GCSE & IGCSE Physics key terms

202 terms with short definitions you can learn for the exam. Type to filter the list.

Command words 27

Add / Label: Put information or names on a diagram, graph or table you are given.
Assess GCSE: Consider all the factors, decide which matter most and make a judgement.
Calculate: Work out a number from the data, showing your working. Include the unit.
Comment on: Look at the data or information given and decide what it shows, linking several factors.
Compare GCSE: Give similarities and differences between two or more things. Write about both in every point.
Complete: Fill in the missing parts of a table, diagram or sentence.
Deduce: Reach a conclusion from the information you are given.
Describe: Give an account of what happens or what something is like. You do not need to say why.
Design IGCSE: Plan a procedure or method from scientific ideas you know.
Determine: Get an answer that includes a number, using the data given, and show how you got it.
Devise GCSE: Plan a method or experiment, using what you know.
Discuss: Pick out the issue, then explore its different sides with reasoning.
Draw: Produce a diagram, with a ruler where lines should be straight (for example ray diagrams and circuits).
Estimate: Find an approximate value from data or a graph, or with a sensible rounding.
Evaluate: Look at the evidence or information, weigh it up and reach a supported conclusion.
Explain: Say why or how something happens. Each point needs a reason, so use linking words such as 'because' and 'so'.
Give / State / Name: Recall a fact or a short piece of information. No explanation needed.
Give a reason / reasons: Say why something is the case. Give only the number of reasons asked for.
Identify: Pick out the key information from the data, diagram or text given.
Justify: Give evidence that supports a statement or an earlier answer.
Plot: Mark points accurately on a grid from data, then usually draw a line or curve of best fit.
Predict: Say what you expect to happen, based on the pattern or your knowledge.
Show that: Prove that the value or statement given is correct. Show every step and give your answer to more significant figures than the value in the question.
Sketch: Draw freehand: for a graph, labelled axes and the correct shape, without plotting exact points.
State what is meant by: Give the meaning of a term. Different wordings can earn the mark if the physics is right.
Suggest: Use your knowledge to give a sensible idea for an unfamiliar situation. There may be more than one acceptable answer.
Write GCSE: Write down an answer such as an equation, in words or symbols.

Key terms 99

Absolute magnitude IGCSE: How bright a star would look from a standard distance (10 parsecs), so stars can be compared fairly. More negative means brighter.
Absolute zero: The lowest possible temperature, 0 K or about -273 °C, where particles have the least kinetic energy.
Acceleration: The rate of change of velocity: change in velocity ÷ time. Unit m/s^{2}.
Activity: The number of nuclei that decay each second in a radioactive sample. Unit becquerel (Bq).
Alpha particle: Two protons and two neutrons (a helium nucleus). Strongly ionising, stopped by paper or a few cm of air.
Alternating current (a.c.): Current that keeps reversing direction. UK mains is a.c. at 50 Hz.
Amplitude: The maximum displacement of a point on a wave from its rest position.
Atomic number: The number of protons in the nucleus of an atom.
Background radiation: Low-level ionising radiation that is always around us, from sources such as rocks, cosmic rays and medical uses.
Beta particle: A fast-moving electron emitted from the nucleus when a neutron turns into a proton. Stopped by a few mm of aluminium.
Big Bang theory: The model that the universe began from a very small, very hot, dense region and has been expanding ever since.
Black hole: What may be left after a supernova of a very massive star: so dense that not even light can escape.
Braking distance: The distance a vehicle travels after the brakes are applied until it stops.
Centre of gravity IGCSE: The point through which the whole weight of an object can be taken to act.
Chain reaction: In fission, neutrons released by one split nucleus go on to split more nuclei.
Charge: A property of particles such as electrons and protons. Charge = current × time. Unit coulomb (C).
Conduction (thermal) IGCSE: Energy transfer through a material by particles vibrating and passing energy on. Metals conduct well because of free electrons.
Contamination: When radioactive material gets onto or into an object or person, so they are exposed to radiation for longer.
Convection IGCSE: Energy transfer in a fluid: heated fluid expands, becomes less dense and rises, and cooler fluid sinks to replace it.
Cosmic microwave background (CMB): Microwave radiation reaching us from every direction in space, left over from the early universe. Evidence for the Big Bang.
Critical angle IGCSE: The angle of incidence inside a denser material that gives an angle of refraction of 90°. sin c = 1 ÷ n.
Current: The rate of flow of charge. Unit ampere (A).
Density: Mass per unit volume: ρ = m ÷ V. Unit kg/m^{3}.
Diode: A component that lets current flow in one direction only.
Direct current (d.c.): Current that flows in one direction only, as from a cell or battery.
Displacement: Distance moved in a stated direction. A vector.
Doppler effect IGCSE: The change in observed frequency and wavelength when a wave source moves towards or away from an observer.
Earthing: Connecting a conductor to the ground so charge can flow away safely. The earth wire protects users from metal cases becoming live.
Efficiency: The fraction of the total energy supplied that is usefully transferred: useful energy ÷ total energy.
Elastic deformation: A change of shape that is fully reversed when the force is removed.
Electromagnet: A coil of wire (solenoid), usually around an iron core, that is magnetic only when a current flows.
Electromagnetic induction: Producing a potential difference in a conductor when it moves through a magnetic field or the field around it changes.
Electromagnetic spectrum: The family of transverse waves that travel at 3 × 10^{8} m/s in a vacuum: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Extension: The increase in length of a spring or wire when it is stretched.
Fleming's left-hand rule: Finger rule for the motor effect: first finger field, second finger current, thumb force (motion).
Frequency: The number of waves passing a point each second. Unit hertz (Hz). f = 1 ÷ T.
Friction: A force that opposes motion between surfaces in contact.
Fuse: A thin wire in the live line that melts and breaks the circuit if the current is too large.
Gamma ray: High-frequency electromagnetic radiation emitted from a nucleus. Weakly ionising and very penetrating; reduced by thick lead or concrete.
Generator: A device that uses electromagnetic induction to transfer energy from a kinetic store to an electrical supply.
Gravitational field strength (g): The force of gravity on each kilogram of mass. Unit N/kg. About 10 N/kg on Earth.
Half-life: The time taken for half the unstable nuclei in a sample to decay, or for the activity to halve.
Hertzsprung-Russell (HR) diagram IGCSE: A graph of stars' absolute magnitude (or luminosity) against surface temperature. Shows the main sequence, giants and white dwarfs.
Hooke's law: The extension of a spring is proportional to the force applied, up to the limit of proportionality: F = k × x.
Inelastic deformation: A change of shape that stays after the force is removed.
Infrared: Electromagnetic radiation just beyond red light. Emitted by all objects; hotter objects emit more.
Ionising radiation: Radiation that can remove electrons from atoms, forming ions. Can damage cells and DNA.
Irradiation: Exposing an object to radiation. It does not make the object radioactive.
Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.
Kelvin scale: Temperature scale starting at absolute zero. Temperature in K = temperature in °C + 273.
Kinetic energy: Energy in the store of a moving object: KE = ½ × m × v^{2}.
Light-dependent resistor (LDR): A resistor whose resistance falls as light intensity increases.
Limit of proportionality: The point beyond which extension is no longer proportional to the force.
Longitudinal wave: A wave in which the particles oscillate parallel to the direction of energy transfer, for example sound.
Magnetic field: The region around a magnet or current-carrying wire where a magnetic material or another magnet feels a force.
Main sequence star: The long, stable stage of a star's life, when the inward pull of gravity is balanced by the outward pressure from fusion.
Mass number: The total number of protons and neutrons in a nucleus.
Moment: The turning effect of a force: force × perpendicular distance from the pivot. Unit N m.
Momentum: Mass × velocity. Unit kg m/s. A vector, conserved in collisions and explosions.
National Grid GCSE: The UK network of cables and transformers that distributes electricity from power stations to consumers.
Nebula: A cloud of dust and gas in space where stars form.
Neutron star: The very dense core left after a supernova of a massive star.
Normal: A line drawn at 90° to a surface where a ray meets it. Angles of incidence and refraction are measured from it.
Nuclear fission: The splitting of a large nucleus, such as uranium-235, into two smaller nuclei, releasing energy and neutrons.
Nuclear fusion: The joining of two light nuclei to form a heavier nucleus, releasing energy. It powers stars.
P-waves GCSE: Longitudinal seismic waves that travel through solids and liquids.
Period: The time for one complete wave or oscillation. Unit second (s).
Potential difference: The energy transferred per unit charge between two points: E = Q × V. Unit volt (V). The International GCSE often calls it voltage.
Power: The rate of energy transfer, or work done per second. Unit watt (W).
Pressure: Force per unit area acting at right angles to a surface: p = F ÷ A. Unit pascal (Pa).
Protostar: A hot, dense ball of gas pulled together by gravity, before fusion starts.
Red giant: The large, cooler stage a Sun-like star reaches when hydrogen in its core runs out.
Red-shift: The increase in wavelength of light from a galaxy that is moving away from us. More distant galaxies show more red-shift.
Refraction: A change in direction of a wave when it crosses a boundary at an angle, caused by a change in speed.
Refractive index IGCSE: n = sin i ÷ sin r. A measure of how much a material slows light and bends it.
Resistance: Opposition to current: R = V ÷ I. Unit ohm (Ω).
Resultant force: The single force that has the same effect as all the forces acting on an object together.
S-waves GCSE: Transverse seismic waves that travel through solids but not liquids.
Scalar: A quantity with size (magnitude) only, such as speed, mass, energy or time.
Solenoid: A long coil of wire. With a current, its field is like a bar magnet's outside and uniform inside.
Specific heat capacity: The energy needed to raise the temperature of 1 kg of a substance by 1 °C. Unit J/kg °C.
Specific latent heat GCSE: The energy needed to change the state of 1 kg of a substance with no change in temperature. Unit J/kg.
Static electricity: Charge that builds up on an insulator, usually by friction transferring electrons.
Step-up transformer: A transformer with more turns on the secondary coil than the primary, so the output voltage is higher.
Supernova: The huge explosion at the end of the life of a star much more massive than the Sun.
Terminal velocity: The constant top speed of a falling object, reached when air resistance equals weight.
Thermistor: A resistor whose resistance falls as its temperature rises.
Thinking distance: The distance travelled during the driver's reaction time, before the brakes are applied.
Total internal reflection IGCSE: When light inside a denser material hits the boundary at more than the critical angle and is all reflected back.
Transformer: Two coils on an iron core that change the size of an alternating voltage. Only works with a.c.
Transverse wave: A wave in which the oscillations are at right angles to the direction of energy transfer, for example light.
Ultrasound GCSE: Sound with a frequency above 20 000 Hz (20 kHz), too high for humans to hear.
Upthrust GCSE: The upward force a fluid exerts on an object in it, equal to the weight of fluid displaced.
Vector: A quantity with size and direction, such as velocity, force, displacement or momentum.
Velocity: Speed in a stated direction. A vector.
Wavelength: The distance from one point on a wave to the same point on the next wave, for example crest to crest.
Weight: The force of gravity on an object: W = m × g. Unit newton (N).
White dwarf: The small, hot, dense core left when a Sun-like star sheds its outer layers.
Work done: Energy transferred when a force moves an object: force × distance moved in the direction of the force. Unit joule (J).

Equations to recall (1PH0) 20

1. distance travelled = average speed × time GCSE: x = v × t. Units: m, m/s, s.
10. work done = force × distance moved in the direction of the force GCSE: E = F × d. Units: J, N, m.
11. power = work done ÷ time taken GCSE: P = E ÷ t. Units: W, J, s.
12. moment of a force = force × distance normal to the direction of the force (Physics only) GCSE: M = F × d. Units: N m, N, m.
13. energy transferred = charge moved × potential difference GCSE: E = Q × V. Units: J, C, V.
14. charge = current × time GCSE: Q = I × t. Units: C, A, s.
15. potential difference = current × resistance GCSE: V = I × R. Units: V, A, Ω.
16. power = energy transferred ÷ time taken GCSE: P = E ÷ t. Units: W, J, s.
17. electrical power = current × potential difference; electrical power = current^{2} × resistance GCSE: P = I × V; P = I^{2} × R. Units: W, A, V, Ω.
18. density = mass ÷ volume GCSE: ρ = m ÷ V. Units: kg/m^{3}, kg, m^{3}.
19. force exerted on a spring = spring constant × extension GCSE: F = k × x. Units: N, N/m, m.
2. acceleration = change in velocity ÷ time taken GCSE: a = (v − u) ÷ t. Units: m/s^{2}, m/s, s.
20. pressure = force normal to a surface ÷ area of that surface (Physics only) GCSE: P = F ÷ A. Units: Pa (N/m^{2}), N, m^{2}.
3. force = mass × acceleration GCSE: F = m × a. Units: N, kg, m/s^{2}.
4. weight = mass × gravitational field strength GCSE: W = m × g. Units: N, kg, N/kg.
5. momentum = mass × velocity (HT) GCSE: p = m × v. Units: kg m/s, kg, m/s.
6. change in gravitational potential energy = mass × gravitational field strength × change in vertical height GCSE: ΔGPE = m × g × Δh. Units: J, kg, N/kg, m.
7. kinetic energy = ½ × mass × (speed)^{2} GCSE: KE = ½ × m × v^{2}. Units: J, kg, m/s.
8. efficiency = useful energy transferred by the device ÷ total energy supplied to the device GCSE: No unit (a decimal, or × 100 for a percentage).
9. wave speed = frequency × wavelength; wave speed = distance ÷ time GCSE: v = f × λ; v = x ÷ t. Units: m/s, Hz, m, s.

Equations given on the equation sheet (1PH0) 11

(final velocity)^{2} − (initial velocity)^{2} = 2 × acceleration × distance GCSE: v^{2} − u^{2} = 2 × a × x. Units: m/s, m/s^{2}, m.
change in thermal energy = mass × specific heat capacity × change in temperature GCSE: ΔQ = m × c × Δθ. Units: J, kg, J/kg °C, °C.
energy transferred = current × potential difference × time GCSE: E = I × V × t. Units: J, A, V, s.
energy transferred in stretching = 0.5 × spring constant × (extension)^{2} GCSE: E = ½ × k × x^{2}. Units: J, N/m, m.
For gases of fixed mass at constant temperature (Physics only, HT) GCSE: P_{1} × V_{1} = P_{2} × V_{2}. Units: Pa, m^{3}.
For transformers with 100% efficiency: p.d. across primary × current in primary = p.d. across secondary × current in secondary GCSE: V_{p} × I_{p} = V_{s} × I_{s}. Units: V, A.
For transformers: p.d. across primary ÷ p.d. across secondary = turns on primary ÷ turns on secondary (Physics only, HT) GCSE: V_{p} ÷ V_{s} = N_{p} ÷ N_{s}.
force = change in momentum ÷ time (HT) GCSE: F = (mv − mu) ÷ t. Units: N, kg m/s, s.
force on a conductor at right angles to a magnetic field = magnetic flux density × current × length (HT) GCSE: F = B × I × l. Units: N, T, A, m.
pressure due to a column of liquid = height of column × density of liquid × gravitational field strength (Physics only, HT) GCSE: P = h × ρ × g. Units: Pa, m, kg/m^{3}, N/kg.
thermal energy for a change of state = mass × specific latent heat GCSE: Q = m × L. Units: J, kg, J/kg.

SI units and prefixes 12

Area and volume: 1 m^{2} = 10 000 cm^{2}. 1 m^{3} = 1 000 000 cm^{3}. 1 g/cm^{3} = 1000 kg/m^{3}.
centi (c): × 10^{-2} (one hundredth). 30 cm = 0.30 m.
Derived units: newton (N) force, joule (J) energy, watt (W) power, pascal (Pa) pressure, coulomb (C) charge, volt (V) potential difference, ohm (Ω) resistance, hertz (Hz) frequency, tesla (T) magnetic flux density, becquerel (Bq) activity.
giga (G): × 10^{9} (one thousand million). 1 GW = 1 000 000 000 W.
kilo (k): × 10^{3} (one thousand). 5 km = 5000 m.
mega (M): × 10^{6} (one million). 2 MJ = 2 000 000 J.
micro (μ): × 10^{-6} (one millionth). 40 μs = 0.000 040 s.
milli (m): × 10^{-3} (one thousandth). 250 mA = 0.25 A.
nano (n): × 10^{-9} (one thousand-millionth). 500 nm = 5 × 10^{-7} m.
SI base units used at GCSE: metre (m) length, kilogram (kg) mass, second (s) time, ampere (A) current, kelvin (K) temperature.
Temperature: K = °C + 273. A change of 1 °C is the same as a change of 1 K.
Time: 1 hour = 3600 s. Convert minutes and hours to seconds before using P = E ÷ t or Q = I × t.

Equations to recall (4PH1) 23

acceleration = change in velocity ÷ time taken (1.6) IGCSE: a = (v − u) ÷ t. Units: m/s^{2}, m/s, s.
average speed = distance moved ÷ time taken (1.4) IGCSE: Units: m/s, m, s.
charge = current × time (2.15) IGCSE: Q = I × t. Units: C, A, s.
density = mass ÷ volume (5.3) IGCSE: ρ = m ÷ V. Units: kg/m^{3}, kg, m^{3}.
efficiency = (useful energy output ÷ total energy output) × 100% (4.4) IGCSE: No unit (a percentage).
electrical power = current × voltage (2.4) IGCSE: P = I × V. Units: W, A, V.
energy transferred = charge × voltage (2.21) IGCSE: E = Q × V. Units: J, C, V.
energy transferred = work done IGCSE: Both in joules (J).
force = mass × acceleration (1.17) IGCSE: F = m × a. Units: N, kg, m/s^{2}.
gravitational potential energy = mass × g × height (4.13) IGCSE: GPE = m × g × h. Units: J, kg, N/kg, m.
input (primary) voltage ÷ output (secondary) voltage = primary turns ÷ secondary turns (6.19) IGCSE: V_{p} ÷ V_{s} = n_{p} ÷ n_{s}.
input power = output power, for 100% efficiency (6.20) IGCSE: V_{p} × I_{p} = V_{s} × I_{s}.
kinetic energy = ½ × mass × speed^{2} (4.14) IGCSE: KE = ½ × m × v^{2}. Units: J, kg, m/s.
moment = force × perpendicular distance from the pivot (1.30) IGCSE: Units: N m, N, m.
momentum = mass × velocity (1.25) IGCSE: p = m × v. Units: kg m/s, kg, m/s.
pressure = force ÷ area (5.5) IGCSE: p = F ÷ A. Units: Pa, N, m^{2}.
pressure difference = height × density × gravitational field strength (5.7) IGCSE: p = h × ρ × g. Units: Pa, m, kg/m^{3}, N/kg.
refractive index = sin i ÷ sin r (3.18) IGCSE: n = sin i ÷ sin r. No unit.
sin c = 1 ÷ n (3.22) IGCSE: c is the critical angle.
voltage = current × resistance (2.13) IGCSE: V = I × R. Units: V, A, Ω.
wave speed = frequency × wavelength (3.5) IGCSE: v = f × λ. Units: m/s, Hz, m.
weight = mass × gravitational field strength (1.18) IGCSE: W = m × g. Units: N, kg, N/kg.
work done = force × distance moved (4.11) IGCSE: W = F × d. Units: J, N, m.

Other equations in the specification (4PH1) 10

(final speed)^{2} = (initial speed)^{2} + (2 × acceleration × distance moved) (1.10) IGCSE: v^{2} = u^{2} + (2 × a × s). Units: m/s, m/s^{2}, m.
change in thermal energy = mass × specific heat capacity × change in temperature (5.13) IGCSE: ΔQ = m × c × ΔT. Units: J, kg, J/kg °C, °C.
change in wavelength ÷ reference wavelength = velocity of a galaxy ÷ speed of light (8.16) IGCSE: Δλ ÷ λ_{0} = v ÷ c.
energy transferred = current × voltage × time (2.5) IGCSE: E = I × V × t. Units: J, A, V, s.
force = change in momentum ÷ time taken (1.28) IGCSE: F = (mv − mu) ÷ t. Units: N, kg m/s, s.
frequency = 1 ÷ time period (3.6) IGCSE: f = 1 ÷ T. Units: Hz, s.
orbital speed = (2 × π × orbital radius) ÷ time period (8.6) IGCSE: v = 2πr ÷ T. Units: m/s, m, s.
power = work done ÷ time taken (4.17) IGCSE: P = W ÷ t. Units: W, J, s.
pressure × volume = constant, for a fixed mass of gas at constant temperature (5.22) IGCSE: p_{1} × V_{1} = p_{2} × V_{2}.
pressure ÷ kelvin temperature = constant, for a fixed mass of gas at constant volume (5.21) IGCSE: p_{1} ÷ T_{1} = p_{2} ÷ T_{2}. T must be in kelvin.

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