Energy and Thermal Physics

Measuring the power of a lamp

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Calculating the energy transferred per second from a lamp.

Apparatus and Materials

For each group of students...

  • Power supply, LV
  • Lamp 12V 6W
  • Lamp holder on base
  • Ammeter (0 - 1 amp), DC
  • DC voltmeter (0 -15 volt)
  • Variable resistor, optional

Health & Safety and Technical Notes

Read our standard health & safety guidance

The rating of the lamp is chosen so as to provide reasonable current and voltage readings. Any lamp that produces similar values to a (12 V 6 W) lamp is suitable. Remember that, on switch-on, a lamp draws several times the rated current: the power supply must be able to supply this.


  1. Connect the circuit shown and take readings of the ammeter and voltmeter. Calculate the energy transferred electrically each second.

Teaching Notes

  • To give more practice in making calculations of power, a variable resistor can be included in the circuit. Students take a series of readings and compare them with the brightness of the lamp.
  • The table could be labelled as shown:
    • Current in amps (charge flowing in coulombs per second)
    • Potential difference in volts (energy transferred in joules by each coulomb)
    • Power (= energy transferred electrically from the power supply per second, in joules/second or watts (Power = VI)
  • Working out the units is a useful check on what is happening in the circuit in terms of the physics.
  • How Science Works extension: Students could be asked to design an experiment whereby they calculate the efficiency of the energy transferred electrically to light. Discussion will likely identify the difficulty in quantifying the amount of light produced. What should also emerge is that the amount of light radiated can be inferred by calculating the energy stored thermally. A possible approach is to put the lamp in a sealed polystyrene cup filled with air (or even water) and measure the temperature rise.
  • The specific thermal capacity of air at constant pressure is about 1,000 J/kg 'C, and that of water is 4,200 J/kg 'C

This experiment was safety-checked in January 2007

appears in the relation P=VI P=I^2R P=V^2/R ΔQ=PΔt
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