Required Practicals/Edexcel/Practical 7
7ASCP7

EMF and internal resistance (CP7)

Determine the EMF and internal resistance of a cell using a V against I graph.

Apparatus

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

Safety

  • Do not reduce the rheostat to near zero; this short-circuits the cell and could cause overheating.
  • Open the switch between readings.

Method

  1. Connect the circuit: cell, ammeter and rheostat in series; voltmeter across the cell terminals.
  2. Start with maximum resistance. Record V and I. Decrease resistance in steps for at least eight readings.
  3. Open the switch between readings to prevent the cell discharging.
  4. Plot V against I: straight line with negative gradient.
  5. Y-intercept $= \varepsilon$ (EMF); magnitude of gradient $= r$ (internal resistance).

Key Variables

Independent
Current I (adjusted via rheostat)
Dependent
Terminal pd V
Controlled
Same cell; Temperature

Analysis and Results

  • $V = \varepsilon - Ir$: y-intercept $= \varepsilon$; gradient $= -r$.
  • The 'lost volts' $= Ir$ increases with current, explaining why terminal pd falls under load.
  • Measure the open-circuit pd directly as a check on the EMF value.

Common Errors

  • Voltmeter placed across external resistor rather than cell terminals.
  • Not opening the switch between readings (cell discharges, EMF changes).
  • Plotting V vs R instead of V vs I.

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

Q13 marks
A V vs I graph gives gradient $-0.55$ V A$^{-1}$ and y-intercept 1.52 V. State the EMF and internal resistance. Calculate the current if the cell is connected to a $2.5$ $\Omega$ resistor.
Q22 marks
Explain why the terminal pd is less than the EMF when current is drawn from the cell.