Pearson International A Level Physics Core Practical 11: Analyse capacitor charging and discharging

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Record capacitor potential difference during charging or discharging and test an exponential model.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use capacitor, resistor, d.c. supply, switch, high-resistance voltmeter or data logger, timer and leads. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Measure component values and discharge the capacitor safely before changing the circuit.
2. Switch at a defined time and record potential difference at frequent intervals, using data logging where possible.
3. Continue for several time constants and repeat after restoring the same initial condition.
4. Plot voltage against time and linearise the discharge data using a logarithm if required.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: elapsed time after switching.
• Dependent variable: capacitor potential difference or current.
• Important controls: resistance, capacitance, supply emf, initial charge, meter loading and temperature.