The Complete
Physics Glossary
Essential definitions, laws, and concepts from Mechanics to Quantum Theory. Master the language of the universe.
A
Absolute Zero
The lowest theoretical temperature possible (-273.15°C or 0 Kelvin), where molecular motion essentially stops.
Acceleration
The rate of change of velocity of an object with respect to time.
Alpha Particle
A positively charged particle consisting of two protons and two neutrons, emitted during radioactive decay.
Angular Momentum
The rotational equivalent of linear momentum, conserved in a closed system.
Antimatter
Matter composed of antiparticles, which have the same mass as ordinary matter but opposite charge.
Atom
The basic unit of a chemical element, consisting of a nucleus surrounded by electrons.
B
Baryon
A subatomic particle, such as a proton or neutron, made of three quarks.
Bernoulli’s Principle
In fluid dynamics, an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure.
Big Bang
The prevailing cosmological model for the universe from the earliest known periods.
Black Body
An idealized physical body that absorbs all incident electromagnetic radiation.
Boson
A subatomic particle, such as a photon, that follows Bose-Einstein statistics.
Buoyancy
The upward force exerted by a fluid that opposes the weight of an immersed object.
C
Capacitance
The ability of a system to store an electric charge.
Centripetal Force
A force that acts on a body moving in a circular path and is directed toward the center.
Classical Mechanics
The branch of physics concerning the motion of bodies under the action of forces (Newtonian mechanics).
Conservation Law
A principle stating that a particular measurable property of an isolated system does not change with time.
Coulomb’s Law
A law stating that like charges repel and opposite charges attract.
D
Dark Matter
Hypothetical matter that constitutes 85% of the matter in the universe but does not interact with light.
Density
The mass per unit volume of a substance.
Diffraction
The process by which a beam of light or other system of waves is spread out as a result of passing through a narrow aperture.
Doppler Effect
An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other.
Dynamics
The branch of mechanics concerned with the motion of bodies under the action of forces.
E
Electric Field
A region around a charged particle or object within which a force would be exerted on other charged particles.
Electromagnetism
The interaction of electric currents or fields and magnetic fields.
Electron
A stable subatomic particle with a charge of negative electricity, found in all atoms.
Energy
The capacity to do work. Measured in Joules (J).
Entropy
A thermodynamic quantity representing the unavailability of a system’s thermal energy for conversion into mechanical work (measure of disorder).
F
Fermion
A subatomic particle, such as a nucleon or electron, that follows Fermi-Dirac statistics.
Force
An interaction that, when unopposed, will change the motion of an object.
Frequency
The rate at which something occurs or is repeated over a particular period of time. Measured in Hertz (Hz).
Friction
The resistance that one surface or object encounters when moving over another.
G
Gamma Ray
Penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei.
General Relativity
Einstein’s theory of gravitation, where gravity is a curvature of spacetime.
Gravity
The force that attracts a body toward the center of the earth, or toward any other physical body having mass.
H
Hadron
A subatomic particle of a type including the baryons and mesons that can take part in the strong interaction.
Half-life
The time taken for the radioactivity of a specified isotope to fall to half its original value.
Heat
Energy in transit from a high temperature object to a lower temperature object.
Higgs Boson
A subatomic particle responsible for the Higgs mechanism, by which particles acquire mass.
I
Inertia
A property of matter by which it continues in its existing state of rest or uniform motion unless changed by an external force.
Ion
An atom or molecule with a net electric charge due to the loss or gain of one or more electrons.
Isotope
Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons.
K
Kinetic Energy
Energy which a body possesses by virtue of being in motion.
Kinematics
The branch of mechanics concerned with the motion of objects without reference to the forces which cause the motion.
M
Magnetism
A physical phenomenon produced by the motion of electric charge, resulting in attractive and repulsive forces.
Mass
A measure of the amount of matter in an object, usually measured in grams or kilograms.
Matter
Physical substance in general, which occupies space and possesses rest mass.
Momentum
The quantity of motion of a moving body, measured as a product of its mass and velocity.
N
Neutrino
A neutral subatomic particle with a mass close to zero and half-integral spin, rarely reacting with normal matter.
Neutron
A subatomic particle of about the same mass as a proton but without an electric charge.
Newton’s Laws
Three laws describing the relationship between a body and the forces acting upon it, and its motion in response to those forces.
Nucleus
The positively charged central core of an atom, consisting of protons and neutrons.
O
Ohm’s Law
A law stating that electric current is proportional to voltage and inversely proportional to resistance.
Optics
The scientific study of sight and the behavior of light.
Oscillation
Movement back and forth at a regular speed.
P
Particle
A minute portion of matter.
Photon
A particle representing a quantum of light or other electromagnetic radiation.
Plasma
An ionized gas consisting of positive ions and free electrons.
Potential Energy
Energy possessed by a body by virtue of its position relative to others, stresses within itself, electric charge, etc.
Pressure
Continuous physical force exerted on or against an object by something in contact with it.
Proton
A stable subatomic particle occurring in all atomic nuclei, with a positive electric charge.
Q
Quantum Mechanics
The branch of mechanics that deals with the mathematical description of the motion and interaction of subatomic particles.
Quark
Any of a number of subatomic particles carrying a fractional electric charge, postulated as building blocks of hadrons.
R
Radiation
The emission of energy as electromagnetic waves or as moving subatomic particles.
Radioactivity
The emission of ionizing radiation or particles caused by the spontaneous disintegration of atomic nuclei.
Refraction
The fact or phenomenon of light, radio waves, etc. being deflected in passing obliquely through the interface between one medium and another.
Relativity
Albert Einstein’s theory of space and time.
Resistance
The refusal to accept or comply with something; the attempt to prevent something by action or argument.
S
Scalar
Having only magnitude, not direction.
Sound
Vibrations that travel through the air or another medium and can be heard when they reach a person’s or animal’s ear.
Speed of Light
The speed at which light travels in a vacuum (approx 299,792,458 m/s).
Spin
An intrinsic form of angular momentum carried by elementary particles.
Standard Model
A theory describing three of the four known fundamental forces (electromagnetic, weak, strong interactions) and classifying all known elementary particles.
Superconductivity
The property of zero electrical resistance in some substances at very low absolute temperatures.
T
Temperature
The degree or intensity of heat present in a substance or object.
Thermodynamics
The branch of physical science that deals with the relations between heat and other forms of energy.
Time Dilation
A difference in the elapsed time measured by two clocks, due to a relative velocity between them or to a difference in gravitational potential.
Torque
A twisting force that tends to cause rotation.
U
Uncertainty Principle
Heisenberg’s principle that the position and the velocity of an object cannot both be measured exactly, at the same time.
Universe
All existing matter and space considered as a whole.
V
Vacuum
A space entirely devoid of matter.
Vector
A quantity having direction as well as magnitude, especially as determining the position of one point in space relative to another.
Velocity
The speed of something in a given direction.
Viscosity
The state of being thick, sticky, and semifluid in consistency, due to internal friction.
Voltage
An electromotive force or potential difference expressed in volts.
W
Wave
A disturbance that travels through a medium, transporting energy from one location (its source) to another location without transporting matter.
Wavelength
The distance between successive crests of a wave, especially points in a sound wave or electromagnetic wave.
Weight
A body’s relative mass or the quantity of matter contained by it, giving rise to a downward force; the heaviness of a person or thing.
Work
Activity involving force and movement in the direction of the force.