Practical and Experimental Skills
What the exam tests
This domain is assessed through school-based coursework and tests planning, measurement, uncertainty analysis, graph work and evaluation. You must identify variables, choose apparatus, record readings to the correct precision, plot an appropriate graph and use its gradient or intercept to find a physical quantity.
Core rules
- Identify the independent variable, dependent variable and controlled variables before planning.
- Take repeat readings and use the mean where appropriate; time multiple oscillations to reduce reaction-time error.
- Record raw readings to the instrument's resolution, with units and consistent significant figures.
- Plot the quantity that gives a straight line, not necessarily the raw variables. Compare y = mx + c with the theoretical equation.
- The gradient's unit comes from the axes; the intercept may reveal a systematic effect.
- Distinguish random errors, which scatter readings, from systematic errors, which shift all readings in one direction.
Worked example
A simple pendulum is used to find g. Lengths l are measured to the centre of the bob and the time for 20 oscillations is recorded. A graph of T² against l gives a straight line through the origin with gradient 4.02 s² m⁻¹. Since T = 2π√(l/g), squaring gives T² = (4π²/g)l. Thus gradient = 4π²/g. g = 4π²/4.02 = 9.82 m s⁻². The intercept should be near zero; a large intercept suggests a systematic error in length measurement, such as excluding the bob radius.
Traps
Timing one oscillation instead of many; measuring length to the top of the bob; using a large swing angle where the simple-pendulum formula fails; forcing the line through the origin when data do not support it; quoting a gradient without its unit; ignoring zero error in calipers or micrometers.