Every 9702 Definition You Must Know (with the Wording That Earns the Mark)
Published Oct 17, 2026
In Cambridge A Level Physics 9702, a definition is usually worth one or two marks, and it is either right or wrong. Examiners are not looking for a rough idea; they are looking for specific words. "Force per unit mass" earns the mark; "the pull of gravity" does not.
The good news is that definitions are the most predictable marks on Papers 2 and 4. The syllabus tells you exactly which quantities you must be able to define, and the wording barely changes from year to year. This guide lists them by topic, gives the precise wording, and points out the slip that most often costs the mark.
How to use this list
- Learn the wording, not just the meaning. Write each definition from memory, then compare it word by word.
- Know the formula behind each one. Many definitions are a ratio in words: "force per unit mass" is g = F/m. If you can write the equation, you can usually rebuild the sentence.
- Test yourself weekly. Cover the right-hand column and recite. Ten minutes a week is enough.
- Use it in context. Definition questions often lead into a calculation, so practise them inside real questions with 9702 topical questions.
A word on "per unit": it means "divided by". Every "per unit" in a definition corresponds to a division in the defining equation.
AS Level definitions (topics 1–11)
Kinematics and dynamics
| Quantity | Definition | Common slip |
|---|---|---|
| Displacement | Distance in a specified direction from a fixed point | Omitting direction |
| Velocity | Rate of change of displacement | Writing "distance" (that defines speed) |
| Acceleration | Rate of change of velocity | Writing "change of speed" |
| Linear momentum | Product of mass and velocity | Forgetting that it is a vector |
| Force | Rate of change of momentum | Only giving F = ma, which is a special case |
Forces, density and pressure
| Quantity | Definition | Common slip |
|---|---|---|
| Moment of a force | Force × perpendicular distance from the pivot to the line of action of the force | Leaving out "perpendicular" |
| Torque of a couple | One of the forces × perpendicular distance between the forces | Using the distance to the pivot |
| Density | Mass per unit volume | — |
| Pressure | Force per unit area, where the force acts normally to the area | Leaving out "normal" |
Work, energy, power and deformation
| Quantity | Definition | Common slip |
|---|---|---|
| Work done | Force × displacement in the direction of the force | Leaving out "in the direction of the force" |
| Power | Work done per unit time | Writing "energy" without "per unit time" |
| Stress | Force per unit cross-sectional area | — |
| Strain | Extension per unit original length | Using the new length |
| Young modulus | Stress ÷ strain | Applying it beyond the limit of proportionality |
Electricity
| Quantity | Definition | Common slip |
|---|---|---|
| Potential difference | Energy transferred (from electrical to other forms) per unit charge | "Energy per unit current" |
| Electromotive force | Energy transferred (from other forms to electrical) per unit charge in driving charge around a complete circuit | Confusing it with p.d.: e.m.f. is energy gained by the charge, p.d. is energy given up |
| Resistance | Potential difference ÷ current | Writing "opposition to current" (a description, not a definition) |
| Ohm's law | The current in a conductor is proportional to the potential difference across it, provided physical conditions (e.g. temperature) are constant | Leaving out the condition |
A Level definitions (topics 12–25)
Motion in a circle and gravitation
| Quantity | Definition | Common slip |
|---|---|---|
| Radian | The angle subtended at the centre of a circle by an arc equal in length to the radius | — |
| Gravitational field strength | Force per unit mass | "Force on a mass" |
| Gravitational potential (at a point) | Work done per unit mass in bringing a small test mass from infinity to the point | Leaving out "from infinity" |
Thermal physics
| Quantity | Definition | Common slip |
|---|---|---|
| Specific heat capacity | Energy required per unit mass to raise the temperature by one degree (1 K) | Leaving out "per unit mass" |
| Specific latent heat | Energy required per unit mass to change the state of a substance without a change in temperature | Leaving out "without change of temperature" |
| Internal energy | Sum of the random kinetic and potential energies of the molecules | Leaving out "random" or the potential energy |
Oscillations
| Quantity | Definition | Common slip |
|---|---|---|
| Simple harmonic motion | Acceleration is proportional to displacement from a fixed point and in the opposite direction | Giving only one of the two conditions |
Fields and capacitance
| Quantity | Definition | Common slip |
|---|---|---|
| Electric field strength | Force per unit positive charge | Leaving out "positive" |
| Electric potential (at a point) | Work done per unit positive charge in bringing a small test charge from infinity to the point | Leaving out "positive" or "from infinity" |
| Capacitance | Charge stored per unit potential difference | "Ability to store charge" |
| Magnetic flux density | Force acting per unit current per unit length on a wire placed at right angles to the magnetic field | Leaving out "at right angles" |
| Magnetic flux | Product of magnetic flux density and the area perpendicular to the field | — |
Quantum and nuclear physics
| Quantity | Definition | Common slip |
|---|---|---|
| Photon | A quantum of electromagnetic energy | — |
| Work function | Minimum energy needed to remove an electron from the surface of a metal | Leaving out "minimum" or "surface" |
| Threshold frequency | Minimum frequency of radiation that causes electrons to be emitted from the surface | — |
| Half-life | Time taken for the number of undecayed nuclei (or the activity) to halve | "Time for the substance to halve" |
Statements of laws and principles
Some questions ask you to state a law rather than define a quantity. The same precision applies: - Newton's first law: a body remains at rest or moves with constant velocity unless acted on by a resultant force. - Conservation of momentum: the total momentum of a system remains constant provided no resultant external force acts. - 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. - Kirchhoff's first law: the sum of currents into a junction equals the sum of currents out (a consequence of conservation of charge). - Kirchhoff's second law: around any closed loop, the sum of e.m.f.s equals the sum of p.d.s (a consequence of conservation of energy). - Lenz's law: the induced e.m.f. acts in a direction that opposes the change producing it.
Worked example (original question)
(a) Define electric potential at a point. [2] (b) Explain why the electric potential near an isolated negative charge is negative. [1]
(a) Work done per unit positive charge [1] in bringing a small test charge from infinity to the point [1].
(b) A positive test charge is attracted to the negative charge, so work is done by the field (energy released) as it is brought from infinity. Potential is zero at infinity and decreases, so it is negative [1].
Note how part (b) depends on knowing the definition in (a) exactly. This pattern is common in Paper 4.
Three habits that protect definition marks
- Don't add extra conditions you're unsure of. A wrong addition can cancel a correct definition.
- Use the quantity names, not symbols alone. "F/m" without words may not be accepted where a definition in words is required.
- Say "per unit", not "per". "Work done per positive charge" is weaker than "per unit positive charge".
Practise by topic
Download topical PDFs of past-paper questions, with mark schemes: Electricity · D.C. circuits · Motion in a circle · Gravitational fields · Temperature · Ideal gases · Oscillations · Electric fields · Capacitance · Quantum physics · Nuclear physics. Or browse all 9702 past papers.
Frequently asked questions
Are these the exact mark-scheme words? They follow the syllabus wording and the phrasing accepted in Cambridge mark schemes. Equivalent wording that carries the same meaning is usually accepted, but the safest approach is to learn one precise version.
Do I need definitions for Paper 1? Paper 1 is multiple choice, so you won't write them, but options are often designed to catch students who half-know a definition.
How many definitions are there? The list above covers every quantity the 2025–2027 syllabus asks you to define, plus the most common "state" items.
Make the definitions stick
Definitions are easy marks only if they are automatic. In lessons, Engr. Muhammad Waseem drills them inside real exam questions, so the wording becomes second nature under time pressure.
9702 past papers · 9702 Paper 4 technique
Sources: Cambridge International AS & A Level Physics 9702 syllabus for examination in 2025–2027 (the "define", "state" and "recall" learning outcomes across topics 1–25): https://www.cambridgeinternational.org/Images/664565-2025-2027-syllabus.pdf
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