How to Answer 'Define' Questions in IGCSE Physics

Use this guide to practise writing definitions that earn full marks in Cambridge IGCSE Physics 0625 exams.

What the examiner expects

A ‘Define’ question asks for a precise statement of what a physical quantity means. The mark scheme usually requires two or three key elements.

For most quantity definitions, the structure is:

[quantity] is the [measurable relationship] per [unit of the denominator]

Examples of strong definitions

Speed

“Speed is the distance travelled per unit time.”

Key elements: distance, per unit time.

Acceleration

“Acceleration is the change in velocity per unit time.”

Key elements: change in velocity (not just velocity), per unit time.

Specific heat capacity

“Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1 $^\circ$C (or 1 K).”

Key elements: energy, 1 kg, temperature change of 1 degree.

Resistance

“Resistance is the ratio of potential difference across a component to the current through it.”

Key elements: potential difference, current, ratio.

Pressure

“Pressure is the force acting per unit area.”

Key elements: force, per unit area.

Common errors and how to correct them

1. Missing the ‘per unit’ structure. Saying “acceleration is how fast velocity changes” loses the mark for “per unit time.” Always include the denominator.

2. Using the wrong variable. Defining velocity as “distance per unit time” is the definition of speed, not velocity. Velocity requires displacement and a specified direction.

3. Circular definitions. “Power is the amount of power something uses” earns zero. Restate the definition using different, more fundamental terms.

A reliable process

1. Identify the quantity being defined.
2. Recall the equation (e.g. $a = \Delta v / t$).
3. Translate the equation into words: “the change in velocity per unit time.”
4. Check that your statement does not repeat the quantity’s name in the definition.
5. Include the SI unit if the question asks for it separately.

How to apply this in an exam

Write the definition in one clear sentence. Use precise physics language. Check against the equation if you are unsure. Do not pad the answer with extra information that risks contradicting the definition.