Quick answer: Electrical power is the rate at which an appliance transfers energy, measured in watts. This grade 8 science 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.
How does the electrical power of a lamp change as the voltage across it changes? In this lab you measure the voltage and current for a small filament lamp, calculate the power at six settings and find the energy it transfers in one minute.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS3
Aim
To measure the current in a 12 V lamp at different voltages and to calculate its electrical power and the energy it transfers.
Equipment
- Variable low-voltage d.c. supply (0–12 V)
- 12 V, 6 W filament lamp in a holder
- Ammeter (0–1 A) and voltmeter (0–15 V)
- Switch, 6 connecting leads
- Stopwatch
Safety
Use only the low-voltage supply, never the mains. Switch off before changing the circuit. The lamp gets hot, so do not touch the glass until it has cooled.
Method
- Build the circuit shown. The ammeter is in series with the lamp and the voltmeter is across the lamp.
- Set the supply to 2.0 V and switch on.
- Read the voltage and the current at once, then switch off.
- Repeat for 4.0, 6.0, 8.0, 10.0 and 12.0 V, switching off between readings.
- At 12.0 V keep the lamp on for exactly 60 s to confirm the energy value.
Results
Sample measurements from one group:
| Voltage V (V) | Current I (A) |
|---|---|
| 2.0 | 0.20 |
| 4.0 | 0.31 |
| 6.0 | 0.38 |
| 8.0 | 0.44 |
| 10.0 | 0.48 |
| 12.0 | 0.52 |
Analysis questions
Question 1 Power at each setting
Calculate the power of the lamp at each voltage using P = V × I.Question 2 Graph of power against voltage
Plot a graph of power (y-axis) against voltage (x-axis) and describe its shape.Question 3 Energy in one minute
Calculate the energy transferred by the lamp in 60 s at 12.0 V.Question 4 Running cost
Calculate the energy in kWh if the lamp runs at 12.0 V for 5 hours, then the cost at 20 pence per kWh.Question 5 Voltage and current ratios
The voltage was multiplied by 6 from the first to the last reading. By what factor did the current change? Suggest why it did not also rise six times.Question 6 Uncertainty
The ammeter reads to ±0.01 A and the voltmeter to ±0.1 V. Find the percentage uncertainty in the power at 4.0 V.Question 7 Better readings
Describe two changes that would give a more trustworthy power value at each setting.
