How to Write Definitions in IGCSE Physics

Definition questions appear on nearly all IGCSE Physics papers. Each mark requires a specific element, and missing one element loses that mark.

Structure of a physics definition

Most definitions follow this pattern: [quantity] is the [what] per [what] or [quantity] is the [relationship between two things].

Key definitions

Speed: The distance travelled per unit time.

Velocity: The speed of an object in a given direction. (Or: the rate of change of displacement.)

Acceleration: The change in velocity per unit time.

Force: That which can cause an object to accelerate, decelerate or change direction. (Newton’s second law version: force = mass x acceleration.)

Weight: The gravitational force acting on an object.

Density: The mass per unit volume of a substance.

Pressure: The force acting per unit area.

Moment: The turning effect of a force about a pivot; moment = force x perpendicular distance from the pivot.

Work done: The product of the force and the distance moved in the direction of the force.

Power: The rate of doing work (or the rate of energy transfer).

Specific heat capacity: The energy required to raise the temperature of 1 kg of a substance by 1 degree C (or 1 K).

Specific latent heat: The energy required to change the state of 1 kg of a substance without a change in temperature.

Wavelength: The distance between two consecutive points in phase on a wave (e.g., crest to crest).

Frequency: The number of complete waves passing a point per second.

EMF: The energy transferred per unit charge by a source in driving charge around a complete circuit.

Resistance: The ratio of the potential difference across a component to the current through it.

Exam tip

Definitions must be memorised word-for-word. A vague answer like “how fast something goes” for speed will lose the mark. The examiner needs “distance” and “per unit time.”