Required Practicals/OCR A/Practical 5
5ASPAG 5

EMF and internal resistance (PAG 5)

Determine EMF and internal resistance from a terminal pd vs current graph.

Apparatus

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

Safety

  • Do not short-circuit the cell.
  • Open switch between readings.

Method

  1. Connect circuit with ammeter in series and voltmeter across cell terminals.
  2. Adjust rheostat to give eight different current values. Record V and I.
  3. Open switch between readings to prevent discharge.
  4. Plot V vs I: y-intercept $= \varepsilon$; gradient $= -r$.

Key Variables

Independent
Current I
Dependent
Terminal pd V
Controlled
Same cell; Temperature

Analysis and Results

  • $V = \varepsilon - Ir$. Y-intercept $= \varepsilon$; gradient $= -r$.
  • X-intercept gives short-circuit current $I_{sc} = \varepsilon/r$.

Common Errors

  • Voltmeter across external resistor not cell terminals.
  • Not opening switch between readings.
  • Plotting V vs R rather than V vs I.

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

Q14 marks
A cell has EMF 1.50 V and internal resistance 0.60 $\Omega$. It drives a current through an external $4.4$ $\Omega$ resistor. Calculate (a) the current and (b) the power dissipated in the internal resistance.
Q22 marks
The student measures a terminal pd of 1.47 V when the circuit is open (no current). Explain whether this equals the EMF.