Pearson International A Level Physics Core Practical 8: Determine a cell's electromotive force and internal resistance

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

Vary load current and use terminal potential difference to determine cell emf and internal resistance.

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 cell, variable resistor or resistance box, switch, ammeter, voltmeter and connecting 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. Connect the ammeter in series and voltmeter across the cell with the switch open initially.
2. Vary external resistance to obtain paired current and terminal-voltage readings without excessive current.
3. Open the switch between readings and repeat points after checking cell condition.
4. Plot terminal voltage against current; intercept gives emf and negative gradient gives internal resistance.

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: load resistance or resulting current.
• Dependent variable: terminal potential difference.
• Important controls: