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A-level Physics key definitions

Definitions for 282 A-level Physics terms, written in precise exam-ready language and linked to the relevant lesson.

282 terms, and the same wording runs on the flashcards. For the one big idea per topic, see the key ideas.

282 terms

ABCDEFGHIKLMNOPQRSTUVWYZ

AAbsolute magnitude, Absolute zero, Accommodation…21 terms
Absolute magnitude
The apparent magnitude a star would have at a distance of 10 parsecs. · The apparent magnitude scale
Absolute zero
The temperature at which the particles of a substance have minimum internal energy. · Ideal gases and the gas laws
Accommodation
The eye changing the power of its lens, by changing the lens shape, so that objects at different distances all form sharp images on a retina that cannot move. · The eye as an optical system
Accuracy
How close a result is to the true value. · Uncertainty and error
Acoustic impedance
The product of a material's density and the speed of sound in it: Z = ρc. · Ultrasound imaging
Action potential
The brief pulse in which a triggered cell depolarises, its membrane p.d. reversing, and then repolarises as the resting state is restored. · Biological measurement
Activity
The number of nuclei in a sample decaying per second: A = λN, in becquerels. · Radioactive decay and the decay constant
Adiabatic change
A change with no heat entering or leaving: Q = 0, pVγ = constant, and ΔU = −W. · The first law and its sign convention
Amplitude
The maximum displacement of a particle from its equilibrium position. · Progressive waves
Angular magnification
The angle subtended by the final image at the eye divided by the angle subtended by the object at the unaided eye. · The astronomical refracting telescope
Angular momentum
The product of moment of inertia and angular velocity, Iω, in kg m2 s−1. · Torque and angular acceleration
Angular speed
The angle swept out per unit time: ω = θ/t = 2πf, in rad s−1. · Circular motion
Angular velocity
The rate of change of angular displacement: ω = Δθ/Δt, measured in rad s−1. · Rotational motion and the angular equations
Annihilation
A particle and its antiparticle meet and their mass is converted to energy. For a pair annihilating at rest this is released as two photons back to back. · Antimatter and photons
Antinode
A point on a stationary wave where the amplitude of oscillation is maximum. · Stationary waves on strings
Apparent magnitude
A star's brightness as seen from Earth, on a scale where smaller numbers mean brighter stars. · The apparent magnitude scale
Astable
A circuit with no stable state: it switches continually between its two output levels on its own, needing no input, and so supplies the clock that steps counters and timers. · Flip-flops, counters and clocks
Astigmatism
A cornea or lens whose curvature differs between two perpendicular directions, so its power is not the same along both and a point object cannot be focused to a point. · The eye as an optical system
Astronomical unit
The mean distance from the Earth to the Sun, 1.50 × 1011 m. · The apparent magnitude scale
Atomic mass unit
One twelfth of the mass of a carbon-12 atom, 1.661 × 10−27 kg. · Mass-energy and binding energy
Attenuation coefficient
The constant μ in I = I₀e−μx, measuring how strongly a material absorbs and scatters X-rays. · The physics of diagnostic X-rays
BBandwidth of a resonant circuit, Baryon, Bertozzi's experiment…9 terms
Bandwidth of a resonant circuit
The width in frequency between the two points either side of the peak at which the current has fallen to 1/21/\sqrt{2} of its maximum, where the power delivered is half its peak. · Tuned circuits and the Q factor
Baryon
A hadron made of three quarks, such as the proton or the neutron. · Classification of particles
Bertozzi's experiment
Measured electron speed by time of flight against kinetic energy from the pd, finding speed levelling off just below c. · Time dilation and muon decay
Binding energy
The energy needed to separate a nucleus into its constituent nucleons. · Mass-energy and binding energy
Black body
A body that absorbs all radiation falling on it, and is therefore the best possible emitter. · Black-body radiation and spectral classes
Boyle's law
At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume. · Ideal gases and the gas laws
Breaking stress
The maximum stress a material can withstand before it fractures. · Stress, strain and the Young modulus
Brittle material
One that fractures at the end of its straight-line region with no plastic deformation. · Stress, strain and the Young modulus
Brownian motion
The continual random motion of visible particles, such as smoke grains, suspended in a fluid. · The kinetic theory model and its derivation
CCapacitance, Centre of gravity, Centre of mass…24 terms
Capacitance
The charge stored per unit potential difference: C = Q/V, in farads. · Capacitors and the dielectric
Centre of gravity
The single point at which the whole weight of a body may be taken to act; where the gravitational field is uniform across the body it coincides with the centre of mass. · Moments and equilibrium
Centre of mass
The single point at which the whole mass of an object can be taken to be concentrated. · Moments and equilibrium
Chandrasekhar limit
About 1.4 solar masses, the greatest core mass electron degeneracy pressure can support as a white dwarf. · The HR diagram and the life of Sun-like stars
Charles's law
At constant pressure, the volume of a fixed mass of gas is proportional to its absolute temperature. · Ideal gases and the gas laws
Chromatic aberration
Glass refracts blue light more than red, so each colour has its own focal length and images carry coloured fringes. · The astronomical refracting telescope
Coefficient of performance of a heat pump
The heat delivered to the warm space divided by the work put in: COPhp = QH/W = QH/(QH − QC), which is exactly one more than the same machine scores as a refrigerator. · Heat pumps and combined heat and power
Coefficient of performance of a refrigerator
The heat taken from the cold space divided by the work put in: COPref = QC/W = QC/(QH − QC). · Heat pumps and combined heat and power
Coherent bundle
A bundle of optical fibres in which each fibre occupies the same position in the cross-section at both ends, so an image passed through it arrives in the right order. · MR scanning and the endoscope
Coherent sources
Sources with the same frequency and a constant phase difference. · Interference and Young's double slit
Coincidence detection
Registering two 511 keV photons arriving at opposite detectors at effectively the same instant. · Radionuclide imaging and the gamma camera
Combined heat and power
Rejecting a plant's waste heat into water hot enough to pipe round nearby buildings rather than into a river. The sink temperature rises, so the electrical output falls, but the share of the fuel doing something wanted rises from about 40% to about 80%. · Heat pumps and combined heat and power
Comet
A small body of ice, dust and rock orbiting a star on a highly elliptical path. Near the star the ice vaporises, and radiation pressure and the stellar wind blow the gas and dust into a tail that always points away from the star. (OCR A.) · The HR diagram and the life of Sun-like stars
Common-mode signal
A signal appearing identically on both inputs, such as mains hum induced equally on both leads of a cable. It appears in both terms of (V+ − V−) and so cancels before the gain is applied. · Summing and difference amplifiers
Control rods
Rods of boron or cadmium that absorb neutrons to control the rate of the chain reaction. · Nuclear reactors and safety
Coolant
The fluid, such as water or carbon dioxide, that transfers thermal energy from the core to raise steam for the turbines. · Nuclear reactors and safety
Cosmic microwave background
Black-body radiation at 2.7 K arriving almost uniformly from every direction. · Hubble's law and the Big Bang
Cosmological principle
The assumption that, on a large enough scale, the universe is homogeneous (the same composition and density everywhere), isotropic (looking the same in every direction) and governed by the same laws of physics everywhere. · Hubble's law and the Big Bang
Coulomb's law
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation. · Coulomb's law and electric field strength
Couple
A pair of equal and opposite coplanar forces whose lines of action do not coincide. · Moments and equilibrium
Coupling gel
The gel between transducer and skin that displaces the air, so the impedances are matched and the pulse enters the body. · Ultrasound imaging
Critical angle
The angle of incidence in the denser medium for which the angle of refraction is 90°. · Total internal reflection and optical fibres
Critical mass
The minimum mass of fissile material in which, on average, one neutron per fission goes on to cause another fission. · Fission and fusion
Cut-off frequency of an RC filter
The frequency at which the reactance equals the resistance, so the output has fallen to 1/21/\sqrt{2}, about 0.71, of its full value. · Band-pass, band-stop and RC filters
DD-type flip-flop, Damping, De Broglie wavelength…19 terms
D-type flip-flop
A one-bit memory: on each rising clock edge it copies D to Q, then holds Q unchanged, ignoring D, until the next edge. · Flip-flops, counters and clocks
Damping
The loss of energy from an oscillating system to resistive forces, reducing the amplitude over time. · Energy in an oscillation and damping
De Broglie wavelength
The wavelength associated with a moving particle: λ = h/p, where p is the particle's momentum. · Wave-particle duality
Decay constant
The probability per unit time that a given nucleus decays: λ, in s−1. · Radioactive decay and the decay constant
Density
Mass per unit volume: ρ = m/V. · Density and Hooke's law
Depolarisation
The collapse and reversal of the p.d. across a cell membrane as ions cross it, which sweeps across the heart muscle as a wave and triggers contraction. · Biological measurement
Derive the equation for hydrostatic pressure
A column of fluid of cross-sectional area A and depth Δh has volume AΔh, mass ρAΔh and weight ρAgΔh. Dividing that weight by A gives Δp = ρgΔh; the area cancels, which is why the container shape does not matter. · Fluids: pressure, upthrust and viscosity
Describe how to find the centre of gravity of an irregular lamina
Hang it freely from a hole near the edge, hang a plumb line from the same pin and rule the line it marks; repeat from a second hole. The centre of gravity is where the lines cross. · Moments and equilibrium
Describe the arrangement and motion of particles in a gas
Particles are far apart, roughly ten times the spacing in a liquid, in no arrangement at all, moving rapidly in random straight lines with negligible forces between collisions. · Internal energy and changes of state
Describe the arrangement and motion of particles in a liquid
Particles are close together, as in a solid, but disordered beyond the nearest few neighbours, and they slide past one another while staying in contact. · Internal energy and changes of state
Describe the arrangement and motion of particles in a solid
Particles are close together in a regular repeating arrangement, held by strong forces, each vibrating about a fixed position and unable to change places with its neighbours. · Internal energy and changes of state
Describe the magnetic field pattern of a flat circular coil
The field runs through the centre of the coil, straightest and strongest there, then spreads out and loops back round the outside, so one face is north and the other south, like a flattened bar magnet. The right-hand grip rule fixes which face is which. (CIE only.) · Magnetic flux density and the force on a wire
Describe the smoke cell demonstration of Brownian motion
Smoke from a smouldering taper is sealed in a small glass cell, lit from the side and viewed through a microscope. Each smoke particle shows as a bright speck jerking about randomly and independently of the rest. · The kinetic theory model and its derivation
Diffraction
The spreading of a wave as it passes through a gap or past an edge. · Diffraction and the single slit
Diffraction grating
A plate ruled with a very large number of equally spaced slits, typically hundreds per millimetre. · Diffraction gratings
Distance modulus
The difference m − M between apparent and absolute magnitude, equal to 5 log(d/10) with d in parsecs. · The apparent magnitude scale
Drag
The resistive force on an object moving through a fluid, acting opposite to the motion. · Drag and terminal speed
Ductile material
One that shows a large plastic deformation beyond its yield point before it fractures. · Stress, strain and the Young modulus
Duty cycle
The high time of a pulse waveform as a fraction of the whole period, t₁/(t₁ + t₂), quoted as a percentage. It is not the mark-to-space ratio, which is t₁/t₂. · Flip-flops, counters and clocks
EEddy currents, Efficiency, Elastic collision…13 terms
Eddy currents
Currents induced in the iron core itself by the changing flux, which waste energy by heating the core. · Transformers
Efficiency
The ratio of useful output power (or energy) to total input power (or energy); it can never exceed 1. · Work, energy and power
Elastic collision
A collision in which kinetic energy is conserved as well as momentum. · Momentum and impulse
Elastic limit
The point beyond which a stretched object no longer returns to its original length when the force is removed. · Density and Hooke's law
Electric current
The rate of flow of charge: I = ΔQ/Δt. · Current, charge and potential difference
Electric field strength
The force per unit charge on a small positive test charge placed in the field: E = F/Q, in N C−1 or V m−1. · Coulomb's law and electric field strength
Electric potential
The work done per unit positive charge to move a small test charge from infinity to that point. · Electric potential
Electromotive force
The energy transferred to each coulomb of charge passing through a source: ε = E/Q. · EMF and internal resistance
Encryption
Combining a message with a secret key by a reversible rule before it is sent, so an interceptor without the key learns nothing while the holder of the matching key recovers the message exactly. · Radio propagation and data security
EOR gate
A two-input gate whose output is 1 when the inputs differ and 0 when they are the same. · Combinational logic
Equal loudness curve
A curve joining the combinations of frequency and intensity level that a listener judges equally loud, labelled by its own intensity level at the 1 kHz reference tone. · The ear and the transmission of sound
Escape velocity
The minimum launch speed at which an object escapes to infinity from a body's gravitational field without further propulsion. · Orbits and satellites
Excitation
An atomic electron gaining exactly the energy needed to jump to a higher energy level. · Collisions of electrons with atoms
FFar point, Faraday's law, First postulate of special relativity…11 terms
Far point
The furthest object an eye can bring into sharp focus, at infinity for a normal eye, and the distance the relaxed eye is focused on. · The eye as an optical system
Faraday's law
The magnitude of the induced emf equals the rate of change of flux linkage. · Electromagnetic induction: Faraday and Lenz
First postulate of special relativity
Physical laws have the same form in all inertial frames. · The Michelson-Morley experiment
Fleming's left hand rule
First finger along the field, second finger along the conventional current, thumb gives the direction of the force, all three at right angles. · Magnetic flux density and the force on a wire
Flux linkage
The flux through a coil multiplied by its number of turns: NΦ. · Magnetic flux and flux linkage
Focal length of a lens
The distance from the centre of the lens to the principal focus. · Lenses and images
Force field
A region in which a body experiences a non-contact force. · The field concept
Fovea
The small patch of retina on the axis of the eye, packed with cones and carrying no rods, where the sharpest vision is formed. · The eye as an optical system
Frequency
The number of complete oscillations per second; equivalently, the number of complete waves passing a point per second. · Progressive waves
Friction
The resistive force between solid surfaces in contact that opposes relative motion, or attempted motion, between them. · Drag and terminal speed
Full-wave rectification
Rectification using a bridge of four diodes, which reverses the negative half-cycles so both halves of the cycle drive current one way through the load. · Rectification and smoothing
GGain-bandwidth product, Galaxy, Geostationary orbit…7 terms
Gain-bandwidth product
The constant product of closed-loop gain and bandwidth for a given op-amp, so a gain raised by a factor of ten cuts the bandwidth by the same factor. · Negative feedback amplifiers
Galaxy
A gravitationally bound collection of stars, gas, dust and dark matter, from millions to hundreds of billions of stars, usually with a supermassive black hole at its centre. (OCR A.) · The HR diagram and the life of Sun-like stars
Geostationary orbit
A circular orbit above the equator with a period of 24 hours, travelling in the direction of the Earth's spin. · Orbits and satellites
Gravitational field strength
The force per unit mass on a small test mass placed in the field: g = F/m, in N kg−1. · Newton's law of gravitation
Gravitational field strength g
The gravitational force per unit mass at a point: g = W/m, in N kg−1. · Mass and weight
Gravitational potential
The work done per unit mass to move a small test mass from infinity to that point. · Gravitational potential
Ground state
The lowest energy level of an atom, in which its electrons normally sit. · Energy levels and photon emission
HHadrons, Half-adder, Half-life…13 terms
Hadrons
Particles that feel the strong interaction. · Classification of particles
Half-adder
A circuit that adds two bits: an EOR gate on the two inputs gives the sum and an AND gate on the same two inputs gives the carry. · Combinational logic
Half-life
The time taken for the number of undecayed nuclei in a sample, and so its activity, to halve. · Radioactive decay and the decay constant
Half-value thickness
The thickness that halves the beam intensity: x = ln 2/μ. · The physics of diagnostic X-rays
Half-wave rectification
Rectification using a single diode, so only one half of each cycle passes and the other half is blocked. · Rectification and smoothing
Hall voltage
The steady pd produced across a current-carrying slab, at right angles to both the current and an applied magnetic field, and proportional to the flux density at constant current. · The MOSFET as a switch
Hertzsprung-Russell diagram
A plot of stellar luminosity or absolute magnitude against temperature or spectral class. · The HR diagram and the life of Sun-like stars
Homogeneous equation
An equation in which every term, on both sides, has the same SI base units. · SI base and derived units
Hooke's law
The force needed to stretch a spring is proportional to the extension, F = kΔL, up to the limit of proportionality. · Density and Hooke's law
Hubble's law
The recession speed of a galaxy is proportional to its distance: v = Hd. · Hubble's law and the Big Bang
Huygens' construction
The rule that every point on a wavefront acts as a source of secondary circular wavelets, and that the wavefront a short time later is the envelope of those wavelets, the surface that touches all of them. · Diffraction and the single slit
Hypermetropia
Long sight: the eye has too little power for its length, so rays from a near object would focus behind the retina and the near point is further away than 0.25 m. · The eye as an optical system
Hysteresis loop
The loop enclosed between the loading and unloading curves of a polymeric material; its area is the energy released as heat in one loading cycle. · Stress, strain and the Young modulus
IIdeal gas, Ideal operational amplifier, Impulse…17 terms
Ideal gas
A gas that obeys pV ∝ T at all pressures, where T is the thermodynamic temperature. · Ideal gases and the gas laws
Ideal operational amplifier
An amplifier of the difference between its two inputs, taken to have infinite open-loop gain, infinite input resistance, zero output resistance and infinite bandwidth. · The ideal op-amp and the comparator
Impulse
The product of a force and the time for which it acts: impulse = FΔt. · Momentum and impulse
Indicated power
The power developed by the gas inside the cylinders: the area of the p-V loop × cycles per second × the number of cylinders. · Heat engines and their efficiency
Induced fission
A heavy nucleus splitting into two smaller nuclei after absorbing a neutron. · Fission and fusion
Inelastic collision
A collision in which momentum is conserved but some kinetic energy is transferred to other forms, such as internal energy. · Momentum and impulse
Inertial frame of reference
A frame moving at constant velocity, in which Newton's first law holds. · The Michelson-Morley experiment
Intensifying screen
A fluorescent layer against the film that absorbs a far larger share of the X-rays than the emulsion alone and turns each absorbed one into thousands of light photons, so a much smaller dose gives the same exposure. · X-ray detectors and CT scanning
Intensity
The power carried by a wave through unit area at right angles to its direction of travel: I = P/A, in W m−2. · The ear and the transmission of sound
Intensity level
A logarithmic measure of a sound against the threshold of hearing, 10 log(I/I0), quoted in decibels. · The ear and the transmission of sound
Internal energy
The sum of the randomly distributed kinetic and potential energies of the particles in a body. · Internal energy and changes of state
Internal resistance
The resistance inside a source, in which energy is dissipated whenever the source drives a current. · EMF and internal resistance
Inverse-square law for gamma
The intensity of gamma radiation from a point source is I = k/x2, so doubling the distance quarters the intensity. · Rutherford scattering and the nuclear model
Ionisation
An electron gaining enough energy to leave the atom entirely, forming an ion. · Collisions of electrons with atoms
Ionisation energy
The minimum energy needed to remove an electron completely from an atom in its ground state. · Collisions of electrons with atoms
Isothermal change
A change at constant temperature: pV = constant and ΔU = 0, so Q = W. · The first law and its sign convention
Isotopes
Atoms with the same number of protons but different numbers of neutrons: the same element with a different nucleon number. · Constituents of the atom
KKepler's third law for circular orbits, Kinetic energy, Kirchhoff's first law…4 terms
Kepler's third law for circular orbits
The square of the orbital period is proportional to the cube of the orbital radius: T2 ∝ r3. · Orbits and satellites
Kinetic energy
The energy of a moving object: Ek = ½mv2. · Conservation of energy
Kirchhoff's first law
The total current into a junction equals the total current out: conservation of charge. · Circuit symbols and Kirchhoff's laws
Kirchhoff's second law
Round any closed loop the sum of the emfs equals the sum of the pds: conservation of energy. · Circuit symbols and Kirchhoff's laws
LLaminar flow, Length contraction, Lenz's law…9 terms
Laminar flow
Smooth flow in layers that do not mix. · Fluids: pressure, upthrust and viscosity
Length contraction
A moving object is measured shorter along its direction of motion: l = l₀√(1 − v2/c2). · Time dilation and muon decay
Lenz's law
The induced emf acts in the direction that opposes the change of flux producing it. · Electromagnetic induction: Faraday and Lenz
Leptons
Fundamental particles that do not feel the strong interaction. · Classification of particles
Lift
The component of the force from a fluid on an object that acts perpendicular to the direction of flow. · Drag and terminal speed
Light year
The distance light travels through a vacuum in one year, 9.46 × 1015 m. · The apparent magnitude scale
Light-dependent resistor (LDR)
A component whose resistance falls as the light intensity falling on it rises. · Sensing circuits and the potentiometer
Limit of proportionality
The point beyond which force is no longer proportional to extension, so Hooke's law ceases to apply. · Density and Hooke's law
Longitudinal wave
A wave whose oscillations are parallel to the direction of energy transfer, as compressions and rarefactions. · Longitudinal and transverse waves
MMagnetic flux, Magnetic flux density, Mass…14 terms
Magnetic flux
The product of flux density and the area it passes through at right angles: Φ = BA, in webers. · Magnetic flux and flux linkage
Magnetic flux density
The force per unit current per unit length on a wire carrying current at right angles to the field: B = F/IL, in tesla. · Magnetic flux density and the force on a wire
Mass
A measure of an object's resistance to acceleration: its inertia. · Mass and weight
Mass defect
The difference between the mass of a nucleus and the total mass of its separate nucleons. · Mass-energy and binding energy
Material dispersion
Pulse broadening caused by different wavelengths travelling at slightly different speeds in the glass. · Total internal reflection and optical fibres
Meson
A hadron made of a quark and an antiquark. · Classification of particles
Moderator
The reactor material that slows fission neutrons to thermal speeds by elastic collisions. · Nuclear reactors and safety
Modulation
Printing a signal onto a higher-frequency carrier wave by varying the carrier amplitude or frequency, so that aerials can be a practical length and each station can be given a frequency of its own. · Modulation and PCM
Moment of a couple
One of the two forces multiplied by the perpendicular distance between their lines of action. · Moments and equilibrium
Moment of a force
The force multiplied by the perpendicular distance from the line of action of the force to the pivot. · Moments and equilibrium
Moment of inertia
The sum of mr2 taken over every particle of a body: its resistance to angular acceleration about that axis, in kg m2. · Rotational motion and the angular equations
Momentum
The product of an object's mass and its velocity: p = mv, a vector. · Momentum and impulse
Myopia
Short sight: the eye is too powerful for its length, so parallel rays focus in front of the retina and the far point is at a finite distance instead of infinity. · The eye as an optical system
NNatural frequency, Near point, Newton's first law…9 terms
Natural frequency
The frequency at which a system oscillates freely after a single displacement, with no driving force. · Forced vibrations and resonance
Near point
The closest object an eye can bring into sharp focus, taken as 0.25 m for a normal adult eye; also called the least distance of distinct vision. · The eye as an optical system
Newton's first law
An object remains at rest or continues at constant velocity unless a resultant external force acts on it. · Newton's laws and the resultant force
Newton's law of gravitation
Two point masses attract with a force F = Gm1m2/r2. · Newton's law of gravitation
Newton's second law
The resultant force on an object equals the rate of change of its momentum; for constant mass this becomes F = ma. · Newton's laws and the resultant force
Newton's third law
When body A exerts a force on body B, body B exerts a force on A of equal magnitude, opposite direction and the same type. · Newton's laws and the resultant force
Node
A point on a stationary wave where the displacement is always zero. · Stationary waves on strings
Normal adjustment
The telescope setting in which the focal planes of the objective and eye lens coincide, so the final image is at infinity and viewed by a relaxed eye. · The astronomical refracting telescope
Nucleon number A
The total number of protons and neutrons in a nucleus. · Constituents of the atom
OOhm's law, Order of magnitude2 terms
Ohm's law
The current through an ohmic conductor is proportional to the potential difference across it, provided physical conditions such as temperature stay constant. · Current-voltage characteristics
Order of magnitude
A value expressed to the nearest power of ten. · Estimation and orders of magnitude
PPair production, Parallax angle, Parsec…27 terms
Pair production
A single high-energy photon passing close to a nucleus converts into a particle and its antiparticle. · Antimatter and photons
Parallax angle
The angle that one astronomical unit subtends at a star, which is half the total sideways shift the star appears to make against the distant background over six months. · The apparent magnitude scale
Parsec
The distance at which one astronomical unit subtends an angle of one arcsecond. · The apparent magnitude scale
Path difference
The difference in the distances from two sources to a point, often expressed in wavelengths. · Interference and Young's double slit
Peak-to-peak value
The full span from minimum to maximum of the waveform, twice the peak value for a sinusoid. · Alternating currents
Period of a wave
The time taken for one complete oscillation: T = 1/f. · Progressive waves
Phase difference
The fraction of a cycle by which one oscillation leads or lags another, in radians or degrees. · Progressive waves
Photodiode
A diode used in reverse bias, in which absorbed photons free electron-hole pairs so that the reverse current, the photocurrent, is proportional to the intensity of the light. · The MOSFET as a switch
Photon
A discrete packet of electromagnetic energy: E = hf. · The photoelectric effect and the photon
Piezoelectric effect
A pd applied across the crystal deforms it, and deforming the crystal produces a pd. · Ultrasound imaging
Planck's quantum hypothesis
Energy is emitted and absorbed in discrete quanta of size E = hf. · Quanta and the photoelectric effect
Planet
A body in orbit around a star, massive enough for its own gravity to have pulled it into a nearly spherical shape and to have swept its orbital neighbourhood clear. It does not fuse, so it shines only by reflected light. (OCR A names this set.) · The HR diagram and the life of Sun-like stars
Planetary nebula
The glowing shell of gas thrown off by a low-mass star at the end of its red giant stage, lit by the hot exposed core it leaves behind. The name is historical and no planets are involved. · The HR diagram and the life of Sun-like stars
Planetary satellite
A body in orbit around a planet rather than around the star directly, covering both natural moons and artificial satellites. (OCR A.) · The HR diagram and the life of Sun-like stars
Polarised wave
A transverse wave whose oscillations are confined to a single plane containing the direction of travel. · Polarisation and electromagnetic waves
Polymeric material
A material built from long tangled molecular chains, such as rubber or polythene, whose stress-strain curve stiffens as the strain grows. · Stress, strain and the Young modulus
Potential difference
The work done per unit charge moving between two points: V = W/Q, in volts. · Current, charge and potential difference
Power
The rate of doing work, equivalently the rate of energy transfer: P = ΔW/Δt. · Work, energy and power
Power of a lens
One over the focal length in metres: P = 1/f, measured in dioptres. · Lenses and images
Precision
How closely repeated readings cluster together. · Uncertainty and error
Pressure
The force acting at right angles to a surface, divided by the area of that surface: p = F/A, measured in pascals. · Fluids: pressure, upthrust and viscosity
Principal focus
The point where rays arriving parallel to the principal axis are brought together by a converging lens (or appear to come from, for a diverging lens). · Lenses and images
Principle of conservation of energy
Energy cannot be created or destroyed, only transferred from one form to another. · Conservation of energy
Principle of moments
For a body in equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point. · Moments and equilibrium
Proper length
The length of an object measured in its own rest frame: l₀. · Time dilation and muon decay
Proper time
The time between two events measured in the frame in which they happen at the same place: t₀. · Time dilation and muon decay
Proton number Z
The number of protons in a nucleus, which determines the element. · Constituents of the atom
QQ factor, Quantum efficiency, Quasar3 terms
Q factor
The resonant frequency divided by the bandwidth. A high Q means a narrow, sharp peak and little energy lost from the circuit each cycle. · Tuned circuits and the Q factor
Quantum efficiency
The fraction of the photons arriving at a detector that are actually registered. · Telescopes beyond the visible
Quasar
The active nucleus of a distant galaxy, powered by matter falling into a supermassive black hole. · Quasars
RRadial velocity method, Random error, Rayleigh criterion…22 terms
Radial velocity method
Detecting an exoplanet from the periodic Doppler shift of its star's spectral lines as star and planet orbit their common centre of mass. · Quasars
Random error
Error that scatters readings unpredictably either side of the true value. · Uncertainty and error
Rayleigh criterion
Two point sources are just resolved when the central maximum of one diffraction pattern falls on the first minimum of the other: θ ≈ λ/D in radians. · Telescopes beyond the visible
Real image
An image formed where rays actually meet, so it can be caught on a screen. · Lenses and images
Rectification
Conversion of alternating current to direct current. · Rectification and smoothing
Red shift
The fractional increase in the wavelength of light from a receding source: z = Δλ/λ. · The Doppler effect and red shift
Refractive index
The ratio of the speed of light in a vacuum to its speed in the material: n = c/v. · Refraction and Snell's law
Relaxation time
The time tipped protons take to settle back into line with the field after the radio-frequency pulse ends. It differs between tissues, and that difference is what an MR image contrasts. · MR scanning and the endoscope
Repeatability
The same experimenter, using the same method and equipment, obtains consistent results. · Uncertainty and error
Reproducibility
A different experimenter, or a different method or equipment, still obtains consistent results. · Uncertainty and error
Reservoir capacitor
The capacitor across the load that charges near each peak of the rectified supply and discharges through the load between the peaks. · Rectification and smoothing
Resistance
The ratio of the potential difference across a component to the current through it: R = V/I. · Charge carriers, drift speed and resistance
Resistivity
The property of a material given by ρ = RA/L: the resistance of a piece of the material of unit length and unit cross-sectional area. · Resistivity and temperature
Resolution
The smallest change in a quantity that an instrument can display. · Uncertainty and error
Resonance
The large-amplitude oscillation of a system when the driving frequency equals its natural frequency. That is the lightly damped case; heavier damping moves the peak slightly below the natural frequency. · Forced vibrations and resonance
Resonance in an LC circuit
The condition in which the inductive and capacitive reactances are equal and opposite, so they cancel: in a series circuit the impedance falls to a minimum and the current is a maximum, while in a parallel circuit the impedance is a maximum. · Tuned circuits and the Q factor
Rest mass
The mass of an object measured in the frame where it is at rest: m₀. · Time dilation and muon decay
Resting potential
The potential difference held across the membrane of a resting muscle or nerve cell by the unequal distribution of ions, about 90 mV in muscle (about 70 mV in nerve) with the inside negative. · Biological measurement
Resultant force
The single force equivalent to the vector sum of all the forces acting on an object. · Newton's laws and the resultant force
Ripple
The small periodic variation left in a smoothed output as the capacitor discharges between peaks. · Rectification and smoothing
Root mean square speed
The square root of the mean of the squares of the molecular speeds. · The kinetic theory model and its derivation
Root-mean-square value
The value of the steady direct current that would dissipate the same average power in a resistor. For a sinusoid, and only for a sinusoid, Irms = I₀/√2. · Alternating currents
SSaturation, Scalar, Schwarzschild radius…19 terms
Saturation
The state in which the output demanded by the gain would pass the supply rails, so the output sticks just inside a rail and no longer follows the input. · The ideal op-amp and the comparator
Scalar
A quantity with magnitude only, such as speed, distance, energy or temperature. · Scalars and vectors
Schwarzschild radius
The radius of the event horizon: Rs ≈ 2GM/c2. · The HR diagram and the life of Sun-like stars
Semiconductor diode
A component that conducts in the forward direction only, with very high resistance in reverse. · Current-voltage characteristics
Sideband
One of the two frequencies produced either side of the carrier by amplitude modulation, at fc + fM and fc − fM; they carry the information, the carrier itself carries none. · Modulation and PCM
Simple harmonic motion
Oscillation in which the acceleration is proportional to the displacement from equilibrium and directed towards it: a = −ω2x. · Simple harmonic motion
Sky wave
A radio wave, roughly 3 to 30 MHz, that is refracted back towards the ground by the ionosphere, so it reaches thousands of kilometres beyond the horizon, sometimes in several hops. · Radio propagation and data security
Smoothing
Reducing the variation of a rectified supply using a reservoir capacitor across the load. · Rectification and smoothing
Solar system
A star together with everything gravitationally bound to it: its planets, their satellites, and the asteroids, comets and dust between them. (OCR A.) · The HR diagram and the life of Sun-like stars
Specific charge
The charge of a particle divided by its mass: Q/m, in C kg−1. · Constituents of the atom
Specific charge of the electron
The magnitude of the electron's charge divided by its mass: e/m. · Cathode rays and the electron gun
Specific heat capacity
The energy needed to raise the temperature of one kilogram of a substance by one kelvin without a change of state. · Specific heat capacity and latent heat
Specific latent heat
The energy needed to change the state of one kilogram of a substance without a change of temperature. · Specific heat capacity and latent heat
Stationary wave
The superposition of two progressive waves of the same frequency and equal amplitude travelling in opposite directions. · Stationary waves on strings
Step-index optical fibre
A thin glass core carrying light by total internal reflection, wrapped in cladding of slightly lower refractive index. · Total internal reflection and optical fibres
Stopping potential
The minimum potential difference that reduces the photoelectric current to zero: eVs = Ek(max). · The photoelectric effect and the photon
Superconductor
A material whose resistivity falls to exactly zero when cooled to or below its critical temperature. · Light-dependent resistors and superconductivity
Superposition
When two waves meet, the resultant displacement is the vector sum of the individual displacements. · Interference and Young's double slit
Systematic error
Error that shifts every reading the same way, such as a zero error. · Uncertainty and error
TTensile strain, Tensile stress, Terminal potential difference…26 terms
Tensile strain
The extension per unit original length: strain = ΔL/L, a ratio with no unit. · Stress, strain and the Young modulus
Tensile stress
The force applied per unit cross-sectional area: stress = F/A. · Stress, strain and the Young modulus
Terminal potential difference
The potential difference across a source's terminals: V = ε − Ir, equal to the emf when no current flows. · EMF and internal resistance
Terminal speed
The constant speed a falling object reaches when the resistive forces have grown to equal its weight. · Drag and terminal speed
The Avogadro constant
The number of particles in one mole: 6.02 × 1023 mol−1. · Ideal gases and the gas laws
The coulomb
The charge passing a point in one second when the current is one ampere: 1 C = 1 A s. · Current, charge and potential difference
The electronvolt
The energy gained by an electron accelerated through a potential difference of one volt. · Collisions of electrons with atoms
The order n of a grating maximum
The whole number in d sin θ = nλ; n = 0 is the straight-through maximum along the normal. · Diffraction gratings
The radian
The angle subtended at the centre of a circle by an arc equal in length to the radius. · Circular motion
The ultraviolet catastrophe
Classical wave theory predicts black-body intensity rising without limit at short wavelengths, contrary to measurement. · Quanta and the photoelectric effect
The weak interaction
The only interaction that can change quark flavour, as when a down quark becomes an up quark in beta-minus decay. · Particle interactions and exchange particles
The weber
The flux through one square metre held at right angles to a field of one tesla: 1 Wb = 1 T m2. · Magnetic flux and flux linkage
Thermal neutron
A fission neutron slowed to the speeds at which it is most likely to be absorbed and cause further fission. · Nuclear reactors and safety
Thermionic emission
Electrons gain enough kinetic energy from a heated metal to escape its surface. · Cathode rays and the electron gun
Thermistor (NTC)
A semiconductor component whose resistance falls steeply as temperature rises. · Resistivity and temperature
Threshold frequency
The minimum frequency of incident radiation that will release electrons from a surface. · The photoelectric effect and the photon
Threshold of hearing
The minimum intensity a normal ear can detect at a frequency of 1 kHz, taken as 1.0 × 10−12 W m−2. · The ear and the transmission of sound
Threshold voltage
The gate-source voltage at which a MOSFET first forms a conducting channel. Below it no channel exists and the drain current is zero, however large the drain voltage. · The MOSFET as a switch
Threshold voltage of a diode
The forward potential difference, about 0.6 V, above which a diode conducts with low resistance. · Current-voltage characteristics
Time constant
The time RC for the charge on a capacitor to fall to 1/e, about 37%, of its initial value. · The time constant and exponential decay
Time dilation
An observer moving relative to the events measures a longer time than the proper time: t = t₀/√(1 − v2/c2). · Time dilation and muon decay
Time-division multiplexing
Sharing one channel by giving each user the whole channel for a brief repeating time slot, which suits digital signals because their samples are short pulses with gaps between them. · Radio propagation and data security
Torque
The turning effect of a force about an axis: T = Fr for a force applied tangentially at radius r, in N m. · Torque and angular acceleration
Tracer
A radionuclide attached to a molecule the body processes, so detectors outside the body can map where it is taken up. A gamma camera uses a gamma emitter; PET uses a positron emitter, whose annihilation photons are what escape. · Radionuclide imaging and the gamma camera
Transit method
Detecting an exoplanet from the periodic dip in a star's brightness as the planet crosses its face. · Quasars
Transverse wave
A wave whose oscillations are perpendicular to the direction of energy transfer. · Longitudinal and transverse waves
UUncertainty, Universe, Upthrust3 terms
Uncertainty
The interval within which the true value can reasonably be expected to lie, quoted after a ± sign. · Uncertainty and error
Universe
Everything that exists: all matter and radiation, together with the space and time they occupy. (OCR A.) · The HR diagram and the life of Sun-like stars
Upthrust
The upward force on a body in a fluid, equal to the weight of fluid displaced by the body. · Fluids: pressure, upthrust and viscosity
VVector, Virtual earth, Virtual image…4 terms
Vector
A quantity with both magnitude and direction, such as velocity, displacement, force or momentum. · Scalars and vectors
Virtual earth
The inverting input of an inverting amplifier, held within microvolts of 0 V by negative feedback although it carries no connection to earth. · The ideal op-amp and the comparator
Virtual image
An image formed where rays only appear to come from when projected backwards; it cannot be caught on a screen. · Lenses and images
Viscosity
The property of a fluid that measures its resistance to flow: the constant η in Stokes' law. · Fluids: pressure, upthrust and viscosity
WWavelength, Weight, Work…4 terms
Wavelength
The distance between two adjacent points on a wave oscillating in phase, such as crest to crest. · Progressive waves
Weight
The gravitational force acting on an object's mass: W = mg, a vector. · Mass and weight
Work
Energy transferred by a force: the force multiplied by the distance moved in the direction of the force, W = Fs cos θ. · Work, energy and power
Work function
The minimum energy needed to release an electron from the surface of a metal. · The photoelectric effect and the photon
YYoung modulus1 term
Young modulus
The ratio of tensile stress to tensile strain for a material within the limit of proportionality. · Stress, strain and the Young modulus
ZZener diode1 term
Zener diode
A diode built to break down at a chosen reverse voltage without damage, holding its pd nearly constant while the current through it varies widely. · The MOSFET as a switch