A Level Physics 9702 · Exam special · AS & A Level · every topic · ~8 min read
A Level Physics 9702: Definitions You Must Know
Every definition 9702 asks for, AS and A Level, in one place: worded precisely enough to earn the mark, with the key words examiners look for and the near-misses that score zero.
MeasurementKinematics & dynamicsForces & materialsWaves & superpositionElectricityParticle physicsFieldsThermal physics
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Paper tag = where it is examinedDATA & FORMULAE given in the examC1A1B1 how marks are awardedhighlight = comes up again and againHANDY worth memorisingTest yourself at the end
In 30 seconds
- Definitions are easy marks, often 1 or 2 each, on Papers 2 and 4. They are marked strictly: the key word must be there.
- Ratio definitions: "X per unit Y" is the safest form. "Force per unit positive charge" scores; "force on a charge" doesn't.
- Watch three little words: "per unit", "positive" (test charge), and "from infinity" (potential).
- Learn the formula and the words together. A formula alone usually scores if every symbol is defined, but words are safer.
By the end you can
- Recall precise definitions for every 9702 topic.
- Include the key words the mark scheme requires.
- Avoid the common near-miss wordings.
1AS Level (Topics 1–11)
Measurement, motion and forces
| Term | Definition |
|---|---|
| Systematic error | an error that makes every reading differ from the true value by the same amount or fraction; reduced by calibration, not by repeating |
| Random error | scatter of readings about the true value; reduced by repeating and averaging |
| Accuracy | how close a value is to the true value |
| Precision | how close repeated readings are to each other |
| Displacement | distance in a stated direction from a fixed point |
| Velocity | rate of change of displacement |
| Acceleration | rate of change of velocity |
| Momentum | product of mass and velocity |
| Newton's second law | the resultant force is proportional to the rate of change of momentum (F = Δp/Δt) |
| Newton's first law | a body stays at rest or at constant velocity unless a resultant force acts on it |
| Principle of conservation of momentum | the total momentum of a system remains constant provided no resultant external force acts |
| Moment | force × perpendicular distance from the pivot to the line of action of the force |
| Couple | a pair of equal and opposite forces whose lines of action do not coincide; torque = one force × perpendicular distance between them |
| Principle of moments | for equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point |
| Centre of gravity | the point at which the whole weight of a body may be considered to act |
| Density | mass per unit volume |
| Pressure | normal force per unit area |
| Work done | force × distance moved in the direction of the force |
| Power | work done per unit time |
| Efficiency | useful energy output ÷ total energy input |
Materials, waves and electricity
| Term | Definition |
|---|---|
| Stress | force per unit cross-sectional area |
| Strain | extension per unit original length |
| Young modulus | stress ÷ strain (in the limit of proportionality) |
| Elastic deformation | the object returns to its original shape when the load is removed |
| Plastic deformation | the object does not return to its original shape when the load is removed |
| Wavelength | the distance between adjacent points on a wave that are in phase |
| Intensity | power per unit area (perpendicular to the direction of travel) |
| Coherent | having a constant phase difference |
| Principle of superposition | when two waves meet, the resultant displacement is the sum of the individual displacements |
| Diffraction | the spreading of a wave as it passes through a gap or around an edge |
| Electric current | the rate of flow of charge |
| Coulomb | the charge passing a point when a current of 1 A flows for 1 s |
| Potential difference | energy transferred from electrical to other forms per unit charge |
| E.m.f. | energy transferred from other forms to electrical per unit charge |
| Resistance | potential difference ÷ current |
| Resistivity | resistance × cross-sectional area ÷ length (ρ = RA/L) |
| Kirchhoff's first law | the sum of currents into a junction equals the sum out (conservation of charge) |
| Kirchhoff's second law | in a closed loop, the sum of e.m.f.s equals the sum of p.d.s (conservation of energy) |
Common mistake"Voltage per unit charge" or "energy per charge" without "per unit". And e.m.f. vs p.d.: the direction of energy transfer is the difference.
2A Level (Topics 12–25)
Fields
| Term | Definition |
|---|---|
| Radian | the angle subtended at the centre of a circle by an arc equal in length to the radius |
| Angular speed | rate of change of angular displacement |
| Gravitational field strength | gravitational force per unit mass |
| Gravitational potential | work done per unit mass in bringing a small test mass from infinity to the point |
| Electric field strength | force per unit positive charge (on a stationary charge) |
| Electric potential | work done per unit positive charge in bringing a small test charge from infinity to the point |
| Capacitance | charge stored per unit potential difference |
| Time constant | the time for the charge (or p.d., or current) to fall to 1/e of its initial value during discharge; τ = RC |
| Magnetic flux density | force per unit current per unit length on a straight conductor at right angles to the field |
| Magnetic flux | flux density × area perpendicular to the field |
| Faraday's law | the induced e.m.f. is proportional to the rate of change of magnetic flux linkage |
| Lenz's law | the induced e.m.f. acts in a direction to oppose the change that produces it |
| r.m.s. value | the steady direct current (or p.d.) that dissipates the same mean power in a resistor |
Thermal physics and oscillations
| Term | Definition |
|---|---|
| Thermal equilibrium | no net transfer of thermal energy between bodies (same temperature) |
| Absolute zero | the temperature at which a substance has minimum internal energy |
| Specific heat capacity | energy per unit mass to raise the temperature by one degree |
| Specific latent heat | energy per unit mass to change state with no change in temperature |
| Internal energy | sum of the random kinetic and potential energies of the molecules |
| Ideal gas | a gas obeying pV ∝ T (T in kelvin) at all pressures, volumes and temperatures |
| Avogadro constant | the number of particles in one mole |
| Simple harmonic motion | acceleration proportional to displacement from a fixed point and directed towards that point |
| Resonance | maximum amplitude of a forced oscillation, when the driving frequency equals the natural frequency |
Quantum, nuclear, medical and astronomy
| Term | Definition |
|---|---|
| Photon | a quantum (discrete packet) of electromagnetic energy |
| Work function | the minimum energy needed to remove an electron from the surface of a metal |
| Threshold frequency | the minimum frequency of radiation that causes photoelectric emission |
| Electronvolt | the energy gained by an electron accelerated through a p.d. of 1 V |
| Mass defect | the difference between the total mass of the separate nucleons and the mass of the nucleus |
| Binding energy | the minimum energy needed to separate a nucleus into its individual nucleons |
| Activity | the number of nuclei decaying per unit time |
| Decay constant | the probability per unit time that a nucleus will decay |
| Half-life | the time for the number of undecayed nuclei (or activity) to halve |
| Acoustic impedance | density × speed of sound in the medium |
| Luminosity | the total radiant power emitted by a star |
| Radiant flux intensity | radiant power passing normally through unit area |
| Standard candle | an astronomical object of known luminosity |
Common mistake"Binding energy is the energy released when a nucleus forms" is acceptable, but "energy stored in the nucleus" isn't. And activity is "number of decays per unit time", not "rate of radiation".
Linking questionEvery definition is used in the topic notes from Physical quantities and units to Astronomy and cosmology.
Test yourself — cover the answers first
Quick check
- Define the radian.
- Define electric potential.
- Distinguish e.m.f. and p.d.
- Define the decay constant.
- Define coherent.
Show answers
- the angle subtended at the centre by an arc equal in length to the radius
- work done per unit positive charge bringing it from infinity
- e.m.f.: other forms → electrical per unit charge; p.d.: electrical → other forms per unit charge
- probability per unit time that a nucleus decays
- constant phase difference
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