Quick answer: The emf of a source is the energy it gives each coulomb of charge. This Physics 11-12 lab on internal resistance gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
A fresh cell and a worn cell may both read 1.5 V on a voltmeter, yet one powers a torch much better than the other. In this lab you vary the current drawn from a cell and use the changing terminal voltage to find its emf and internal resistance.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB B.5
Aim
To find the emf and the internal resistance of a cell from the graph of terminal potential difference against current.
Equipment
- one 1.5 V cell in a holder (use a fresh one)
- rheostat (0–10 Ω, rated at 2 A or more)
- digital ammeter (0–1 A range)
- digital voltmeter (0–2 V range)
- switch and connecting leads
Safety
Keep the circuit switched off between readings and do not let the current exceed 0.6 A, because the cell and the rheostat warm up and a short circuit can burn the leads. Never connect the cell terminals directly to each other.
Method
- Connect the circuit with the voltmeter directly across the terminals of the cell.
- Set the rheostat to its largest resistance and close the switch.
- Adjust the rheostat to give a current of 0.10 A and record the ammeter and voltmeter readings.
- Reduce the resistance in steps to give currents of 0.20, 0.30, 0.40, 0.50 and 0.60 A, switching off for a few seconds between readings.
- Plot V (vertical axis) against I (horizontal axis) and draw the line of best fit.
- Determine the intercept and the gradient.
Results
Sample measurements:
| Current I (A) | Terminal p.d. V (V) |
|---|---|
| 0.10 | 1.46 |
| 0.20 | 1.40 |
| 0.30 | 1.35 |
| 0.40 | 1.29 |
| 0.50 | 1.23 |
| 0.60 | 1.18 |
Analysis questions
Question 1 Graph
Plot V against I for the sample data and describe the relationship you see.Question 2 Emf
Use the line of best fit to state the intercept on the V axis and say what it represents.Question 3 Internal resistance
Calculate the gradient from the points (0.10 A, 1.46 V) and (0.60 A, 1.18 V) and state the internal resistance.Question 4 Power in the cell
Calculate the power dissipated inside the cell when the current is 0.60 A.Question 5 Short-circuit current
Use your values of ε and r to predict the short-circuit current, and explain why it cannot be tested directly.Question 6 Switching off between readings
Explain why the cell should be disconnected between readings.Question 7 Uncertainty
The voltmeter reads to ±0.01 V. Calculate the percentage uncertainty in the p.d. at 0.60 A.Question 8 Reliability of r
Give two procedural changes that would reduce the scatter in the measured internal resistance.
