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A-level Physics required practicals

Cell emf and internal resistance

AQA 3.5.1.6 · RP6

A-level Physics (7408) · Required practical 6 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.

Board and spec code confirmed against AQA 7408 · registry checked 2026-07-11How this checking works

Determine a cell's emf and internal resistance from the variation of terminal potential difference with current.

Apparatus

  • Cell or battery whose emf is to be measured
  • Ammeter, voltmeter and switch
  • Variable resistor and fixed protective resistor
  • Connecting leads

Apparatus & techniques (AT)

AT b · digital instruments

Digital ammeter and voltmeter readings are recorded as matched I-V pairs for each external load setting.

AT f · circuit construction

The student constructs the series load circuit and connects the voltmeter directly across the cell terminals.

AT g · circuit design/checking

The circuit is checked for polarity, meter placement, protective resistance and a safe initial current before measurements begin.

Safety

Hazard

A low-resistance load can produce a large current and heat the cell, resistor or leads.

Control

Include a fixed protective resistor, begin with high variable resistance, limit the current and open the switch between readings.

Hazard

A short circuit can damage the cell.

Control

Check the circuit with the switch open and never connect the ammeter directly across the cell.

Method

  1. 1Connect the ammeter, variable resistor, protective resistor and cell in series, with the voltmeter directly across the cell terminals; check meter polarities and ranges before closing the switch.
  2. 2Set the variable resistor to high resistance, close the switch briefly and record terminal pd V and current I together.
  3. 3Open the switch, reduce the load resistance to obtain a new current and repeat over a safe, useful range.
  4. 4Take each pair quickly and leave the circuit open between readings so the cell and resistor can recover and cool.
  5. 5Plot terminal pd V on the vertical axis against current I on the horizontal axis and fit a straight line without forcing it through a chosen point.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 2: Editorial focus: choose a safe current range that is wide enough to determine both intercept and gradient reliably.
  • CPAC 4: Editorial focus: record paired V and I readings quickly while controlling the cell's load time and temperature.
  • CPAC 5: Editorial focus: interpret intercept and negative gradient with uncertainties and explain physical drift in the data.

Variables

Independent

Current I, varied by changing the external load resistance

Dependent

Terminal potential difference V across the cell

Control

  • The same cell and meter arrangement
  • Cell temperature and time under load
  • Meter ranges and lead/contact configuration

Results & processing

  • Use V = epsilon - Ir: the V-axis intercept is emf epsilon and the gradient is -r, so internal resistance is the magnitude of the gradient.
  • Use the spread of acceptable graph lines to attach uncertainties to both intercept and gradient.
  • Explain a drift in later readings through cell heating, chemical polarisation or depletion rather than treating every departure as meter noise.

Analysis skills

  • Fit V = epsilon - Ir and obtain emf from the V intercept and internal resistance from the magnitude of the gradient.
  • Use best and acceptable extreme graph lines to estimate uncertainty in both the intercept and gradient.

Uncertainty

Sources

  • Ammeter and voltmeter resolution and calibration
  • Cell heating, polarisation or depletion while current flows
  • Contact resistance and an insufficient range of current values

Calculations

  • Use half the difference between steepest and shallowest acceptable gradient magnitudes as the absolute uncertainty in r.
  • Use the spread of acceptable V-axis intercepts for emf uncertainty; when predicting V = epsilon - Ir, add absolute contributions from epsilon and Ir for a worst-case estimate.

Interpretation

  • A systematic downward drift with time means the fixed-epsilon, fixed-r model may no longer describe one stable cell state.
  • A narrow current range can give a deceptively straight graph but a poorly constrained intercept and gradient.

Exam angles

  • Correct a circuit so that terminal pd and current are measured in the intended places.
  • Extract emf and positive internal resistance from intercept and negative gradient, including graph uncertainty.
  • Explain why readings drift and distinguish internal power loss I squared r from useful external power VI.

Where students lose marks

Assuming the terminal pd is always equal to the cell emf while current flows.

Fix: Account for the lost volts Ir inside a practical cell and use V = epsilon - Ir.

  • Examiner report: P1-19 · PDF p. 6

Putting the voltmeter across the variable resistor instead of directly across the cell.

Fix: The measured dependent quantity is the cell's terminal pd, so connect the voltmeter to the cell terminals.

Reporting the negative graph gradient as a negative internal resistance.

Fix: State that gradient = -r and quote r as the positive magnitude with unit ohms.

Improve the method

  • Use a protective resistor and start at high external resistance so the maximum current stays safe.
  • Open the switch between readings and take V-I pairs promptly to limit heating, depletion and polarisation.
  • Use a wide safe current range and sufficient points so intercept and gradient are constrained by data rather than extrapolated from a narrow cluster.

Source references

  • Specification: PSpec 3.5.1.6 · PDF p. 29

Try it — exam-style

Medium
ORIGINAL

A V-against-I graph has intercept (1.58 +/- 0.02) V and best gradient -0.82 V A−1. The steepest and shallowest acceptable gradients are -0.87 V A−1 and -0.77 V A−1. Determine the emf and internal resistance with absolute uncertainties.

[4 marks]

Total for this question: 4

Medium
ORIGINAL

Later V-I readings lie below the original best-fit line even at similar currents. Give one physical cause and one procedural change that would reduce the effect.

[2 marks]

Total for this question: 2

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

AQA 7408 specification coverage

Cells, internal resistance and every linked 7408 specification point.

A-level Physics flashcards

EMF, lost volts, power and circuit analysis drilled.

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