Quick answer: In a series circuit the current is the same everywhere and the voltages add up to the supply; in a parallel circuit every branch has the full supply voltage and the branch currents add up. This Physics 9-10 lab on electrical power gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Does the power in a resistor really grow with the square of the current? In this lab you measure voltage and current for a 10 Ω resistor, calculate the power two ways and use a graph to find the resistance from the gradient. It takes about 30 minutes with standard school equipment.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P2, HS-PS3-3
Aim
To measure the power in a fixed resistor at different currents, and to show that P = V I and that P is proportional to I².
Equipment
- variable DC power supply, 0–6 V (or a battery pack with a rheostat)
- 10 Ω wire-wound resistor rated 5 W
- digital ammeter, 0–1 A, resolution 0.01 A
- digital voltmeter, 0–10 V
- switch and six connecting leads
- graph paper or a spreadsheet
Safety
The resistor becomes hot at the highest current, so do not touch it and keep the switch open between readings. Never go above 6 V. Check all leads for damaged insulation.
Method
- Build the circuit shown, with the ammeter in series and the voltmeter across the resistor. Keep the switch open.
- Set the supply to 1.0 V, close the switch and read the ammeter.
- Record V and I, then open the switch.
- Repeat for 2.0 V, 3.0 V, 4.0 V, 5.0 V and 6.0 V.
- Calculate P = V I and I² for each row.
Results
Sample measurements from one group:
| V (V) | I (A) | P = V I (W) | I² (A²) |
|---|---|---|---|
| 1.0 | 0.10 | 0.10 | 0.010 |
| 2.0 | 0.20 | 0.40 | 0.040 |
| 3.0 | 0.29 | 0.87 | 0.084 |
| 4.0 | 0.39 | 1.56 | 0.152 |
| 5.0 | 0.49 | 2.45 | 0.240 |
| 6.0 | 0.59 | 3.54 | 0.348 |
Analysis questions
Question 1 Planning the readings
Name what you change on the supply, what you calculate from the meters, and two things that must stay fixed for a fair set of readings.Question 2 Check one row
For the row with V = 4.0 V, check the power P = V I. Then calculate I²R using R = 10 Ω and find the percentage difference between the two powers.Question 3 Plot and gradient
Plot P (y-axis) against I² (x-axis) and draw the best-fit line. Use the first and last points to calculate the gradient, with its unit.Question 4 Resistance from V ÷ I
Calculate V ÷ I for each row and find the mean. Compare this value with the gradient.Question 5 Heating effect
Explain why the switch should be opened between readings and why the resistance might rise slightly at 6.0 V.Question 6 Uncertainty
The ammeter resolution is 0.01 A. Calculate the percentage uncertainty in the current at 0.10 A and at 0.59 A. Which readings are more reliable?Question 7 Improve the lab
Suggest two improvements that would make the value of the resistance more precise.
