Required Practicals/AQA/Practical 6
6AS3.5.2.1

EMF and internal resistance of a cell

Determine the EMF and internal resistance of a cell by measuring terminal pd and current for different external resistances.

Apparatus

  • Cell or battery under test
  • Ammeter (in series) and voltmeter (across cell terminals)
  • Variable resistor (rheostat) or set of fixed resistors
  • Switch and connecting leads

Safety

  • Do not short-circuit the cell (rheostat at zero resistance): this causes excessive current, overheating and possible damage to the cell.
  • Open the switch between readings to prevent the battery discharging and its EMF changing.

Method

  1. Connect the ammeter in series with the cell and rheostat; connect the voltmeter directly across the cell terminals.
  2. Adjust the rheostat to its maximum resistance and close the switch. Record terminal pd V and current I.
  3. Decrease resistance in steps; record V and I for at least eight settings.
  4. Plot V against I: straight line with negative gradient.
  5. Y-intercept gives EMF; magnitude of gradient gives internal resistance r.

Key Variables

Independent
Current I (varied by adjusting external resistance)
Dependent
Terminal pd V
Controlled
Same cell throughout; Temperature of cell

Analysis and Results

  • From $V = \varepsilon - Ir$: plot V vs I gives gradient $= -r$ and y-intercept $= \varepsilon$.
  • The x-intercept (where V = 0) gives the short-circuit current $I_{sc} = \varepsilon/r$.
  • Compare measured EMF with the open-circuit pd measured directly with the voltmeter (should be approximately equal).

Common Errors

  • Connecting the voltmeter across the external resistor rather than the cell terminals.
  • Leaving the circuit closed between readings, causing the EMF to change as the cell discharges.
  • Plotting V against R (external resistance) rather than V against I; gradient is not simply r.
  • Not considering that the voltmeter draws a small current itself.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q14 marks
A cell of EMF 1.50 V and internal resistance 0.80 $\Omega$ is connected to an external resistance of 5.2 $\Omega$. Calculate the current and the terminal pd.
Q23 marks
A V against I graph gives a y-intercept of 1.48 V and a gradient of -0.62 V A$^{-1}$. State the EMF and internal resistance, and explain what happens to the terminal pd as current increases.
Q32 marks
Explain why the open-circuit pd across the cell terminals equals its EMF.