Definitions

absolute temperature scale: The Kelvin or thermodynamic temperature scale with zero at -273 °C, the temperature at which the pressure of an ideal gas becomes zero

absolute zero: The temperature at which the pressure of an ideal gas becomes zero. This is 0 K on the Kelvin temperature scale, which is equivalent to -273 °C.

absorption coefficient: A measure of how rapidly the intensity of radiation is attenuated, or reduced, with distance.

Accelerate: To increase velocity.

Acceleration: The rate of change of an object’s velocity.

acceleration due to gravity: The rate of increase in velocity due to the force of gravity acting on a mass. On Earth the acceleration due to gravity is 9.81 m sˉ¹.

acceleration of free fall: The acceleration due to gravity

acceptor: Material that accepts electrons from a semiconductor lattice so that electric charge is carried as positive 'holes'. Doping with an acceptor impurity results in a p-type semiconductor.

active noise control: Noise reduction method that combines the noise with its antiphase signal so the two cancel out

A-D converter: A device that converts analogue signals with a range of values to their digital equivalent consisting of a sequence of 0s and 1s.

aerodynamic drag: The frictional force experienced by an object as it moves through air.

Aerodynamics: The study of how air flows around a moving object and the forces created.

alpha decay: Radioactive decay in which the nucleus releases an alpha particle (a helium nucleus). In alpha decay the mass number decreases by 4 and the proton number by 2.

Ampere: The SI unit of current, symbol A. A current of 1 A is equal to a flow of 1 coulomb of charge per second (1 A = 1 C sˉ¹).

Amplitude: The maximum displacement of a particle from its equilibrium position.

amplitude modulation: Transmission of a signal by varying the amplitude of the carrier wave.

Analogue: A signal that varies over a continuous range of values.

angle of incidence: The angle between the incident ray to a surface and the normal at the point of contact.

angle of reflection: The angle between the reflected ray from a surface and the normal at the point of contact.

angle of refraction: The angle through which a ray is bent as it crosses the boundary between two media. The angle is measured from the normal at the point of contact

angular diameter: Of a star, moon or planet – the angle formed between lines drawn from the observer to opposite sides of an object.

angular displacement: The angle through which a rotating object has turned.

angular velocity: The rate of rotation of an object, measured as the angle turned per unit time, for example radians per seconds (rad sˉ¹).

annihilation reaction: A reaction between a particle and its antiparticle in which the particles turn into electromagnetic radiation.

Antimatter: Matter with equal mass but opposite charge and spin to ordinary matter, for example the positron is the antiparticle to the electron.

Antinode: Position along a standing wave at which there is greatest displacement.

Antiphase: Two oscillations are in antiphase when they are half a cycle (180° or π radians) out of step.

Archimedes' principle: An object wholly or partly immersed in a fluid experiences an upthrust equal to the weight of fluid displaced.

Astable: A circuit that has no stable output state.

atomic number: The number of protons in the nucleus of an atom.

attenuation: A loss of intensity of a signal.

avalanche: An effect when a charged particle or photon interacts with other particles to produce a much greater number of charged particles or photons, for example in a photomultiplier tube or in a laser.

average speed: Value for speed obtained by dividing the total distance covered by the time taken. This may be very different from the instantaneous speed.

baryon: Hadron made from three quarks bound together. The proton and the neutron are both baryons.

base units: The seven fundamental units on which the SI system of units is based. These include units for mass (kilogram, kg), length (metre, m), time (second, s), electric current (ampere, A) and thermodynamic temperature (kelvin, K).

beta-minus decay: Radioactive decay in which an electron and an antineutrino are emitted as a neutron in the nucleus turns into a proton. The atomic number of the parent atom is increased by 1.

beta-plus decay: Radioactive decay in which a positron and a neutrino are emitted as a proton in the nucleus turns into a neutron. The atomic number of the parent atom is decreased by 1.

Big Bang theory: The theory that the Universe evolved from a single very hot explosion.

Big Crunch: A possible fate for the Universe, where it collapses back on itself to a singularity.

binary star: A system of two stars orbiting around their common centre of gravity.

binding energy: The minimum external energy required to separate all the neutrons and protons of a nucleus.

bit: Short for 'binary digit', the smallest unit of data used in a digital system such as a computer.

black body: An object that is a perfect emitter and absorber of radiation. A black body absorbs all wavelengths that fall upon it and radiates all wavelengths. The spectrum of wavelengths emitted depends only on its temperature.

black body radiation: The radiation that is emitted from an object because of its temperature.

black hole: A remnant of a massive star formed when its matter collapses to a point (singularity). The density of a black hole is so great that even light cannot escape from it.

Boltzmann constant: The constant that relates the average energy of a molecule to its absolute temperature.

breaking stress: The maximum stress (force per unit area) that can be applied to a material before it is pulled apart (also called the tensile strength).

brittle: A material that shows no plastic deformation and breaks just beyond its elastic limit.

bulk modulus: The ratio of the stress applied to an object and the change in volume per unit volume.

capacitance: The amount of charge stored per unit potential difference across the plates of a capacitor.

capacitor: A device that stores electric charge.

carrier wave: A wave used to carry a signal, which may be added to the carrier wave as frequency modulation, amplitude modulation or pulse code modulation.

centre of gravity: The point through which all the weight of an object appears to act.

centre of mass: The point through which all the mass of an object appears to be concentrated.

centripetal acceleration: Acceleration of a moving object towards the centre of a circle as a result of a centripetal force.

centripetal force: A force that acts towards the centre of a circle.

charge: A fundamental property of matter that exists as positive and negative forms.

chemical potential energy: The energy in chemical bonds holding the atoms of a compound together, which can be released by a chemical reaction.

circuit element: Part of an electrical circuit.

circular waves: Waves that spread out in concentric circles from the source of the disturbance.

cladding: Material used to surround the core of an optical fibre that has a refractive index slightly less than that of the fibre, to help reduce dispersion.

closed Universe: A model of the Universe in which gravity is strong enough to halt its expansion and reverse the process, so that the Universe eventually comes together in a Big Crunch.

coefficient of viscosity: A measure of the resistance to flow of a fluid.

coherent: Waves that are of the same frequency and phase are coherent.

coloumb: The SI unit of electric charge, symbol C. The coulomb is derived from the base unit the ampere: 1 A = 1 C sˉ¹.companding: A way of arranging quantum levels in which they are compressed and expanded.

compression: Squashing of a material. The part of a longitudinal wave where the pressure is greatest is known as a compression.

compressive force: The force on a material which tends to reduce the volume - to squash the material.

compressive strain: The amount of deformation per unit length of a material that is being squashed.

compressive strength: The resistance to deformation of a material when subjected to a load that squashes it (compression).compressive stress: The compressive force per unit area.

conservation of charge: In a circuit, the amount of charge flowing in at a point must be equal to the amount of charge flowing out. In nuclear physics, conservation of electric charge applies to nuclear reactions, requiring the electric charge at the end to be the same as the electric charge at the start.

conserved: A quantity that stays the same is conserved.

constructive interference: When two waves at the same point are in phase with each other so the resultant is a wave with increased intensity.

continuous spectrum: A spectrum that contains an entire range of frequencies.

continuous waves: Waves that go on forever, that is they have infinite length.

cosmic background radiation: Electromagnetic radiation in the microwave region of the spectrum that corresponds to a temperature of the Universe of 2.7 K.

cosmological principle: The principle that the large-scale structure is the same throughout the Universe.

cosmological redshift: The shifting of the entire spectrum of a receding source to longer wavelengths.

cosmology: The study of the Universe.

Coulomb's law: Law relating the force between two charged particles to their charges and the distance between them.

critical angle: For light travelling from a denser medium to a less dense medium, the largest possible internal angle of incidence that will allow light to emerge. For angles larger than the critical angle the light is totally internally reflected.

critical density: The density of the Universe that just allows it to stop the expansion of space after infinite time.

critical Universe: A model in which the Universe has a density equal to the critical density. Also called a 'flat' Universe because the geometry of space in a critical Universe is not curved, but flat.

cycle: The repeating 'unit' of a periodic motion.

cyclotron: A particle accelerator that uses magnetic fields to bend the beam of particles into a circle.

D-A converter: An electronic device used to convert digital signals to analogue ones.damping: Energy loss of a vibration due to friction.

dark matter: Matter in the Universe that we cannot detect by emission or absorption of radiation. Evidence for its existence comes from its gravitational effect on other objects in the Universe.

daughter nucleus: The product nucleus of a radioactive decay.de Broglie wavelength: The wavelength of an electron when it behaves as a wave rather than as a particle. λ = h/p (where λ is the wavelength, h is the Planck constant and p is the electron's momentum).

decay constant: The constant relating the number of atoms of a radionuclide that decay in a given period of time and the total number of atoms of the same kind present at the beginning of that period. dN/dt = -λN.

demodulation: The process of extracting the information from a modulated carrier wave.

demultiplexer: A device that extracts and reassembles multiplexed signals at the receiving end.

density: Mass per unit volume: ρ = m/V.

dependent variable: The variable that is being investigated in an experiment. The value of the dependent variable is measured for each value of the independent variable chosen by the experimenter.

derived units: SI units can be combined to give derived units, such as speed (m sˉ¹) or force (N = kg m sˉ²).destructive interference: Cancelling out that occurs when two waves arrive at a point in antiphase.

diffraction: The interaction between waves and solid obstacles. If the obstacle is similar in size to the wavelength of the wave, then the wave spreads out significantly.

digital: A signal consisting of 1s and 0s only.

direction of an electric field: The direction of the force experienced by a positive electric charge placed in that field.

dispersion: Spreading out of a signal such as a beam of light.

displacement: Distance moved from a reference point. Displacement is a vector quantity.

donor: Material that release free electrons into a semiconductor lattice to act as negative charge carriers. Doping with a donor impurity results in an n-type semiconductor.

Doppler effect: The change in the frequency and wavelength of a wave caused by the relative movement of source and detector.

drift velocity: The average velocity of the electrons in a conductor due to an electric field.

driver frequency: The frequency of an external force that causes a system to oscillate at that frequency.

ductile: A material that shows plastic deformation without breaking is ductile. A ductile material can be pulled into wires, for example copper.

eddy current: Currents induced in the body of a material by a changing magnetic field.

efficiency: The ratio of energy output to energy input. Efficiency is usually given as a percentage: efficiency = useful energy (or power) output / total energy (or power) input x 100% .

elastic collision: A collision in which there is no net loss of kinetic energy.

elastic limit: The load on a spring beyond which the extension increases more rapidly than expected, and the spring remains permanently deformed when the load is removed.

elastic strain energy: The energy stored in a stretched spring.

electric dipole: A pair of equal and opposite electric charges separated by a small distance.

electric field: A region in which a charged object experiences a force.

electric field lines: A way of showing an electric field diagrammatically to indicate the strength of the field and the direction of the field.

electric field strength: The electric field strength of an electric field is the force experienced by a charge of 1 coulomb placed in that field.

electromagnet: A type of magnet made using an electric current in coils around a core. .electromagnetic force: The force experienced by a current-carrying conductor in a magnetic field.

electromagnetic induction: The production of an emf using magnetism.

electromagnetic relay: An enclosed mechanical switch that can use one electrical circuit to operate another.

electromagnetic spectrum: The entire range of electromagnetic radiation, from the longest radio waves to the shortest gamma rays.

electromagnetic waves: Transverse waves that combine oscillating magnetic and electric fields. Electromagnetic waves do not need a medium through which to travel, and so can travel through empty space.

electromotive force: The electromotive force (or emf) of a cell is the amount of energy supplied by the cell to each unit of charge.

electron: Subatomic particle with a negative charge and very small mass.

electron gas: The free electrons in a metal, given this name because they have random movements like the particles in a gas.

electron gun: Source of electrons inside a cathode ray tube. The electrons are released from a negative electrode (a cathode) and accelerated in an electric field.

electronvolt: The energy gained by an electron travelling through a potential difference of 1 volt.

emf: The energy gained per unit charge by charges passing through an electrical supply. The unit for emf is J Cˉ¹ or volts (V).emission spectra: Characteristic colours (frequencies) of light given out by an element when its atoms are excited.

energy flux: The rate at which energy crosses unit area. Also called intensity.

energy level: Discrete quantities of energy that an electron can possess, electrons can only possess these energy values.

energy transfer: The process by which energy is transferred from one object to another, for example by heating or working. An object with energy has the capacity to do work.

equilibrium: A steady state. The forces on a body in equilibrium are balanced.

equilibrium position: The rest position of an object.

escape velocity: The minimum velocity needed by an object to escape from the pull of gravity of the object from which it is launched.

exponential: A type of relationship in which equal steps result in equal fractional changes.

extrinsic semiconductor: Semiconductor in which extra charge carriers are introduced as a result of doping the semiconductor with an impurity

Faraday's law of electromagnetic induction: The size of the emf induced in a loop of a conductor is proportional to the rate at which the magnetic flux in the loop is changed.

field lines: Imaginary lines representing the influence of a field. Where the lines are close together the field is strong, and where they are farther apart the field is weaker. Also called lines of force.

fission: Radioactive decay in which the nucleus breaks up into smaller particles.

Fleming's left-hand rule: Uses the left hand to find the direction of the electromagnetic force on a current-carrying conductor. With thumb, first finger and second finger at right angles to each other, the first finger represents the direction of the magnetic field, the second finger the direction of the current and the thumb the direction of motion (the direction of the force).

Fleming's right-hand rule: Uses the right hand to find the direction of the induced current when a conductor is moved in a magnetic field. With thumb, first finger and second finger at right angles to each other, the first finger represents the direction of the magnetic field, the second finger the direction of the current and the thumb the direction of movement of the wire.

fluid: A medium that can flow. Gases and liquids are fluids.

force: A push or pull that can cause an object with mass to accelerate. Force has both magnitude and direction, making it a vector quantity.

free-body diagram: A diagram that shows all the forces acting on a body in a certain situation.

frequency: The number of cycles of an oscillation that occur in unit time. The unit is the hertz (Hz) or sˉ¹.

frequency division multiplexing: A method of transmitting several analogue signals along the same cable in which the different signals are modulated onto a carrier wave at different frequencies. At the receiving end a band-pass filter is used to separate the signals, which are then demodulated.

frequency modulation: Transmission of a signal by varying the frequency of the carrier wave.

friction: Resistance to movement caused by the small-scale roughness of the surfaces in contact.

fringes: Interference pattern of bright and dark bands.

fundamental force: A force that cannot be explained in terms of anther force. Gravity and the electromagnetic force are fundamental forces.

fundamental mode: The vibration with the lowest frequency.

fundamental particles: Particles that cannot be divided into smaller constituents.

fusion: Nuclear reactions in which nuclei fuse to form a new, larger nucleus

gamma decay: Radioactive decay in which a nucleus becomes more stable by emitting a photon of gamma radiation.

gamma rays: Electromagnetic radiation emitted when a nucleus decays. Gamma rays are photons with very high energy.

graded index: Optical fibres in which the refractive index changes gradually across the fibre so it is greater in the centre than at the edges. Rays travelling in different parts of the fibre have different speeds, so the rays meet at the end without dispersion. Also known as multimode fibres.

gradient: The slope of a graph (gradient = change in y value ÷ change in x value). The gradient is the value of m in the equation y = mx + c.

Grand Unified Theories: Theories that bring the fundamental forces that act between fundamental particles together into one theory.

graph: A plot of one variable against another. The shape or slope of the graph provides information about the relationship between the two variables.

gravitation: The attractive force between two bodies due to their mass.

gravitational field: The field around a mass which causes it to attract another mass which is close to it.

gravitational field strength: Gravitational field strength g at a point in a gravitational field is defined as the force per unit mass acting at that point.

gravitational potential: is the amount of work done per unit mass to move an object from an infinite distance to that point in the field

gravitational potential energy: Potential energy stored in an object due to its position.

ground state: The state in which an atom has its lowest energy level

hadron: Groups of quarks held together by strong forces (baryons and mesons).half-life: The time taken for half the radioactive nuclei of a radioactive substance to decay.

hard: Hard materials are those which resist plastic deformation. This means the are difficult to dent, scratch or cut. .harmonic motion: Vibrations and any kind of motion that repeats itself, such as the swinging of a simple pendulum or a mass bouncing on a spring.

harmonics: The set of vibrations associated with a system, such as the vibrations of a musical instrument.

heating: One of the ways in which energy can be transferred. Energy is transferred in heating as the result of a temperature difference.

heliocentric: A model of the solar system with the Sun at the centre.

Hertzsprung–Russell diagram: Graph of luminosity of stars against their surface temperature. On this diagram stars appear in groups related to the age and size of the star.

hole: A positive charge in a semiconductor lattice which can act as a charge carrier

horsepower: A unit of power that dates back from before the industrial revolution, but is still used to describe the power of a car's engine. 1 HP is about 750 W.

Hubble constant: The ratio of the speed v of a receding galaxy to its distance d from an observer.

Hubble's law: The speed of a receding galaxy is proportional to its distance from the observer.

hydrometer: A device with a weighted base and a tall stem used to determine the density of the fluid. The hydrometer floats in the fluid, and the density is read at from the level of the fluid on the graduated stem.

Huygens' construction: An explanation for the way in which a circular wave spreads out that considers each point on a wavefront to be a source of a new set of disturbances. This eventually leads to a plane wave as the radius of the circular wave becomes very large.

hypothesis: A proposal put forward to explain facts or observations that can be tested by further investigation

ideal gas: A gas that exactly obeys the gas laws.

impulse: The product of force and the time for which it acts.in phase: When two oscillations are exactly in step.in quadrature: When two oscillations are out of phase by a quarter of a cycle.

incoherent: Waves are incoherent if the phase difference between them keeps changing.

independent variable: In an experiment, the variable that is chosen to be changed by the experimenter.

induced current: The current induced in a conductor as a result of electromagnetic induction.

induced emf: The emf induced in a conductor as a result of electromagnetic induction.

infrared: Electromagnetic waves with wavelengths longer than those of visible light, just beyond the red end of the spectrum.

instantaneous speed: The speed at which an object is travelling at a particular time. .insulator: A material that does not conduct electricity is an electrical insulator. A thermal insulator does not conduct heat.

intensity: The rate at which energy crosses unit area. Also called energy flux.

intercept: The point on the axis of a graph where a line drawn through the plotted points meets the axis.

interference: The interaction of two or more waves superimposed upon each other.

internal energy: The total random distribution of kinetic and potential energy of a collection of particles.

internal resistance: The resistance to the flow of charge within a source of emf.

interstellar material: Matter that exists in the space between stars.

intrinsic semiconductor: Pure semiconductor in which the charge carriers arise from within the semiconductor and whose conductivity is not affected by external factors.

inverse square law: A relationship in which one parameter is proportional to the inverse of the square of the other, for example the gravitational force between two objects (gravitational force is proportional to 1/distance², so when the distance doubles the force decreases by a factor of 4).

ionisation: When an electron is given enough energy to escape from the attraction of the nucleus.

isotope: Atoms with the same number of protons, but differing numbers of neutrons.

joule: Unit of energy defined as the energy transferred when a force of 1 newton is displaced by a distance of 1 metre

kinetic energy: Energy due to motion. Kinetic energy of an object of mass m moving at speed v is given by KE = ½mv².

Kirchhoff's first law: The total current into any point in a circuit is equal to the total current out of that point.

Kirchhoff's second law: The sum of potential rises and falls around a closed path in a circuit is zero.

laminar flow: Smooth fluid flow in which layers have a constant velocity.

law: A statement that explains observations of physical behaviour, based on repeated scientific observations and experiments over many years that have become accepted by other scientists.

law of conservation of energy: Energy cannot be created or destroyed.

law of reflection: The angle of incidence is equal to the angle of reflection.

Lenz's law: The direction of the current induced in a conductor by moving it in a magnetic field is such that its own field opposes the motion.

lepton: Fundamental particle with a very small mass. The electron is one of the six types of lepton.

light year: The distance travelled by light in one year.

line spectra: Characteristic colours (frequencies) of light given out by an element when its atoms are excited.

linear: Two quantities that are related by a linear relationship are proportional to each other. If the value of one quantity doubles, then so does the other.

linear accelerator: A series of electrodes used to accelerate particles to very high speeds in a straight line in order to carry out particle physics experiments.

lines of force: Imaginary lines representing the influence of a field. Where the lines are close together the field is strong, and where they are farther apart the field is weaker. Also called field lines.

liquid crystal: A liquid in which the molecules arrange themselves in some sort of ordered pattern.log-linear graph: A graph in which one of the variables is plotted as its logarithm. This type of graph is useful when there is an exponential relationship between the variables.

longitudinal wave: Wave motion in which the disturbance occurs in the same direction as the wave is travelling.

luminosity: The rate at which a source such as a star radiates energy

magnetic field: The three-dimensional region of space in which a magnet has influence.

magnetic flux: The 'amount' of magnetic field within a loop or coil.

magnetic flux density: A measure of the strength of a magnetic field.

magnetic flux linkage: The magnetic flux through a coil with a number of turns. This is the product of the magnetic flux through the area of the loop of the coil and the number of turns of the coil.

magnitude: Size. A scalar quantity has magnitude, but a vector quantity has both magnitude and direction.

main sequence: The main group of stars on a Hertzsprung–Russell diagram. These are stars that are burning hydrogen. Our Sun is a main sequence star.

malleable: Materials which show large plastic deformation before cracking or breaking are called malleable. Such materials can be hammered or rolled into flat sheets, for example gold.

mass: The amount of matter in an object. The unit is the kilogram (kg).

mass defect: The difference between the mass of a nucleus and the mass of the individual, separate nucleons from which it is made. Also called mass deficit.

mass deficit: The difference between the mass of a nucleus and the mass of the individual, separate nucleons from which it is made. Also called mass defect.

mass number: The mass of a chemical element, which is the sum of the number of protons and neutrons in its nucleus.

mean square speed: The average (mean) of the squares of the speeds of the particles in a gas.

mechanical waves: Waves that require a medium through which to travel. Sound waves, water waves and seismic waves are all mechanical waves.

medium: The material through which a wave travels.

meson: Hadron made from two quarks bound together.

metre: SI base unit of length.

metres per second: Unit for speed or velocity.

microwaves: Electromagnetic waves with wavelengths in the centimetre region.

modulation: Adding of an information signal to a carrier wave. .momentum: The product of mass and velocity. Momentum is a vector quantity.

monochromatic: Light that is of a single frequency is monochromatic.

monomode: An optical fibre on which the rays can only travel down the centre.

monostable: A circuit that has one stable output state.

multimode: Optical fibres in which the refractive index changes gradually across the fibre so it is greater in the centre than at the edges. Rays travelling in different parts of the fibre have different speeds, so the rays meet at the end without dispersion of the signal.

multipath dispersion: The spreading out of a signal of light or infrared due to the different paths taken by individual rays as they travel through a medium (such as an optical fibre).

multiplexer: The device that splits and assembles signals to be sent together down a cable as a multiplexed signal.

multiplexing: When several signals are combined together, to be separated at the receiving end.

natural frequency: The frequency at which an object vibrates or oscillates most easily. If it is driven at this frequency the object will oscillate with a large amplitude.

nebula: A cloud of gas or dust in interstellar space.

negative feedback: When a signal is used to reduce the output of a device.

neutron star: A dense ball of neutrons that remains at the core of a star after a supernova explosion.

newton: The SI unit of force. The newton is a derived unit: 1 N = 1 kg m sˉ².Newton's First Law of Motion: An object remains at rest or continues moving along a straight line at constant velocity unless acted on by an external force.

Newton's Second Law of Motion: The acceleration of an object is directly proportional to the magnitude of the net force on the object in the same direction as the net force, and inversely proportional to the mass of the object. F = ma.

Newton's Third Law of Motion: If body A exerts a force on body B, then body B exerts a force of the same size on body A, but in the opposite direction.

node: Position along a standing wave at which there is no displacement.

noise: Unwanted electrical signals that can degrade a signal.

non-ohmic: A conductor for which the current is not proportional to the voltage applied (it does not obey Ohm's law). The graph of voltage against current is not a straight line. .normal ray: The ray that meets a boundary at right angles. Angles of incidence, reflection and refraction are all measured from this normal ray.

nucleon: A particle found in an atomic nucleus, i.e. a neutron or a proton.

nucleon number: The number of nucleons in the nucleus of an atom, i.e. the number of neutrons and protons.

ohmic conductors: A conductor for which the current is proportional to the voltage applied (it obeys Ohm's law). The graph of voltage against current is a straight line through the origin.

open circuit voltage: The voltage of a power source when the circuit is not complete, so no current is flowing.

open Universe: A model of the Universe in which gravity cannot halt the expansion of matter. The Universe will expand forever.

operational amplifier: A type of digital to analogue converter

parallax: The apparent shifting of the position of a star relative to the background of distant stars when observed from different positions in the Earth’s orbit round the Sun.

parallel: When components in an electrical circuit are connected so the current is split between the components. When connected in parallel the components will have the same potential difference across them.

parallelogram rule: The resultant of two vectors can be found by drawing a parallelogram with the two vectors to be added as the sides. The diagonal of the parallelogram is the resultant.

parsec: Distance that gives a parallax angle of 1 arc second.

path difference: Difference in distance travelled by two waves at a point. Depending on their path difference, the waves may be in phase or out of phase, and so interfere constructively or destructively.

peer review: Process by which research is scrutinised for accuracy and quality by others working in the field before it is published.

period: In circular motion, the time taken to complete one orbit. When describing oscillations, the period is the time taken to complete one cycle.

periodic oscillation: A motion that repeats itself over and over again, at regular intervals.

permittivity of free space: A physics constant that describes how an electric field affects and is affected by the medium through which it acts.

persistent current: Flow of charge in a superconductor that continues as long as the material is kept below the critical temperature at which it becomes superconducting.

phase: The stage an oscillation has reached in its cycle.

phase difference: The phase difference between two oscillations is the fraction of a cycle that their peaks and troughs are out of step.

photoelectric effect: The emission of electrons from the surface of a metal when a beam of light of sufficient energy is shone on the metal.

photoelectron: Electron emitted from the surface of a metal as a result of interaction between light and the electrons in the metal.

photomultiplier tube: A device in which a single incident photon produces a pulse of current in an external circuit. This type of tube can be used in an image intensifying camera.photon: A 'particle' of electromagnetic radiation.

pions: Pions are the lightest of the meson family of fundamental particles and are often produced in particle physics experiments.

Planck's constant: The constant h relating the energy E of a photon to its frequency ν. E = hν.

plane polarised: Transverse waves that oscillate in one plane only are said to be plane polarised.

plane waves: Waves in which the wavefronts form a series of parallel straight lines.

planetary nebula: The hot gases and dust ejected from a low-mass star as it evolves into a white dwarf.

plasma: An ionised gas.

plastic collision: A collision in which there is a net loss of kinetic energy.

plastic deformation: Permanent deformation of a material that has been loaded beyond its elastic limit.

polar: A polar molecule has a neutral charge overall, but the charge is not evenly distributed, so one end is slightly negative and the other end slightly positive.

polarisation: Phenomenon only exhibited by transverse waves in which the oscillations (which are at right angles to the direction of propagation) can be restricted to one plane.

positron: The antimatter version of the electron

potential: The potential at a point in an electric field is the work done per unit positive charge moving from infinity to that point.

potential difference: The difference in electrical potential between two points in a circuit, measured in volts (V). It is a measure of the work done by the current.

potential divider: A device that allows a desired fraction of potential difference to be taken.

potentiometer: Another name for a potential divider.

power dissipation: The rate at which energy is transferred to an element in a circuit.

power-law relationship: A relationship where one variable is directly proportional to the other variable raised to a power, or exponent.

pressure: Force per unit area. The unit of pressure is the pascal, Pa, or N mˉ².pressure law: At constant volume, the pressure of a fixed mass of an ideal gas is proportional to its temperature (on the Kelvin scale).primary source: A report of original findings or ideas, for example a research article.

principle of conservation of linear momentum: The total momentum of a system remains constant provided no external forces act on the system. This applies to all objects moving in a straight line.

principle of superposition: Where two or more waves meet, the total displacement at any point is the sum of the displacements that each individual wave would cause at that point.progressive wave: A wave that travels, transferring energy as it moves.

projectile: An object given an initial velocity that then moves under a constant force, for example the force of gravity.

propagation: The movement of a wave through a medium.

proton number : The number of protons in the nucleus of an atom; the same as atomic number.

pulsar: A rotating neutron star that emits energy in the form of radio waves at regular intervals.

pulse: A very short wave motion that does not have any repeated patterns.

pulse code modulation: A process by which a continuously varying voltage signal is represented as a series of numbers.

quality: The particular nature of a musical sound, which is the result of the frequencies present in the sound and their relative amplitudes.

quantisation: Splitting a signal into a number of discrete levels known as quantum levels.

quantised: A signal that has been split into discrete levels so it does not vary continuously.

quantum level: A discrete level. The electrons in an atom exist in discrete energy levels called quantum levels.

quantum mechanics: Laws of physics that apply to particles on the atomic and subatomic scale, where the laws of large-scale physics do not apply.

quark: The fundamental particles from which protons, neutrons (and some other particles) are made. There are six types of quark.

radiation flux: A measure of the amount of energy landing on a unit area in a unit time.

radio waves: Electromagnetic waves with wavelengths ranging from centimetres to several kilometres.

radioactive decay: Process by which an unstable nucleus changes spontaneously to a more stable one, through emission of ionising radiation.

range : The horizontal displacement of a projectile when it reaches the ground once more.

rarefaction: The part of a longitudinal wave where the pressure is least.

rate of change of velocity with time: The definition of acceleration. Given by (final velocity - initial velocity) ÷ time taken for change .

rays: Straight lines drawn to show the direction of travel of a wavefront. The rays are perpendicular to the wavefronts.

reaction equation: Equation representing the interaction of subatomic particles to form new particles.

red giant: A giant star with a relatively low surface temperature, so that it glows with a red colour.

red shift: In cosmology, the shifting of the entire spectrum of a receding source to longer wavelengths.

reflection: Bouncing back of a wave as it arrives at a barrier.

refraction: Change in direction of a wave as it passes from one medium to another.

refractive index: The constant in Snell's law that relates the sine of the angle of incidence to the sine of the angle of refraction for light at the boundary between two different media. The higher the refractive index the more a ray is bent as it passes from air into the medium.

relative motion: The motion of one object in relation to another.

resistivity: A measure of the electrical resistance of a material for a given size and shape: ρ = RA/l (unit Ω m).

resonance: When an object is driven at its natural frequency so that it oscillates with a large amplitude.

rest mass: The mass of a particle when it is not moving.

rest-mass energy: The energy that would be released if the entire rest mass of a particle were to dematerialise. This is also the energy that would be needed to create the particle 'from nothing'. This comes from Einstein's equation E = mc².

restoring force: The force acting towards the central point of the oscillation for an object moving with simple harmonic motion.

resultant: The result of combining two or more vectors, for example forces.

resultant force: The total force on an object that results from the combination of the forces acting on it.

root-mean-square speed: Used when looking at the behaviour of gas particles. This is an 'average' speed that is directly related to the pressure exerted by the particles in a gas.

scalar: A quantity that has a magnitude or size, but no direction. Mass is a scalar quantity.

secondary source: Comments or criticisms written about a primary source. A secondary source does not provide original data, but analyses the results of previously published primary research.

seismic waves: Waves in the Earth caused by earthquakes or volcanic eruptions.

semiconductor: A material that has a resistivity between those of conductors and insulators. Silicon and germanium are semiconductors.

series: When components in an electrical circuit are connected so the current passes through one and then the other.

shear modulus: The ratio of shear stress to shear strain. Shear modulus is a measure of the resistance of a material to shear deformation.

shear stress: A stress in which the top and bottom of a material are pushed in opposite horizontal directions.

short circuit: A low-resistance connection between two parts of a circuit.

simple harmonic motion: A type of harmonic motion in which the frequency is independent of the amplitude and the force that causes the motion is directed towards the central point of the motion.

Snell's law: Law relating the angle of incidence to the angle of refraction for light at the boundary between two different media: sin i/sin r = constant.

special relativity: When objects are travelling at speeds close to the speed of light the equations of special relativity are needed to explain their motion.

spectrometry: The study of spectra, used to determine the composition of materials. Spectra from distant galaxies can be used to determine their age and distance.

spectrum: The range of frequencies emitted by radiating object.

speed: Rate of change of position. Speed = Δs/Δt.

spring constant: For a spring that obeys Hooke's law, the constant k relating the distance the extension of the spring Δx and the force F applied to it: F = -kΔx.

standard model of particle physics: The current theory, which identifies 12 fundamental particles from which all matter is made.

standing wave: A wave that does not travel in any direction, but remains where it is with constant nodes and antinodes. Also called a stationary wave.

statics: The study of forces on objects that are not moving.

stationary wave: A wave that does not travel in any direction, but remains where it is with constant nodes and antinodes. Also called a standing wave.

step-down transformer: Transformer that has an output voltage lower than the input voltage.

stepped index: Optical fibre in which the core is surrounded by cladding that has a refractive index slightly less than the core. This reduces dispersion of the signal.

step-up transformer: Transformer that has an output voltage higher than the input voltage.

stiffness: A measure of the resistance of a material to deformation by a tensile (pulling or stretching) force.

stimulates: When emission of a photon from a transition within an atom triggers the emission of another photon of the same energy, we say that the photon stimulates the emission. Such stimulated emission results in an intense, coherent beam of light (the laser).

stopping potential: For photoelectrons, the value of the stopping potential (the potential difference that just stops all emitted electrons) allows the maximum kinetic energy of the photoelectrons to be calculated.

streamlines: Lines of laminar fluid flow. At any point on a streamline the velocity of the fluid will be constant over time.

strength: The ability of a material to withstand stress.

superconductor: Material with zero resistivity, so that there is no resistance to the flow of charge.

supergiant: A massive, hot star formed when a star of mass at least five times that of the Sun has used up its hydrogen and starts to burn helium.

supernova: The explosion that occurs when a massive star uses up its fuel and collapses. Depending on the size of the star, the remains may become a neutron star or a black hole.

supernova remnant: The rapidly expanding cloud of very hot gases blown out when a massive star explodes in a supernova.

superposition: The combination of two or more waves. At any point the resultant displacement is equal to the sum of the displacements of the individual waves at that point.

symmetry: Property of the Gell-Mann model of elementary particles that predicted a particle that was later found.

Systeme International: The standard system of units used in most countries of the world. The SI system was developed to help scientists and engineers exchange ideas and data with colleagues more easily.

tensile strain: The extension per unit length of a material subjected to a tensile (pulling or stretching) force

tensile strength: The maximum force per unit area that can be applied to a material before it is pulled apart (also called the breaking stress).

tensile stress: The tensile (pulling or stretching) force per unit area.

terminal potential difference: The potential difference across the terminals of a source.

terminal velocity: Constant velocity of a falling object in which the force of gravity downward is balanced by the drag force.

theory: An explanation or model of the way matter behaves that can be tested through experiment, and which is capable of predicting future behaviour.

thermionic emission: Process in which electrons are emitted from a heated filament.

thermistor: Electronic component that indicates temperature by the value of its resistance.

threshold frequency: The minimum frequency of light that will release a photoelectron from the surface of a metal.

timbre: The quality of a musical sound. This depends of the harmonics present in the sound and is a characteristic of the instrument making the sound.

time constant: A measure of how long it takes for a resistor–capacitor combination to discharge.

time division multiplexing: A method of multiplexing in which input data is sampled and sliced up into sections of equal time duration. Sections are then assembled in order, one after the other, for transmission. They are extracted and reassembled at the receiving end.

time period: The time taken for an oscillating system to complete one cycle.

torque: The turning effect of a force.

total energy of a simple harmonic oscillator: The combined kinetic and potential energy for an oscillator undergoing simple harmonic motion.

total internal reflection: When all the light incident at the boundary between a denser and a less dense medium is reflected back into the denser medium.

tough: Tough materials are able to withstand impact forces without breaking and require a large force to produce a small plastic deformation.

transformer: A device consisting of two electrically insulated coils wound on a common iron core that converts electricity at one ac voltage to another.

transient: Only remaining for a short time. This can apply to oscillations or to electric or magnetic fields.

transmitted: Energy that is transferred with a wave across a boundary is transmitted.

transport equation: Equation used to calculate the speed at which charge carriers travel through a wire: I = nAvq (where I is the current, n the number of free charge carriers, A is the cross-sectional area of the wire, v is the velocity of the charge carriers and q is the charge on each charge carrier).

transverse wave: Wave motion in which the disturbance occurs at right angles to the direction in which the wave is travelling.

turbulent flow: Irregular, chaotic fluid flow in which eddies form. The fluid velocity is not constant over time.

ultrasound: Sound waves with frequencies above the threshold of human hearing (frequency > 20 000 Hz).

ultraviolet: Electromagnetic waves with wavelengths shorter than the visible region, just beyond the violet end of the spectrum.

uncertainty: Measure of the possible error in a measurement.

universal gravitation: The concept that there is an attractive force of gravity between all masses in the Universe.

unpolarised: Transverse waves that oscillate in all planes.

upthrust: The upward force exerted on an object immersed in a fluid, equal to the weight of the fluid displaced.

vector: A quantity that has both magnitude and direction. Velocity is a vector quantity.

virtual image: One that cannot be formed on a screen.

viscosity: A measure of the resistance of a fluid to flow. Viscosity is due to frictional forces between the molecules of the fluid.

viscous drag: The friction force acting to oppose motion through a fluid.

volt: The unit for emf and pd. A cell is said to have an emf of 1 volt when it supplies 1 joule of energy to each coulomb of charge flowing through it: 1 V = 1 J Cˉ¹.

watt: Unit of power, which is the rate of use of energy: 1 W = 1 J sˉ¹.

wave train: A wave that has a beginning and an end.

wavefront: On a diagram the crests of a wave are represented by lines known as wavefronts.

wavelength: The distance between two points on a wave that are in phase, such as between two crests or two troughs.

wave-particle duality: Theory which suggests that any particle can also exhibit wave-like properties.

wave: A periodic motion or disturbance that carries energy away from its source.

wavicle: A wave–particle object which behaves sometimes like a classical wave and at other times like a classical particle.

weak: A weak material has a low tensile strength.

weight: The force of gravity on an object. Weight is the product of mass m and gravitational field strength g: W = mg.

white dwarf: The very hot remnant of a low-mass star.

work: Work is done when a force is used to transfer energy to an object.

Work done = force x distance moved in direction of force .work function: In the photoelectric effect, the energy required to remove an electron completely from the surface of a metal

work function: the minimum energy needed to release an electron from the surface of a metal

working: One of the ways in which energy can be transferred. Work is done when a force is used to transfer energy to an object.

X-rays: Short-wavelength, high-energy electromagnetic radiation. X-rays can be used the probe beneath the surface of materials (such as in medical X-rays) and in cancer therapy. X-rays are used in X-ray diffraction to identify the composition of materials.