the group velocity of an overall wave packet.
quantum · intro
Phosphorescence
A type of light emission similar to fluorescence, in which a substance absorbs energy and later re-emits it as light, but with a much longer delay, ranging from fractions of a second to many hours, because the excited electrons become temporarily trapped in a metastable energy state before eventually releasing their energy. Glow-in-the-dark materials rely on phosphorescence to continue emitting light well after the original light source is removed.
quantum · intro
Photoelectric Effect
The emission of electrons from a metal surface when light of sufficiently high frequency shines on it, a phenomenon that could not be explained by classical wave theories of light. Albert Einstein explained the effect in 1905 by proposing that light consists of discrete packets of energy called photons, and that an electron is emitted only if a single photon carries enough energy to overcome the metal's binding energy, regardless of the light's overall intensity.
quantum · intro
Photon
The elementary particle that carries the electromagnetic force and constitutes light and all other forms of electromagnetic radiation. Photons have no rest mass, always travel at the speed of light in a vacuum, and carry energy proportional to their frequency, as described by Planck's relation. Photons exhibit both particle-like and wave-like behavior, a duality central to quantum mechanics.
particle · intermediate
Pion
The lightest type of meson, composed of an up or down quark paired with an up or down antiquark, playing a historically important role as the particle proposed by Hideki Yukawa to explain the strong force that holds atomic nuclei together at short range. Pions come in three varieties, distinguished by their electric charge, and are unstable, decaying quickly into lighter particles such as muons and neutrinos.
cosmology · intro
Planck Satellite
A European Space Agency space observatory launched in 2009 that mapped the cosmic microwave background across the full sky with unprecedented precision, measuring tiny temperature and polarization fluctuations left over from the early universe. Data from the Planck satellite has provided some of the most precise measurements to date of key cosmological parameters within the standard Lambda-CDM model, including the universe's composition, geometry, and expansion history.
quantum · intro
Planck's Constant
A fundamental physical constant that sets the scale of quantum effects, relating the energy of a photon to its frequency, with an extremely small value of approximately 6.626 times ten to the minus thirty-four joule-seconds. Introduced by Max Planck in 1900 to explain blackbody radiation, this constant appears throughout quantum mechanics, including in the Heisenberg uncertainty principle and the de Broglie wavelength relation.
thermo · intermediate
Planck's Law
A formula, derived by Max Planck in 1900, that precisely describes the intensity of electromagnetic radiation emitted by a blackbody at each wavelength as a function of its temperature. By assuming that energy could only be emitted or absorbed in discrete packets, later called quanta, Planck's law successfully matched experimental observations where earlier classical theories had failed, marking the historical birth of quantum theory.
astro · intro
Planet
A celestial body that orbits a star, is massive enough for its own gravity to pull it into a roughly spherical shape, and has cleared the neighborhood around its orbit of other significant debris, according to the definition adopted by the International Astronomical Union in 2006. Planets are distinguished from smaller bodies like asteroids and dwarf planets, such as Pluto, which fails to meet the orbital-clearing criterion.
thermo · intro
Plasma
A state of matter consisting of a gas that has been heated or energized enough that a significant fraction of its atoms have been ionized, separating electrons from nuclei and creating a mix of free-moving charged particles. Plasma conducts electricity and responds strongly to electric and magnetic fields, unlike ordinary gases. It is often called the fourth state of matter and makes up the vast majority of visible matter in the universe, including the interiors and atmospheres of stars.
waves · intermediate
Polarization
A property of transverse waves, especially light, describing the orientation of the wave's oscillation relative to its direction of travel. Unpolarized light contains waves oscillating in all directions perpendicular to its motion, while polarized light has its oscillations restricted to a single plane or a specific rotating pattern. Polarizing filters, used in sunglasses and photography, block light oscillating in unwanted directions to reduce glare.
particle · intro
Positron
The antiparticle of the electron, having the same mass as an electron but a positive electric charge instead of a negative one. Positrons are produced in certain types of radioactive decay, in pair production from high-energy photons, and are used in medical imaging through positron emission tomography, commonly known as a PET scan. When a positron meets an electron, the pair annihilates into gamma-ray photons.
mechanics · intro
Potential Energy
Stored energy that an object possesses because of its position, configuration, or state, which has the potential to be converted into kinetic energy or another form of energy. Gravitational potential energy depends on an object's height within a gravitational field, while elastic potential energy depends on the deformation of a spring or similar material. Potential energy is always defined relative to a chosen reference point.
mechanics · intro
Power (Physics)
The rate at which energy is transferred, converted, or work is done, measured in watts, where one watt equals one joule of energy per second. Power is distinct from energy itself; a small amount of energy delivered very quickly, or a large amount delivered slowly, can both correspond to significant or modest power depending on the timescale involved. Power ratings are used to compare the performance of engines, electrical appliances, and biological systems alike.
mechanics · intro
Pressure
The amount of force applied perpendicular to a surface per unit area, measured in pascals, where one pascal equals one newton per square meter. Pressure explains why a sharp knife cuts more easily than a dull one, since the same force is concentrated over a much smaller area, and why deep-sea creatures can withstand the immense hydrostatic pressure found at great ocean depths. Pressure plays a central role in fluid mechanics, thermodynamics, and atmospheric science.
nuclear · intro
Proton
A subatomic particle with a positive electric charge equal in magnitude to that of the electron, found in the nucleus of every atom alongside neutrons, and defining the chemical identity of an element through its count, the atomic number. Protons are composed of three quarks, two up quarks and one down quark, bound together by the strong nuclear force via gluons, and are classified as baryons.
astro · intro
Pulsar
A rapidly rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles, which sweep across space like a lighthouse beam as the star spins. If one of these beams happens to point toward Earth periodically, the pulsar appears to flash, or pulse, at extremely regular intervals, sometimes hundreds of times per second. Pulsars were first discovered in 1967 and have since been used as extraordinarily precise natural clocks for testing general relativity.
