In Augment’s notes, a marked term shows its definition when you hover or tap it, and each page ends with the terms it introduces.
660 terms from the Physics syllabus, each defined in a sentence or two, with the part of the syllabus where you first meet it. 131 of them are HL only, and marked.
Temperature measured from absolute zero, in kelvin: .
The uncertainty in a quantity expressed in the same units as the quantity, written as ± a value.
The lowest possible temperature, 0 K or −273 °C, at which the particles of a substance would have minimum (in the ideal gas model, zero) kinetic energy.
A continuous spectrum crossed by dark lines, produced when light passes through a cooler gas that absorbs photons whose energies match its energy-level differences.
A device that converts kinetic energy into electrical energy by rotating a coil in a magnetic field (or a magnet inside coils), producing an alternating emf.
Producing genuinely one's own work and acknowledging every idea, method, value, image or text taken from another source, including AI tools.
The rate of change of velocity, ; a vector quantity measured in .
The acceleration of a body falling under gravity alone, ; near the Earth's surface , directed downwards.
A graph of acceleration against time; the area between the line and the time axis is the change in velocity.
How close a measurement, or the mean of a set of measurements, is to the true value; it is limited by systematic errors.
The number of nuclei in a sample that decay per unit time, measured in becquerels (Bq).
A change in which no thermal energy is transferred to or from the gas (), so ; for a monatomic ideal gas is constant.
The total power scattered (reflected) by a surface or planet divided by the total power incident on it; a number from 0 to 1. The Earth's mean albedo is about 0.30.
A helium-4 nucleus (two protons and two neutrons) emitted by an unstable nucleus in alpha decay.
An emf that reverses direction periodically; a coil rotating at constant speed in a uniform field gives .
A meter that measures current, connected in series with the component; an ideal ammeter has zero resistance.
The number of particles in a sample divided by the Avogadro constant, ; measured in moles.
The maximum displacement from the equilibrium position, of an oscillating body or of the particles of a medium carrying a wave; symbol .
The angle between an incident ray and the normal at the point where the ray meets a boundary.
The rate of change of angular velocity, , measured in .
The angle through which a body has turned about an axis, measured in radians, with a sense (clockwise or anticlockwise).
times the frequency of an oscillation, , measured in .
The product of a torque and the time for which it acts, , equal to the change in angular momentum; the area under a torque–time graph.
The product of moment of inertia and angular velocity, ( for a particle), measured in .
The angle swept out by the radius per unit time for a body moving in a circle, , measured in ; it is related to the speed by .
The meeting of a particle and its antiparticle, which disappear and are replaced by energy; an electron and a positron at rest produce two 0.511 MeV γ photons travelling in opposite directions.
A neutral particle of almost zero mass, the antiparticle of the neutrino, emitted with the electron in β⁻ decay.
A point on a standing wave where the amplitude of oscillation is a maximum, midway between two adjacent nodes.
A particle with the same mass as a given particle but the opposite charge; the positron is the antiparticle of the electron and the antineutrino that of the neutrino.
Two oscillations of the same frequency are in antiphase when their phase difference is rad (half a cycle), so one is at its positive maximum when the other is at its negative maximum.
An opening, such as a gap or slit, through which a wave passes.
The point of an orbit around the Sun that is farthest from the Sun, where the orbiting body moves slowest.
The power received per unit area from a star at the observer, ; measured in .
The force with which a support (such as bathroom scales) pushes up on a body; it differs from the true weight when the support accelerates vertically.
A unit of angle equal to 1/3600 of a degree.
A body wholly or partly submerged in a fluid experiences an upward buoyancy force equal to the weight of the fluid it displaces, .
The mean distance between the Earth and the Sun, .
The fluid resistance a satellite meets from the thin upper atmosphere; it removes energy, so the orbit shrinks and the satellite's speed increases.
The range of infrared wavelengths, about 8–13 μm, that the atmosphere absorbs only weakly, so surface radiation in this range escapes directly to space.
Light emitted high in the atmosphere near the polar regions by atoms excited by charged particles from the Sun, guided there by the Earth’s magnetic field.
The displacement of a body divided by the time taken, ; on a displacement–time graph, the gradient of the chord between two times.
The number of particles in one mole of a substance, .
The line about which a body rotates; points on it do not move as the body turns.
Ionising radiation present everywhere from natural and artificial sources (radon, rocks, cosmic rays, food, medical uses), which must be subtracted from measured count rates.
The narrow region of a plot of neutron number against proton number where the stable nuclides lie; it follows for light nuclei and curves to for heavy ones.
Two or more cells connected in series.
The SI unit of activity: one decay per second.
A decay in which a neutron in the nucleus changes into a proton, and an electron (β⁻ particle) and an antineutrino are emitted.
A decay in which a proton in the nucleus changes into a neutron, and a positron (β⁺ particle) and a neutrino are emitted.
The energy needed to separate a nucleus completely into its individual protons and neutrons; equal to the mass defect times .
The binding energy of a nucleus divided by its nucleon number; the larger it is, the more tightly bound the nucleus.
An idealised object that absorbs all radiation falling on it and emits the maximum possible radiation at each wavelength, with a spectrum set only by its temperature.
A region of space where matter has collapsed so far that its gravity prevents anything, even light, from escaping; formed from the cores of the most massive stars.
A decrease in the observed wavelength (and increase in frequency) of electromagnetic waves from a source moving towards the observer.
Bohr's model of the hydrogen atom: the electron moves in circular orbits whose angular momentum is quantised, , and emits or absorbs a photon only when it jumps between orbits.
The radius of the smallest () orbit in the Bohr model of hydrogen, about m.
The change from liquid to gas throughout the liquid, with bubbles of vapour forming, at a fixed temperature for a given pressure.
The gas constant per molecule, .
The condition a standing wave must meet at the end of its medium: a node at a fixed or closed end, an antinode at a free or open end.
For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume: = constant.
The distance a vehicle travels from the moment the brakes are applied until it stops; for a constant deceleration it is proportional to the square of the initial speed.
The random, jerky movement of small particles suspended in a fluid, caused by uneven bombardment by the fluid's molecules.
The upward force of a fluid on a body in it, equal to the weight of fluid displaced: .
A reversible cycle of two isothermal and two adiabatic changes; the most efficient possible cycle between two reservoir temperatures.
The maximum efficiency of a heat engine between reservoirs at and : , with temperatures in kelvin.
The bright region at the centre of a single-slit diffraction pattern, twice as wide as each side maximum and much brighter.
The point at which the whole mass of a body can be treated as concentrated; the weight of a body acts through it.
The acceleration of a body moving in a circle, directed towards the centre: .
The resultant force on a body moving in a circle, directed towards the centre and perpendicular to the velocity: . It is provided by real forces such as tension, friction or gravity.
A sequence of fissions in which neutrons released by one fission cause further fissions; it is self-sustaining when, on average, one neutron from each fission causes another.
The largest mass, about 1.4 solar masses, that a white dwarf can have while being supported by electron pressure.
A charged particle that is free to move through a material and so carry a current, such as a free electron in a metal or an ion in an electrolyte.
The charge of a particle divided by its mass, , in ; it sets the radius and period of the particle’s path in a magnetic field.
Charging a conductor by touching it with a charged object, so that some of the charge spreads on to it; the conductor gains charge of the same sign.
Charging two different materials by rubbing them together, which transfers electrons from one to the other and leaves them with equal and opposite charges.
For a fixed mass of gas at constant pressure, volume is proportional to absolute temperature: = constant.
A source of emf that converts chemical energy into electrical energy through reactions at its two electrodes.
A drawing of how components are connected, using standard symbols and straight wires, without showing the physical layout.
The form of the second law stating that thermal energy cannot spontaneously transfer from a colder region to a hotter one.
A pipe closed at one end and open at the other; the closed end is a displacement node, the first harmonic has and only odd harmonics are present.
A system that can exchange energy with its surroundings but not matter, such as a gas sealed in a cylinder with a movable piston.
The ratio of the friction force to the normal force for two surfaces: for the maximum static friction, while sliding. It has no unit.
Describes waves or sources with the same frequency and a constant phase difference.
The instruction word that starts a question part (state, calculate, explain, discuss …) and defines the kind and depth of answer expected.
One of two perpendicular vectors that together add up to a given vector; a vector at angle to the horizontal has components and .
A region of a longitudinal wave where the particles of the medium are closer together than normal, so the pressure is higher.
The scattering of an X-ray or gamma-ray photon by a free or loosely bound electron, in which the photon loses energy, so its wavelength increases, and the electron recoils.
The increase in wavelength of a Compton-scattered photon: .
The constant , equal to the Compton shift for a photon scattered through 90° by an electron.
The change from gas to liquid, during which energy equal to the latent heat of vaporization is released.
The transfer of thermal energy through a material, without bulk movement of the material, by collisions between particles and, in metals, by free electrons.
A material with a large number of mobile charge carriers, so that it carries a large current for a small pd.
The total angular momentum of a system about an axis stays constant if no resultant external torque acts on it about that axis.
The principle that the total electric charge of an isolated system stays constant: charge can be transferred but never created or destroyed.
The total momentum of a system stays constant provided no resultant external force acts on it.
A force, such as gravity or an elastic force, whose work on a body moving between two points depends only on those points and not on the path taken.
Superposition of waves that meet in phase, giving a resultant amplitude larger than either wave's; for in-phase sources it occurs where the path difference is .
A force that acts only while two bodies touch, such as the normal force, friction, tension, an elastic force, fluid resistance or buoyancy.
A spectrum containing all wavelengths over a wide range with no gaps, emitted by a hot, dense object such as a lamp filament or a star's interior.
A rod of a strongly neutron-absorbing material, such as boron or cadmium, moved into or out of a reactor core to control the rate of fission.
A variable that could affect the dependent variable and is therefore kept constant (or monitored) so that only the independent variable changes.
The transfer of thermal energy by the bulk movement of a fluid, driven by differences in density between warmer and cooler parts of the fluid.
A continuous circulation of fluid in which warmer, less dense fluid rises and cooler, denser fluid sinks, carrying energy round the loop.
The direction of current used in circuit diagrams: the direction positive charge would flow, from the positive terminal of a source round the circuit to the negative terminal. Electrons in a metal move the opposite way.
A fluid pumped through a reactor core to carry away the energy released by fission.
The SI unit of charge: the charge that passes a point when a current of 1 A flows for 1 s.
The electrostatic repulsion that two positively charged nuclei must overcome to get close enough for the strong nuclear force to bind them.
The constant in Coulomb's law, .
The electric force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation: .
The number of decays registered by a detector per unit time; proportional to the activity for a fixed arrangement, but smaller than it.
Two equal and opposite parallel forces whose lines of action are separated; it produces a torque about any axis but no resultant force.
A point of maximum positive displacement on a transverse wave.
The angle of incidence, in the medium where a wave is slower, for which the angle of refraction is 90°. For light, .
The amount of damping at which a displaced system returns to equilibrium in the shortest time without oscillating.
The smallest mass of fissile material, in a given shape and arrangement, in which a chain reaction is self-sustaining.
Uniform electric and magnetic fields at right angles to each other, arranged so that their forces on a moving charge act in opposite directions.
A part of a vehicle designed to fold up in a controlled way in a collision, lengthening the stopping time and so reducing the average force on the occupants.
A pivoted frame that measures a magnetic flux density by balancing the magnetic force on a current-carrying wire against the weight of a known mass.
One complete oscillation, ending when the body is next at the same position moving in the same direction.
A series of changes that returns a gas to its starting state, so and the net work equals the area enclosed on a P–V diagram.
The loss of energy from an oscillating system to its surroundings because of resistive forces, which makes the amplitude decrease.
The booklet of equations, constants, prefixes, unit conversions and circuit symbols provided for every physics paper.
An organised, published collection of measurements from which an investigator selects data to answer a research question.
The nucleus left after a parent nucleus has decayed.
The wavelength of a particle of momentum : .
The constant in : the fraction of undecayed nuclei that decay per unit time, related to the half-life by .
The energy released by the radioactive decay of fission products, which continues after a reactor's chain reaction has stopped.
A sequence of radioactive decays in which each daughter is itself unstable, ending at a stable nuclide.
A store for long-lived radioactive waste, hundreds of metres underground in stable rock, with several barriers between the waste and groundwater.
Mass per unit volume, ; measured in .
The variable that is measured to find how it responds to changes in the independent variable.
A unit formed by multiplying and dividing base units according to the defining equation of a quantity, such as the newton, .
Superposition of waves that meet in antiphase, so that their displacements cancel; for in-phase sources it occurs where the path difference is .
The spreading of a wave as it passes through an aperture or past the edge of an obstacle; greatest when the aperture is about the same size as the wavelength.
A large number of identical, equally spaced parallel slits, which produces very sharp maxima at angles given by .
A current that flows in one direction only, such as the current from a cell or battery.
Describes two quantities whose ratio is constant: a graph of one against the other is a straight line through the origin.
A tube of gas at low pressure that glows when a high potential difference is applied across it, giving the emission spectrum of the gas.
The change in position of a body, from its initial to its final position; a vector quantity measured in metres.
A graph of displacement (or position) against time; its gradient at any time is the velocity.
The spreading of energy into the surroundings, mostly as internal energy, in a form that can no longer do useful work.
The total length of the path travelled by a body; a scalar quantity measured in metres.
The minimum separation of a charged particle from a nucleus in a head-on collision, where all its initial kinetic energy has become electric potential energy: .
A wrong option in a multiple-choice question, designed so that a specific, common mistake leads to it.
The change in the observed frequency and wavelength of a wave when the source and the observer move relative to each other.
A medical technique that measures the speed and direction of blood flow from the frequency shift of ultrasound reflected by moving blood cells.
The forward force that propels a vehicle, exerted by the road on the driven tyres. At constant velocity it equals the total resistive force.
The frequency of the periodic external force acting on an oscillating system.
Friction between surfaces that are sliding over each other, ; usually less than the maximum static friction.
Connecting a conductor to Earth so that electrons can flow between them, which neutralises the conductor or removes charge of one sign from it.
Loops of induced current that flow inside a solid piece of metal when the magnetic flux through it changes; they oppose the change and dissipate energy in the metal.
The curving and spreading of field lines near the ends of parallel plates, where the field is weaker and no longer uniform.
The ratio of useful work (or power) output to total work (or power) input, ; it has no unit and can never exceed 1 (100 %).
: the maximum kinetic energy of a photoelectron equals the energy of the absorbed photon minus the work function of the metal.
A collision in which both momentum and the total kinetic energy of the bodies are conserved.
The energy stored in a body because it is deformed. For a spring obeying Hooke's law, .
A scalar property of particles such as protons and electrons that makes them exert electric forces on each other; it is positive or negative and measured in coulombs (C).
A closed conducting path through which charge can flow, containing a source of emf and components that transfer energy.
The rate of flow of charge past a point, ; measured in amperes (A), where .
A region in which a charge experiences an electric force.
The electric force per unit charge on a small positive test charge, ; a vector measured in or .
The work done per unit charge in bringing a small positive test charge from infinity to a point, for a point charge; a scalar measured in volts.
The work done to assemble a system of charges from infinite separation; for two point charges , positive for like charges and negative for unlike charges.
The sudden conduction of an insulator such as air when the electric field becomes strong enough to ionise it, producing a spark; for dry air about .
The production of an emf in a circuit when the magnetic flux linking it changes.
The continuous range of electromagnetic waves ordered by wavelength: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves.
A transverse wave of electric and magnetic fields oscillating in phase, perpendicular to each other and to the direction of travel; it travels through a vacuum at .
The spreading and interference of an electron beam passing through or reflecting off a crystal, which shows that electrons have a wave nature.
A device that produces a narrow beam of electrons: electrons released from a heated cathode are accelerated through a potential difference towards an anode and pass through a hole in it.
A unit of energy: the energy transferred when one electron moves through a pd of 1 V; .
The separation of charge in a conductor caused by a nearby charged object; with earthing it charges the conductor, without contact, with the opposite sign to the object.
The magnitude of the charge on one electron or proton, .
A closed oval curve with two points inside it called foci; planetary orbits are ellipses with the Sun at one focus.
The energy transferred from other forms into electrical energy per unit charge passing through a source, symbol ; measured in volts.
A set of bright lines at particular wavelengths, produced by a hot gas at low pressure; each line comes from electrons dropping between two energy levels.
The power radiated per unit area by a surface divided by the power per unit area a black body at the same temperature would radiate, ; a number from 0 to 1.
A law found from experimental measurements that describes a pattern without explaining its cause.
The capacity of a system to do work; a scalar quantity measured in joules (J) that is conserved in every process.
The condition in which a planet (or part of it) radiates away energy at the same rate as it absorbs it, so its temperature stays constant.
The energy released by a fuel per unit volume, measured in .
One of the discrete values of energy that an electron in an atom is allowed to have.
The additional warming of the Earth's surface caused by increases in greenhouse gas concentrations due to human activity, mainly the burning of fossil fuels.
Uranium in which the fraction of uranium-235 has been raised above the natural 0.7%, to about 3–5% for most power reactors.
A property of the state of a system that measures its disorder; it changes by and equals ; measured in .
The four equations linking , , , and for a body moving in a straight line with uniform acceleration.
The position where the resultant force on an oscillating body is zero; displacement in an oscillation or a wave is measured from it.
The temperature at which a planet radiates as much power as it absorbs; without an atmosphere it satisfies .
A surface on which every point has the same potential; no work is done moving along it, and field lines cross it at right angles.
A line through a plotted point that extends by the absolute uncertainty on each side, showing the range of the value.
The marking rule by which a wrong value is penalised only once: later work that uses it with a correct method still earns its marks.
The minimum launch speed for an unpowered object to escape completely from a body's gravitational field, .
The change from liquid to gas at the surface of a liquid, at any temperature, as the fastest molecules escape; it cools the remaining liquid.
Something that happens at a single point in space at a single instant, described in a frame by one position and one time coordinate.
Any energy level of an atom above the ground state; an atom in an excited state soon drops to a lower level and emits a photon.
A planet orbiting a star other than the Sun; many are found from the periodic Doppler shift of their star's spectral lines.
An interaction in which bodies fly apart from rest or from each other; momentum is conserved while kinetic energy increases at the expense of another store.
A conducting enclosure; because the electric field inside a conductor's cavity is zero, it shields its contents from external electric fields.
The emf induced in a circuit equals the rate of change of the magnetic flux linkage through it: .
A force that acts across a distance without contact, even through a vacuum: gravitational, electric and magnetic forces.
A line showing the direction of the force on a test mass (or charge) at each point; the closer the lines, the stronger the field.
A lamp in which a thin tungsten wire glows when heated by a current; its resistance rises as it heats up, so it is non-ohmic.
A glass bulb containing an electron gun and gas at low pressure, in which an electron beam bent into a circle by a magnetic field is made visible by the glowing gas.
The standing wave of lowest frequency that a system can support.
Conservation of energy for a closed system: , where is thermal energy supplied to the system and is work done by it.
The first dark direction either side of the centre of a single-slit pattern, at (radians, small angles), where is the slit width.
Describes a nuclide, such as uranium-235 or plutonium-239, that can undergo fission after absorbing a slow (thermal) neutron.
A nucleus formed when a heavy nucleus splits; fission products are neutron-rich and so are usually radioactive β⁻ emitters.
An end of a string that cannot move, such as a clamp; it is always a node of a standing wave.
A rule for the direction of the magnetic force: with the first finger along the field and the second finger along the conventional current, the thumb points along the force.
The force exerted by a gas or liquid on a body moving through it, opposite to the body's velocity relative to the fluid; it increases with speed.
The product of the number of turns of a coil and the magnetic flux through each turn; measured in webers.
A heavy wheel with a large moment of inertia, used to store energy as rotational kinetic energy or to smooth out changes in rotation.
A push or a pull exerted by one body on another; a vector quantity measured in newtons (N).
An oscillation maintained by a periodic external force; in the steady state the system oscillates at the driving frequency.
A fuel formed over millions of years from the remains of living things, such as coal, oil or natural gas; burning it releases carbon dioxide.
The absolute uncertainty divided by the value, ; it has no unit.
The dark absorption lines in the spectrum of the Sun, produced by elements in its cooler outer layers.
An end of a string that can move freely, such as a light ring sliding on a smooth rod; it is always an antinode of a standing wave.
Motion under the influence of gravity alone, with no other force (such as fluid resistance) acting.
A diagram of one body on its own showing every force that acts on it as a labelled arrow, with lengths roughly in proportion to the forces.
The change from liquid to solid at the melting point, during which energy is released at constant temperature.
The number of oscillations or cycles per unit time; symbol , measured in hertz (Hz), with .
The component of the contact force parallel to two surfaces, which opposes their relative sliding or their tendency to slide.
One of the bright or dark bands in an interference or diffraction pattern.
The distance between the centres of neighbouring bright (or dark) fringes; for a double slit, .
A substance that releases energy in a controlled way, usually by burning or by nuclear reactions.
A sealed metal tube filled with pellets of nuclear fuel (usually uranium dioxide), which keeps the fission products inside.
The principle that the laws of mechanics (Newton's laws) are the same in all inertial reference frames, with time the same for all observers.
The equations and linking the coordinates of an event in two inertial frames in Newtonian physics, with velocities related by .
A sensitive ammeter, usually with its zero in the centre of the scale, used to detect small currents in either direction.
High-energy photons emitted when a nucleus falls from an excited state to a lower energy state; the numbers of protons and neutrons do not change.
The pressure of a fluid above atmospheric pressure, as read by a tyre gauge; absolute pressure = gauge pressure + atmospheric pressure.
A detector of ionising radiation: each particle or photon that ionises the gas inside produces a pulse of current, which a counter records.
A sensor that detects ground vibrations: relative motion between a coil and a magnet, one of them held by a spring, induces an emf proportional to the speed of the ground.
A circular equatorial orbit with a period of one day, travelling west to east, so the satellite stays above the same point on the Earth; it is about 36 000 km above the surface.
The distance between neighbouring slits of a diffraction grating: for lines per metre.
The constant in Newton's law of gravitation, .
A region of space in which a mass experiences a force because of its mass.
The gravitational force per unit mass at a point, in ; near the Earth's surface.
The work done per unit mass to bring a small test mass from infinity to a point, ; a scalar measured in , zero at infinity.
The energy stored by a mass because of its position in a gravitational field. Near the Earth's surface its change is .
The warming of a planet's surface by atmospheric gases that let through short-wavelength sunlight but absorb the infrared emitted by the surface and re-emit it in all directions, partly back to the surface.
A gas in the atmosphere, such as water vapour, carbon dioxide, methane or nitrous oxide, that absorbs and emits infrared radiation.
A body whose emissivity is less than 1 but about the same at all wavelengths, so it radiates a fixed fraction of the black-body power at its temperature.
The lowest energy level of an atom, the state in which it is most stable.
The time taken for the number of undecayed nuclei of a nuclide, and so the activity of a sample, to fall to half its initial value.
One of the standing waves a system can support; the nth harmonic has a frequency n times that of the first harmonic.
The energy needed to raise the temperature of a particular object by 1 K, equal to ; measured in .
A device that runs a cycle, taking thermal energy from a hot reservoir, converting part of it to work and rejecting the rest to a cold reservoir.
A device that transfers energy from the hot primary coolant to water in a separate loop, turning it into steam, without the two fluids mixing.
A device that uses work to transfer thermal energy from a cold reservoir to a hot one, used to heat buildings.
A graph of temperature against time for a substance heated at constant power: sloping sections where one phase warms, flat sections where the phase changes.
A resistor, usually of high-resistivity wire such as nichrome, designed to transfer electrical energy to its surroundings as thermal energy.
Damping greater than critical: the system does not oscillate and returns to equilibrium more slowly than with critical damping.
A graph of stellar luminosity (vertical, log scale) against surface temperature (horizontal, log scale, decreasing to the right), on which stars fall into groups such as the main sequence, giants and white dwarfs.
An equation in which every term has the same units; any physically correct equation is homogeneous.
The elastic restoring force of a spring is proportional to its extension and opposite in direction, , up to the limit of proportionality.
A graph of the current through a component against the pd across it.
A positive feedback in which warming melts ice and snow, exposing darker ocean and land that absorb more sunlight, which causes further warming.
A gas that obeys exactly; modelled by kinetic theory as point molecules with no forces between them except in elastic collisions.
The equation linking the pressure, volume, temperature and amount of an ideal gas.
A meter that does not change the circuit it measures: an ideal ammeter has zero resistance and an ideal voltmeter infinite resistance.
The perpendicular distance between the line along which a particle approaches and a parallel line through the centre of the nucleus; the smaller it is, the larger the deflection.
The product of the average resultant force on a body and the time it acts, ; it equals the change in momentum and the area under a force–time graph.
Two oscillations of the same frequency are in phase when their phase difference is zero, so they reach their maxima at the same time.
The variable that the investigator deliberately changes (or selects) to see its effect on another variable; it is plotted on the horizontal axis.
A current driven round a complete circuit by an induced emf, .
A cooker in which a rapidly alternating magnetic field from a coil induces eddy currents in the metal base of a pan, heating the pan directly.
A collision in which momentum is conserved but kinetic energy decreases; in a perfectly inelastic collision the bodies stick together.
The tendency of a body to resist changes to its velocity; the mass of a body is a measure of its inertia.
A reference frame that is not accelerating, in which a body with no resultant force on it stays at rest or moves with constant velocity.
Electromagnetic radiation with wavelengths from about 700 nm to 1 mm, longer than visible light; bodies at everyday temperatures emit mostly infrared.
A narrow, nearly vertical band of the HR diagram, at surface temperatures of roughly 5000–8000 K, in which evolved stars pulsate, regularly changing in size and luminosity.
The velocity of a body at one instant; on a displacement–time graph, the gradient of the tangent at that time.
A material with almost no mobile charge carriers, so that the current through it is negligible.
The power carried by a wave per unit area perpendicular to its direction of travel, in ; for a given frequency it is proportional to the square of the amplitude.
The superposition of waves from coherent sources, giving a steady pattern of maxima and minima.
The forces between the particles of a substance: attractive at moderate separations and strongly repulsive when particles are pushed very close together.
The total of the random kinetic energies and intermolecular potential energies of the particles in a substance.
The resistance of the materials inside a source of emf, symbol , which causes the terminal pd to fall below the emf when a current flows.
A quantity that has the same value in every inertial reference frame, such as the speed of light or the space-time interval.
A curve on a space-time diagram joining events with the same interval from the origin, used to mark equal units on the axes of different frames.
A relationship in which a quantity is inversely proportional to the square of the distance from a point source, as when radiation spreads over a sphere of area .
The minimum energy needed to remove an electron from an atom in its ground state; 13.6 eV for hydrogen.
Radiation with enough energy to remove electrons from atoms it passes, leaving ions; α, β and γ radiation are ionising.
A real process that cannot be exactly undone, because of friction, free expansion or heat flow across a temperature difference; it increases the total entropy.
A change at constant pressure, in which the work done by the gas is .
Describes an oscillation whose period does not depend on its amplitude, as in simple harmonic motion.
A system on which no resultant external force acts, so its total momentum stays constant.
A curve on a P–V diagram joining states of a gas at the same temperature; for an ideal gas, = constant along it.
A change at constant temperature; for an ideal gas , so .
One of two or more nuclides of the same element: the same proton number but different numbers of neutrons, and so different nucleon numbers.
A change at constant volume, in which the gas does no work, so .
The SI unit of work and energy: the work done when a force of 1 N moves its point of application 1 m in the direction of the force. .
The continuous record of atmospheric carbon dioxide concentration measured at Mauna Loa Observatory, Hawaii, since 1958.
The form of the second law stating that no heat engine working in a cycle can take thermal energy from a single reservoir and convert it completely into work.
Each planet moves in an ellipse with the Sun at one focus.
The line joining a planet to the Sun sweeps out equal areas in equal intervals of time, so the planet moves fastest when closest to the Sun.
For bodies orbiting the same central mass, the square of the orbital period is proportional to the cube of the semi-major axis (the radius for a circular orbit): .
A unit of energy: the energy transferred at a power of 1 kW for one hour, equal to .
The description of motion in terms of position, velocity and acceleration, without considering its causes.
The energy a body has because of its motion: .
The model of a gas as a very large number of tiny molecules in random motion, colliding elastically, with no forces between them except in collisions.
The angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.
The shortening of a moving object along its direction of motion: , where is the proper length.
The direction of an induced emf is such that the current it would produce opposes the change in flux that causes it; a consequence of the conservation of energy.
The region of a space-time diagram bounded by the 45° light lines through an event, containing all the events it can affect or be affected by.
Damping small enough that the system still oscillates, with an amplitude that decreases gradually.
A semiconductor resistor whose resistance falls as the intensity of light falling on it rises.
A semiconductor device that conducts in one direction only, once the pd exceeds a threshold, and emits light when it conducts.
The distance light travels in a vacuum in one year, ; in light-years and years, .
The largest extension (or force) up to which the force on a spring or wire is proportional to its extension.
A feature of an investigation that restricts how far its conclusion applies, such as a narrow range, a single material or an untested assumption.
The straight line through the point where a force acts, in the direction of the force.
A straight line or smooth curve drawn through plotted data to show their trend, with the points spread evenly on either side and passing through the error bars.
A straight line on an HR diagram joining stars of equal radius; since , it slopes down from top left to bottom right.
The straight line joining an observer and the body observed; only motion along it produces a Doppler shift.
The product of the mass and the velocity of a body, ; a vector measured in or N s.
Rearranging a predicted relationship into the form and plotting the matching quantities, so that the relationship gives a straight line if it holds.
A published, traceable value of a quantity, from a data book, reliable database or peer-reviewed source, used to judge the accuracy of an experimental result.
A graph of against ; a straight line shows a power law , with gradient .
An axis on which equal distances represent equal factors, usually powers of ten, used for data spanning many orders of magnitude.
A wave in which the particles of the medium oscillate parallel to the direction of energy transfer.
The part of a standing wave between two adjacent nodes; it is half a wavelength long and all its points oscillate in phase.
, the factor by which times are dilated and lengths contracted between frames moving at relative speed ; always at least 1.
The equations and linking the coordinates of an event in two inertial frames in special relativity.
An orbit a few hundred to about two thousand kilometres above the Earth's surface, with a period of roughly 90 to 120 minutes.
The total power radiated by a star or other body, in watts; for a black body .
The state of a system described by its bulk properties, such as pressure, volume and temperature, or the total number of heads in a set of coins.
A region in which a magnet, a magnetic material or a moving charge experiences a magnetic force.
A line showing the direction in which the north pole of a small compass would point at each point; the lines form closed loops, running from N to S outside a magnet.
For a flat area in a uniform field, , where is the angle between the field and the normal to the area; measured in webers.
The force per unit charge per unit speed on a charge moving at right angles to a magnetic field, ; a vector measured in tesla.
The force a magnetic field exerts on a moving charge, , perpendicular to both the velocity and the field.
One of the two ends of a magnet, north or south, where its field is most concentrated; like poles repel and unlike poles attract, and poles always come in pairs.
The band on the HR diagram, from hot luminous stars to cool dim ones, of stars fusing hydrogen into helium in their cores; stars spend most of their lives on it.
One level of performance in an assessment criterion, described in words together with the marks it can earn.
The difference between the total mass of the separated nucleons of a nucleus and the mass of the nucleus itself.
The mass of matter passing a point per unit time, , in ; the force needed to change the velocity of a stream of matter by is .
An instrument that separates ions by their charge-to-mass ratio, using the different radii of their paths in a magnetic field, to measure the masses and abundances of isotopes.
The principle that mass and energy are equivalent, : any change in the energy of a system changes its mass by .
A mass attached to a spring that oscillates with simple harmonic motion of period .
The wave associated with a moving particle, whose wavelength is the de Broglie wavelength .
The steepest and least steep straight lines that pass through every error bar on a graph; half the difference of their gradients is the uncertainty in the gradient.
The solar power per unit area averaged over a planet's whole surface, , because the planet intercepts sunlight over a disc of area but has a surface area .
The sum of the kinetic, gravitational potential and elastic potential energy of a system. It is conserved when no friction or other resistive force does work.
A wave that needs a material medium whose particles oscillate to pass on the disturbance, such as sound, water waves or waves on a string.
The material through which a mechanical wave travels, whose particles oscillate as the wave passes.
The change from solid to liquid at the melting point, during which energy is absorbed at constant temperature.
A way of finding a specific heat capacity, latent heat or temperature by mixing a hot and a cold substance in an insulated container and equating the energy lost by one to the energy gained by the other.
A feature of a method that reduced the accuracy or precision of the data, such as heat loss in calorimetry or variable contact resistance.
The way data are collected or selected to answer a research question, together with the reasons for each choice of technique, apparatus, range and number of measurements.
An 1887 interferometer experiment that looked for a change in the speed of light with the direction of the Earth's motion and found none.
One particular arrangement of all the individual particles of a system that produces a given macrostate; the number of microstates is .
An experiment (1909–1913) in which charged oil drops were held stationary in a uniform electric field, showing that charge is quantised and measuring the elementary charge.
A double-slit or grating maximum that does not appear because it falls on a minimum of the single-slit envelope; for a double slit it happens when equals the order .
Re-marking of a sample of a school's internal assessment by an external examiner, so that every school's marks follow the same standard.
A material of light nuclei, such as water or graphite, that slows fast neutrons to thermal speeds by elastic collisions so that they are more likely to cause fission.
The constant in the ideal gas equation .
The mass of one mole of a substance, ; usually quoted in .
The SI unit of amount of substance: one mole contains exactly particles, about .
A measure of a body's resistance to angular acceleration about an axis, depending on how its mass is distributed about that axis; for point masses, measured in .
Made of single atoms rather than molecules of several atoms, as in helium, neon and argon.
Describes light (or another wave) of a single wavelength and frequency.
The force on a current-carrying conductor in a magnetic field, , used in electric motors and loudspeakers.
An unstable subatomic particle, like a heavy electron, with a mean lifetime of at rest; muons made by cosmic rays provide evidence for time dilation.
The frequency at which a system oscillates when displaced and left to oscillate freely, with no driving force.
A huge cloud of gas, mostly hydrogen and helium, and dust in space, in which stars form.
A process in which a change causes effects that reduce the original change, such as a warmer surface radiating more energy away.
A point where the fields of two or more bodies cancel, so the resultant field strength is zero.
A neutral particle of almost zero mass that interacts extremely weakly with matter; emitted with the positron in β⁺ decay.
The number of neutrons in a nucleus, .
The extremely dense remnant core of a massive star after a supernova, about 20 km across, made mostly of neutrons and supported by the pressure of the closely packed neutrons.
Fission that happens after a heavy nucleus, such as uranium-235, absorbs a neutron and becomes highly unstable.
The ratio of the numbers of neutrons and protons in a nucleus; stable nuclei have values from about 1 (light) to about 1.5 (heavy).
The SI unit of force: the resultant force that gives a mass of 1 kg an acceleration of , so .
A body stays at rest or moves with constant velocity in a straight line unless a resultant external force acts on it.
Every point mass attracts every other with a force proportional to the product of the masses and inversely proportional to the square of their separation: .
The resultant force on a body equals its rate of change of momentum, ; for constant mass, .
The resultant torque on a rigid body equals its moment of inertia multiplied by its angular acceleration, .
When body A exerts a force on body B, B exerts a force of the same type on A that is equal in size and opposite in direction.
A point on a standing wave where the displacement is always zero. Adjacent nodes are half a wavelength apart.
A force, such as friction or fluid resistance, whose work depends on the path taken and which transfers mechanical energy to internal energy.
A component whose resistance changes with the current or pd, such as a filament lamp or an LED.
A line drawn perpendicular to a surface or boundary at the point where a ray meets it. Angles of incidence, reflection and refraction are measured from it.
The component of the contact force between two surfaces that is perpendicular to them, .
The mass of a nucleus divided by its volume; about , the same for all nuclei.
One of the discrete energies a nucleus can have; transitions between them give γ photons and α particles of definite energies.
The splitting of a heavy nucleus into two smaller nuclei of comparable mass, usually with the release of two or three neutrons and energy.
The joining of two light nuclei to form a heavier nucleus, which releases energy when the product has a higher binding energy per nucleon.
Rutherford's model of the atom (1911): a tiny, massive, positive nucleus surrounded by electrons in mostly empty space.
The symbol for a nuclide, where X is the chemical symbol, the proton number and the nucleon number.
The radius of a nucleus, given by with m.
A device in which a controlled, self-sustaining fission chain reaction releases energy at a steady rate.
A particle in the nucleus: a proton or a neutron.
The total number of protons and neutrons in a nucleus.
The formation of chemical elements by nuclear reactions, mainly fusion in stars and reactions in supernovae.
The small, dense, positively charged centre of an atom, made of protons and neutrons, which holds nearly all of the atom's mass in a region about to m across.
A particular kind of nucleus, with a definite number of protons and neutrons, written .
In relativity, a reference frame with clocks at every position that records where and when each event happens, after allowing for the time light takes to arrive.
The current in a conductor is directly proportional to the pd across it, provided its temperature and other physical conditions stay constant.
A component that obeys Ohm's law, so its resistance is constant at every pd; a metal at constant temperature is ohmic.
A pipe open at both ends; each end is a displacement antinode, the first harmonic has and all harmonics are present.
A thin strand of glass with a core surrounded by cladding of lower refractive index, which guides light along it by repeated total internal reflection.
Describes the medium with the higher refractive index of two, in which light travels more slowly.
The total (kinetic plus gravitational potential) energy of a body in a circular orbit, ; it is negative because the body is bound.
The time for one complete orbit; for a circular orbit .
The speed of a body in a circular orbit of radius around a mass , , found by setting the gravitational force equal to the centripetal force.
The whole number of wavelengths in the path difference between light from neighbouring slits at a grating or double-slit maximum.
The power of ten closest to a quantity; two quantities differ by one order of magnitude when their ratio is about ten.
The fixed reference point from which positions are measured.
A repeated back-and-forth motion of a body about a fixed equilibrium position.
The idealised cycle of a petrol engine: adiabatic compression, heating at constant volume, adiabatic expansion and cooling at constant volume.
A longitudinal seismic wave; it travels faster than S waves and through solids and liquids.
A graph of pressure against volume on which each point is a state of a gas and each line a change of state.
Components connected across the same two points, so that they share the same pd and the branch currents add to the total.
The original nucleus that undergoes radioactive decay.
The distance at which 1 AU subtends an angle of one arc-second; .
The SI unit of pressure: .
The difference between the distances travelled by two waves from their sources to a given point.
How far a radiation travels into matter before it is absorbed, measured by the thickness of material needed to absorb it.
The difference between an experimental and an accepted value as a percentage of the accepted value; it measures accuracy.
The fractional uncertainty expressed as a percentage, .
A collision in which the bodies stick together and move off with a common velocity; the loss of kinetic energy is the largest that momentum conservation allows.
The point of an orbit around the Sun that is closest to the Sun, where the orbiting body moves fastest.
The time for one complete cycle of a repeating motion, such as one revolution in circular motion, in seconds.
The constant that sets the strength of the magnetic field produced by a current in a vacuum.
A property of a medium that determines the electric force between charges in it; the larger the permittivity, the weaker the force.
The constant that sets the strength of electric forces between charges in a vacuum.
The stage an oscillation has reached in its cycle, expressed as an angle in radians, .
The angle , in radians, that fixes the position of a simple harmonic oscillator in its cycle at .
A change between solid, liquid and gas, which happens at constant temperature while energy changes the intermolecular potential energy of the particles.
The difference in phase between two oscillations or two points on a wave of the same frequency, in radians: .
An evacuated tube containing a photosensitive cathode and an anode, used to measure the current and energy of photoelectrons.
The electric current carried by photoelectrons that travel from the cathode to the anode of a photocell.
The emission of electrons from a metal surface when electromagnetic radiation above a threshold frequency falls on it.
An electron released from a surface by the photoelectric effect.
A discrete packet (quantum) of electromagnetic radiation, with energy and momentum ; it travels at in a vacuum and has no rest mass.
The number of photons arriving per second; at a fixed frequency it is proportional to the intensity of the beam.
A quick trial of a method before the main data collection, used to check that it works, to find a suitable range and to estimate the scatter in the results.
How high or low a sound is heard to be; a higher frequency is heard as a higher pitch.
Presenting another person's words, ideas or work as one's own without clear acknowledgement, deliberately or by accident.
The constant that links the energy of a photon to its frequency () and the momentum of any particle to its wavelength ().
A glowing shell of gas thrown off by a low-mass star at the end of its red-giant stage, leaving a white dwarf at its centre.
A gas so hot that its atoms are fully or largely ionised, made of free nuclei (or ions) and free electrons.
Thomson's model of the atom (1904): a sphere of evenly spread positive charge with electrons embedded in it. Alpha-particle scattering showed it to be wrong.
A charge whose size is negligible compared with the distances involved, so that it can be treated as concentrated at a single point.
A body whose size is negligible, so that all its mass can be treated as being at one point.
Where a body is relative to a chosen origin, given as a distance with a direction (or sign).
A process in which a change causes effects that increase the original change, such as warming that reduces albedo and so causes more warming.
The antiparticle of the electron, with the same mass and a charge of ; emitted in β⁺ decay.
(1) The laws of physics are the same in all inertial reference frames; (2) the speed of light in a vacuum is the same for all inertial observers.
The work done per unit charge in moving a positive charge between two points, ; measured in volts, where .
Two or more resistors in series across a supply, sharing its pd in the ratio of their resistances; the output is the pd across one of them.
The rate of change of potential with distance, ; the field strength is minus the potential gradient.
A resistor with a sliding contact; with a supply across its whole length it acts as a potential divider giving any output pd from zero to the supply pd.
The rate of doing work or of transferring energy: , and for a force moving at velocity . Measured in watts.
How closely repeated measurements of the same quantity agree with one another; it is limited by random errors.
The force acting perpendicular to a surface per unit area of the surface, ; measured in pascals (Pa).
For a fixed mass of gas at constant volume, pressure is proportional to absolute temperature: = constant.
The commonest type of power reactor, in which water under high pressure acts as both moderator and coolant and transfers energy to a separate steam loop.
A chemical cell that is used once and discarded when its reactants are used up, such as an alkaline cell.
Data collected or generated by the investigator, by measurement or from a simulation they run.
The positive whole number that labels an energy level; in the Bohr model the hydrogen energy is eV.
Energy cannot be created or destroyed, only transferred from one form or body to another, so the total energy of an isolated system is constant.
For a body in rotational equilibrium, the sum of the clockwise torques about any axis equals the sum of the anticlockwise torques about that axis.
When two or more waves meet at a point, the resultant displacement is the vector sum of the displacements that each wave would produce on its own.
Values calculated from raw data, such as means, derived quantities and their uncertainties.
A body that has been launched and then moves under gravity (and, in real cases, fluid resistance) with no other force acting.
The length of an object measured in the inertial frame in which it is at rest; the longest length any observer measures for it.
The time interval between two events measured in the inertial frame in which they happen at the same place; the shortest time any observer measures between them.
The number of protons in a nucleus; it identifies the element.
The sequence of fusion reactions by which the Sun converts hydrogen into helium-4, releasing 26.7 MeV per helium nucleus.
A collapsing, heating ball of gas that has not yet started hydrogen fusion in its core.
A single short disturbance that travels through a medium, as opposed to a continuous wave.
The restriction of a quantity, such as the energy of an electron in an atom, to certain discrete values rather than any value.
The fact that electric charge exists only in whole-number multiples of the elementary charge, .
The physics of systems in which energy and other quantities are exchanged in discrete amounts (quanta), and in which radiation and matter both show wave and particle behaviour.
A system that sends out microwaves or radio waves and detects their reflections; the Doppler shift of the reflected waves, , gives the speed of the target.
A field whose lines point straight towards (or away from) a single central point, such as the field outside a spherical mass; its strength falls with distance.
The component of a body's velocity along the line of sight, towards or away from the observer; it is the part of the motion that a Doppler shift measures.
The angle at the centre of a circle subtended by an arc equal in length to the radius; a full circle is rad.
The pressure exerted by electromagnetic radiation on matter that absorbs or reflects it, because photons carry momentum.
The random and spontaneous change of an unstable nucleus into a more stable one, with the emission of radiation (α, β or γ).
Unwanted radioactive material, such as spent fuel, that must be isolated from people and the environment until its activity has fallen to safe levels.
Finding the time since an organism died from the activity of carbon-14 per gram of carbon in its remains, which falls with a half-life of 5730 years once intake of carbon stops.
An error that makes readings scatter unpredictably above and below the true value; its effect is reduced by repeating measurements and averaging.
The horizontal distance travelled by a projectile between its launch point and the point where it lands.
A region of a longitudinal wave where the particles of the medium are further apart than normal, so the pressure is lower.
Measurements exactly as they were read from the instruments, before any calculation.
A line showing the direction in which a wave travels and transfers energy; it is always perpendicular to the wavefronts.
The time between a person seeing (or hearing) a signal and starting to respond to it, such as a driver seeing a hazard and pressing the brake.
An actual gas, whose molecules have volume and attract each other, so it deviates from ideal behaviour at high pressure, low temperature and high density.
A large, luminous star with a cool surface, formed when a low-mass star has used up the hydrogen in its core; its main energy source is helium fusion in the core.
An increase in the observed wavelength (and decrease in frequency) of electromagnetic waves from a source moving away from the observer.
A circle of radius round which a point moves at angular speed ; the projection of the point onto a diameter moves in simple harmonic motion, and the angle of its radius is the phase .
A coordinate system, with an origin, axes and synchronised clocks, used to give the position and time of events.
The change in direction of a wave at a boundary so that it returns into the medium it came from, with its speed, frequency and wavelength unchanged.
The change in direction of a wave as it passes from one medium into another in which its speed is different.
The ratio of the speed of light in a vacuum to the speed of light in a medium, . It has no unit and is at least 1.
A heat pump used to keep a space cold: work is done to remove thermal energy from its interior and release it, plus the work, to the surroundings.
The ratio of a medium's permittivity to that of a vacuum; the factor by which the medium reduces the force between charges.
The velocity of one body as measured by an observer moving with another body; for light, only the relative velocity of source and observer along the line joining them sets the Doppler shift.
The rule for the velocity of a body in a frame moving at ; it never gives a speed greater than .
Two events at different places that are simultaneous in one inertial frame are not simultaneous in a frame moving along the line joining them.
The chemical separation of spent fuel into reusable uranium and plutonium and the remaining high-level waste.
The focused question an investigation sets out to answer, naming the independent and dependent variables (or two correlated variables) and the system studied.
The ratio of the potential difference across a component to the current through it, ; measured in ohms (Ω).
A property of a material defined by for a sample of resistance , length and cross-sectional area ; measured in Ω m.
The smallest change in a quantity that a measuring instrument can show, such as 1 mm for a metre rule.
The large increase in the amplitude of a forced oscillation when the driving frequency equals, or is very close to, a natural frequency of the system.
A graph of the amplitude of a forced oscillation against the driving frequency, with a peak near the natural frequency.
A force that acts on a displaced body in the direction of its equilibrium position.
The vector sum of all the forces acting on a body: the single force that would have the same effect as all of them together.
The sum of all the torques on a body about one axis, with clockwise and anticlockwise torques given opposite signs.
An idealised process that can be run backwards, returning the system and its surroundings exactly to their original states; the total entropy is unchanged.
A unit of rotation rate used for machines; dividing by 60 gives revolutions per second, and multiplying by gives the angular velocity in .
A rule for the field of a current: grip the wire with the right hand, thumb along the conventional current, and the curled fingers give the direction of the magnetic field lines.
An extended object whose shape and size do not change: the distance between any two of its points stays fixed.
A planned review of the hazards in a method, how likely and serious each one is, and the precautions that reduce the risk.
Rolling in which the contact point has zero velocity relative to the surface, so the centre moves at .
The square root of the mean of the squares of the speeds of the molecules in a gas; the speed in .
Motion in which every point of a body moves in a circle about the same line, the axis of rotation.
The state of a body on which the resultant torque about any axis is zero, so its angular velocity does not change.
The kinetic energy a body has because it rotates, .
The deflection of alpha particles by the electric repulsion of nuclei, first measured by Geiger and Marsden by firing alpha particles at a thin metal foil; it showed that atoms have a nucleus.
A transverse seismic wave; it travels more slowly than P waves and cannot pass through liquids.
A diagram of energy transfers in which the width of each arrow is proportional to the energy or power it represents; useful output runs straight on and wasted energy branches away.
The largest photocurrent from a photocell, reached when every photoelectron emitted is collected by the anode; it is proportional to the photon flux.
A quantity that has magnitude only, such as distance, speed, mass or time.
The entropy of an isolated system never decreases; equivalently, thermal energy does not flow spontaneously from cold to hot, and no cyclic engine converts thermal energy completely into work.
A rechargeable chemical cell, such as a lithium-ion or lead–acid cell, whose reactions are reversed by driving a current backwards through it.
Data collected by someone else and selected by the investigator, for example from a published database.
A wave that carries energy through the Earth from an earthquake or explosion.
The emf induced in a coil by a change in its own current, which changes its own flux linkage; by Lenz’s law it opposes the change in current.
Half the longest diameter of an ellipse; for an orbit, the mean of the closest and farthest distances from the central body.
A material such as silicon with few mobile charge carriers, whose number increases when it is heated or illuminated, so its resistance falls.
Components joined end to end so that there is one path for the current: they share the same current and their pds add.
Material such as steel, water and concrete around a reactor that absorbs neutrons and gamma radiation to protect people.
The cone-shaped pile-up of wavefronts produced by a source moving faster than the waves it emits, heard for sound as a sonic boom.
A connection of negligible resistance across a source, giving the largest possible current, .
One of the seven independent units of the International System (kg, m, s, A, K, mol, cd) from which every other SI unit is derived.
A symbol placed before a unit to multiply it by a power of ten, such as k for or μ for .
A digit in a number that carries information about its precision; leading zeros are not significant, trailing zeros after a decimal point are.
Oscillation in which the acceleration is proportional to the displacement from a fixed equilibrium position and always directed towards it: .
A small dense bob on a light inextensible string that, for small angles, oscillates with simple harmonic motion of period .
A computer model of a physical system in which the investigator sets the inputs and records the outputs as data.
A simple energy-balance model in which the atmosphere is one layer of uniform temperature and emissivity, transparent to sunlight, that exchanges infrared with the surface and space.
A metal ring on the axle of a generator, connected to one end of the coil, on which a fixed brush slides so that the coil stays connected to the external circuit as it turns.
For an angle in radians that is small (below about 0.2 rad), .
For a wave refracted at a boundary, , or equivalently , with angles measured from the normal.
A semiconductor device that converts the energy of light directly into electrical energy; a source of emf.
The intensity of the Sun's radiation at the Earth's mean distance from the Sun, on a surface perpendicular to the radiation outside the atmosphere: .
The continuous stream of charged particles, mainly protons and electrons, flowing outwards from the Sun.
A long coil of wire wound as a helix; a current in it produces a strong, uniform magnetic field inside and a field like a bar magnet's outside.
A longitudinal mechanical wave of compressions and rarefactions; it needs a medium and cannot travel through a vacuum.
Space and time combined into one framework in which every event is a point, needed because the Lorentz transformation mixes position and time.
A graph of events with position on the horizontal axis and on the vertical axis, on which light travels at 45°.
The quantity given by for two events, which has the same value in every inertial frame.
A space-time interval with : not even light can link the events, so neither can cause the other and observers can disagree on their order.
Einstein's theory, based on two postulates, of how measurements of space and time made in different inertial reference frames are related.
The energy released by a fuel per unit mass, measured in .
The energy needed to raise the temperature of 1 kg of a substance by 1 K, ; measured in .
The energy needed to change the phase of 1 kg of a substance at constant temperature, ; measured in .
The energy needed to melt 1 kg of a solid at its melting point without changing its temperature (released again on freezing).
The energy needed to turn 1 kg of a liquid into gas at its boiling point without changing its temperature (released again on condensing).
A narrow bright or dark line in a spectrum at a wavelength emitted or absorbed by a particular element; its shift from the laboratory wavelength measures the radial velocity of the source.
The measurement of the wavelengths and intensities in a spectrum, used to identify the elements in a source of light or in a gas the light has passed through.
The range of wavelengths present in light (or other radiation) when they are spread out and separated, for example by a prism or a diffraction grating.
The rate of change of distance travelled; a scalar quantity measured in . It is the magnitude of the velocity.
Fuel removed from a reactor after use, containing unused uranium, highly radioactive fission products and plutonium.
Fission of an unstable heavy nucleus that happens on its own, randomly, without being triggered by an incoming particle.
The force per unit extension of a spring obeying Hooke's law, , measured in ; a stiffer spring has a larger .
The pattern formed when two waves of the same frequency, wavelength and amplitude travelling in opposite directions superpose; it oscillates in place and transfers no net energy.
One of the measurable quantities that fix the state of a gas: pressure, volume, temperature and amount of substance.
The state of a rigid body that is at rest with zero resultant force and zero resultant torque on it.
Friction between surfaces that are not sliding; it matches the applied force up to a maximum value, .
The constant in the Stefan–Boltzmann law.
The total power radiated by a black body is proportional to its surface area and to the fourth power of its kelvin temperature: .
The state of a stable star in which, at every depth, the outward force from gas and radiation pressure balances the inward gravitational force.
The apparent shift of a nearby star against distant stars as Earth orbits the Sun; the parallax angle is the angle subtended at the star by 1 AU.
The viscous drag on a small sphere moving slowly through a fluid in laminar flow, , proportional to the radius, the speed and the viscosity.
The total distance a vehicle travels from the moment the driver sees a hazard until it stops: thinking distance plus braking distance.
The minimum reverse potential difference between anode and cathode that reduces the photocurrent to zero; .
The short-range (about ) attractive force between nucleons that holds the nucleus together against the electric repulsion of the protons.
A very large and very luminous star, at the top of the HR diagram, formed when a massive star leaves the main sequence.
The explosion of a massive star at the end of its life, when its iron core collapses, leaving a neutron star or a black hole.
Everything outside the thermodynamic system being studied, with which the system can exchange energy.
The positive feedback in which pedestrians on a swaying footbridge fall into step with the sway, so their sideways foot forces increase it.
A measure of how hot a body is; on the particle model, the kelvin temperature is proportional to the mean random kinetic energy of the particles.
The change in temperature per unit distance through a material, ; measured in .
The pulling force exerted by a stretched string, rope, cable or chain, acting along it and away from the body it is attached to.
The pd between the terminals of a source, ; it equals the emf only when no current flows.
The constant speed reached by a body falling through a fluid when the upward forces on it balance its weight, so that its acceleration is zero.
The SI unit of magnetic flux density: the field that exerts a force of 1 N on a 1 m length of wire carrying 1 A at right angles to it; .
A small positive charge used to detect and measure an electric field, small enough not to disturb the charges producing the field.
A mass small enough not to disturb the field it is placed in, used to define the field strength or potential at a point.
The rate of thermal energy transfer per unit area per unit temperature gradient through a material, in ; measured in .
Energy transferred from one body to another because of a temperature difference between them.
The state of two bodies between which there is no net transfer of thermal energy, because they are at the same temperature.
A neutron slowed until its kinetic energy is similar to that of the surrounding particles, about 0.025 eV; it is far more likely than a fast neutron to cause fission of uranium-235.
Electromagnetic radiation, mainly infrared at everyday temperatures, emitted by every body above absolute zero because of the thermal motion of its charged particles.
A body so large that its temperature does not change noticeably when thermal energy is transferred to or from it.
The release of electrons from the surface of a heated metal, when some electrons gain enough energy to escape.
A semiconductor resistor whose resistance changes with temperature; the common type's resistance falls as its temperature rises.
The distance a vehicle travels at constant speed during the driver's reaction time: speed × reaction time.
Two forces of the same type that two bodies exert on each other, equal in size and opposite in direction, acting on different bodies.
The minimum frequency of electromagnetic radiation that releases electrons from a given metal surface: .
The longest wavelength of electromagnetic radiation that releases electrons from a given metal surface: .
The forward force on a rocket or jet engine from expelling gas backwards, approximately the mass flow rate times the exhaust speed, .
The lengthening of a time interval measured by an observer for whom a clock is moving: , so a moving clock runs slow.
The total time a projectile spends in the air, from launch until it lands.
A space-time interval with : the events can be linked by a body slower than light, so one can cause the other and all observers agree on their order.
The turning effect of a force about an axis: the force multiplied by the perpendicular distance from the axis to its line of action, , measured in .
An instrument that measures a very small force from the angle through which it twists a rod hanging on a thin wire; Cavendish used one to measure in 1798.
The complete reflection of a wave at a boundary with a medium in which it would travel faster, when the angle of incidence exceeds the critical angle.
The path followed by a moving body; for a projectile with no fluid resistance it is a parabola.
Motion in which every point of a body moves through the same displacement, without turning.
The state of a body on which the resultant force is zero, so that it is at rest or moves with constant velocity.
The kinetic energy of a body due to the motion of its centre of mass, .
The passage of a wave across a boundary from one medium into another. The frequency is unchanged; the speed and wavelength change in the same ratio.
A wave in which the oscillations are perpendicular to the direction of energy transfer.
A wave whose disturbance moves through a medium or through space, transferring energy without any net transfer of matter.
A point of maximum negative displacement on a transverse wave.
A mass on a spring (with damping) attached to a structure and tuned to the structure's natural frequency, so that it oscillates out of phase with the structure and absorbs energy from it.
Sound with a frequency above the upper limit of human hearing, about 20 kHz.
The range, either side of a measured value, within which the true value is expected to lie.
One-twelfth of the mass of a carbon-12 atom: .
Acceleration that stays the same in magnitude and direction, so that the velocity changes by equal amounts in equal times.
Motion in a circle at constant speed; the velocity changes direction continuously, so the body accelerates towards the centre.
A field with the same magnitude and direction at every point, shown by parallel, equally spaced field lines.
A quantity that has both magnitude and direction, such as displacement, velocity, acceleration or force.
The rate of change of displacement; a vector quantity measured in .
An arrangement of crossed electric and magnetic fields that lets through undeflected only particles with speed .
A graph of velocity against time; its gradient is the acceleration and the area between the line and the time axis is the displacement.
A measure of how strongly a fluid resists flowing, symbol , measured in Pa s.
The resistive force on a small sphere moving slowly through a fluid in laminar flow, given by Stokes' law ; a special case of fluid resistance.
A meter that measures potential difference, connected in parallel across the component; an ideal voltmeter has infinite resistance.
A positive feedback in which warmer air holds more water vapour, a greenhouse gas, which increases the warming.
The SI unit of power: a rate of energy transfer of one joule per second.
A disturbance that transfers energy from one place to another without a net transfer of matter.
The speed at which a wave transfers energy through a medium or through space, .
The behaviour of both radiation and matter as waves in some experiments (diffraction, interference) and as particles in others (photoelectric effect, Compton scattering, detection at a point).
A line or surface joining neighbouring points of a wave that are vibrating in phase, such as all the points on one crest. Neighbouring wavefronts are usually drawn one wavelength apart.
The shortest distance between two points on a wave that are in phase, such as adjacent crests or adjacent compressions; symbol , in metres.
The SI unit of magnetic flux: , equivalent to .
The gravitational force on a body, , measured in newtons and acting at its centre of mass.
The sensation of having no weight when a body and its surroundings are in free fall together, so no support force acts on it; gravity still acts.
The hot, dense, Earth-sized remnant core of a low-mass star, in which no fusion takes place; it is supported by the pressure of closely packed electrons and slowly cools.
The peak wavelength of a black body's spectrum is inversely proportional to its kelvin temperature: .
The energy transferred when a force moves its point of application: for a constant force, where is the angle between the force and the displacement. Measured in joules.
The minimum energy needed to remove an electron from the surface of a metal, symbol ; usually given in eV.
The work done by the resultant force on a body equals the change in its kinetic energy.
The line on a space-time diagram joining all the events in a body's history; for constant velocity it is straight, at angle to the axis with .