A-Level Physics Comprehensive Glossary

A comprehensive vocabulary review of core physics definitions, laws, units, and constants covering classical mechanics, astrophysics, electricity, medical imaging, thermal physics, and quantum phenomena.

Absolute zero

Zero on the thermodynamic temperature scale, or $0\,\text{K}$, where a substance has minimum internal energy, and is the coldest possible temperature. It is equal to $-273.15\,^\circ\text{C}$.

Absorption spectrum

A spectrum of dark lines across the pattern of spectral colours produced when light passes through a gas and the gas absorbs certain frequencies depending on the elements in the gas.

Acceleration of free fall ($g$)

The acceleration of a body falling under gravity ($9.81\,\text{m}\,\text{s}^{-2}$ on Earth).

Attenuation coefficient

A measure of how much an incident energy beam (ultrasound or x-ray) is attenuated through a medium, given by $I = I_0 e^{-\mu x}$.

Simple scattering

An attenuation mechanism in which a low-energy x-ray photon encounters an electron in an atom, causing it to be scattered without a change in energy.

Compton scattering

An attenuation mechanism where an X-ray is deflected from interaction with an electron, causing the wavelength of the photon to increase as well as the emittance of an electron from the atom at high speed.

Pair production

An attenuation mechanism where a high-energy x-ray photon interacts with the nucleus of an atom, causing the production of an electron ($e^-$) and a positron ($e^+$).

Astronomical Unit (AU)

The average distance from the Earth to the Sun, approximately equal to $1.5 \times 10^{11}\,\text{m}$.

Atomic mass unit ($u$)

A unit of mass equal to $\frac{1}{12}\text{th}$ the mass of one carbon-12 atom.

Archimedes' Principle

States that for an object submerged in a fluid, the up-thrust is equal to the weight of fluid that the object displaces.

Acoustic impedance ($Z$)

The product of the density ($\rho$) of a substance and the speed of ultrasound ($c$) in it ($Z = \rho c$). The fraction of ultrasound intensity reflected at a boundary depends on the acoustic impedance of both media.

Baryon

A hadron composed of $3$ quarks.

Brownian motion

The random motion of a larger particle in a gaseous system due to collisions with smaller particles.

Bremsstrahlung Radiation

A form of X-ray radiation (also known as braking radiation) produced when high-energy electrons interact with a nucleus.

Boyle's Law

A gas law stating that a gas's pressure is inversely proportional to its volume, provided that temperature is constant.

Boltzmann constant ($k$)

A constant describing the ratio between the ideal gas constant and Avogadro's constant ($\frac{R}{N_A}$), approximately $1.38 \times 10^{-23}\,\text{m}^2\,\text{kg}\,\text{s}^{-2}\,\text{K}^{-1}$.

Binding Energy

The minimum energy required to completely separate a nucleus into its constituent protons and neutrons, given by $\Delta E = \Delta m c^2$.

Binding energy per nucleon

A measure of nuclear stability. The greater the value, the more tightly bound the nucleons, and the more energy needed to break them apart.

Capacitance ($C$)

The amount of charge per volt that can be stored, measured in Farads ($\text{F}$).

Characteristic Radiation

A form of X-ray radiation produced when high-energy electrons interact with electrons in orbitals around a nucleus, causing a transition of another electron to fill the resulting gap.

Cosmological Principle

The principle stating that when viewed on a large scale, the universe is homogeneous and isotropic, and the laws of physics are universal.

Coulomb's Law

States that the electrostatic force between two charges is directly proportional to the product of the charges and inversely proportional to the distance between them squared.

Control rods

Rods in a nuclear reactor that absorb thermal neutrons to control the rate of reaction and maintain it at a steady rate.

Critical Angle

The angle of incidence beyond which no more refraction will occur, resulting only in total internal reflection.

Critical Temperature

The temperature at which the electrical resistance of a material falls to zero.

Charles' Law

A gas law stating that the volume of a gas is directly proportional to the temperature of the gas, provided pressure remains constant.

Drift velocity ($v$)

The average velocity of electrons travelling within a material, measured in $\text{m}\,\text{s}^{-1}$.

Ductility

The ability of a material to plastically deform under tensile forces without fracturing.

Doppler effect

The apparent shift in frequency of a wave caused by relative motion between the source and the observer.

Decay Constant

The probability of decay of a nucleus per unit time.

Electric field strength ($E$)

The force per unit charge that a positive point charge would experience at a specified point in an electrical field, measured in $\text{V}\,\text{m}^{-1}$ or $\text{N}\,\text{C}^{-1}$.

Electric potential ($V$)

The energy per unit charge that a positive point charge would require to move from infinity to a specified point in an electrical field, measured in Volts ($\text{V}$).

Electromotive Force (e.m.f)

The energy available to be transferred from other forms (e.g., chemical) into electrical energy, per unit charge.

Escape Velocity

The minimum velocity of an object required to escape the gravitational field of a larger body.

Faraday's Law

States that the emf induced in a conductor is directly proportional to the rate of change of magnetic flux linkage.

Gravitational Potential

The work done per unit mass to move an object from infinity to a specified point in a gravitational field, measured in $\text{J}\,\text{kg}^{-1}$.

Geostationary orbit

An orbit with a period equal to that of the Earth (i.e. equal angular velocity) that is positioned directly above the equator and rotates in the same direction as the Earth.

Hadron

A particle composed of quarks that interacts via the strong force.

Hooke's Law

An approximation stating that the extension of a spring is in direct proportion with the load added to it, as long as this load does not exceed the elastic limit.

Hubble's Law

States that the recessional velocity of a galaxy is almost directly proportional to the distance, $d$, from Earth ($v \approx H_0 d$).

Internal energy

The sum of random potential and kinetic energies of atoms within a system, measured in joules ($\text{J}$).

Kirchhoff's 1st Law

States that the sum of current entering a junction in a circuit equals the total current leaving it; represents the conservation of charge law.

Kirchhoff's 2nd Law

States that the sum of e.m.f's equals the sum of potential differences (p.d.'s) across components in a closed loop of a circuit; represents the conservation of energy law.

Kepler's 3rd Law

States that the square of the orbital period, $T$, of a planet is directly proportional to the cube of its average distance from the Sun.

Lenz's Law

States that an induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.

Lepton

A fundamental particle not composed of quarks that interacts via the weak force.

Magnetic flux density

The force acting per unit charge per unit length on a wire placed at right angles to the magnetic field, measured in Teslas ($\text{T}$).

Magnetic flux linkage

The product of magnetic flux and the number of turns in a current-carrying coil, measured in Weber Turns ($N\Phi$).

Mass defect ($\Delta m$)

The difference between the mass of the completely separated nucleons and the mass of the nucleus.

Meson

A hadron composed of a quark and an antiquark.

Newton's 2nd Law of motion

States that the resultant force acting on an object is directly proportional to the rate of change of momentum with respect to time, and acts in the same direction.

Number density ($n$)

The number of free charge carriers within a material per unit volume, with unit $\text{m}^{-3}$.

Parsec (pc)

The distance at which a radius of $1\,\text{AU}$ subtends an angle of one arcsecond, approximately equal to $3.1 \times 10^{16}\,\text{m}$.

Photoelectric Effect

The emission of photoelectrons from a material into a vacuum due to the absorption of electromagnetic radiation above the threshold frequency.

Piezoelectric effect

The electric charge that accumulates in piezoelectric materials when subjected to applied mechanical stress (e.g. sound waves).

Resistivity

A property of a material measuring its resistance to electric current, defined as the resistance a wire of the material would have if it had a cross-sectional area of $1\,\text{m}^2$ and a length of $1\,\text{m}$.

Refractive Index ($n$)

The ratio of the speed of light in a vacuum to the speed of light in a material, given by $n = \frac{c}{v}$, where $n > 1$.

Stefan's Law (Stefan-Boltzmann Law)

States that the total power radiated per unit surface area of a black body is directly proportional to the fourth power of absolute temperature ($L = 4\pi r^2 \sigma T^4$).

Specific Heat Capacity

The amount of heat energy required to raise the temperature of a substance per unit of mass by $1$ degree, without going through a change in state.

Specific Latent Heat of Fusion

The amount of heat energy required for a phase change of $1\,\text{kg}$ of the mass of a substance from solid to liquid or liquid to solid, without a change in temperature.

Specific Latent Heat of Vaporization

The amount of heat energy required for a phase change of $1\,\text{kg}$ of the mass of a substance from liquid to gas or gas to liquid, without a change in temperature.

Threshold frequency

The lowest frequency of electromagnetic radiation that will result in the emission of photoelectrons from a specified metal surface.

Time Constant

The time taken for current, charge, or voltage to fall to $\frac{1}{e}$ of its initial value.

Wien's Displacement Law

States that the product of the maximum intensity wavelength and the temperature of a body is constant ($\lambda_{\text{max}} T = \text{constant}$, where the constant is approximately $2.9 \times 10^{-3}\,\text{m}\,\text{K}$).

Work function energy ($\Phi$)

The minimum energy that is required for a material to release an electron, measured in joules ($\text{J}$).

Young Modulus

The ratio of tensile stress to tensile strain (stress per unit strain), with units of Pascals ($\text{Pa}$) or $\text{N}\,\text{m}^{-2}$.