Energy and Thermal Physics

Comparing the powers of lamps

Practical Activity for 14-16 PRACTICAL PHYISCS


The intensity of illumination from different lamps is compared.

Apparatus and Materials

  • Lamp holders (SBC) on base, 2 (Safety pattern batten lamp holders are best)
  • Mains lamp, 230V 25W, pearl
  • Mains lamp, 230V 60W, pearl
  • Screen and post

Health & Safety and Technical Notes

Safety pattern lamp holders incorporate a switch so that when the bulb is removed the contacts become dead. The lamp holders must use a double-insulated mains lead fitted with a proper (13 A) mains plug, and a suitable fuse.

Read our standard health & safety guidance


  1. Switch the lamps on and compare the brightness of their illumination.
  2. Stand the lamps in front of the post so that the shadow of the post falls on the screen. If the lamps are lit one at a time, the apparent intensity of the shadow gives a good comparison of the lamp intensities.

Teaching Notes

  • The two lamps are rated at different wattages. This means that they will transfer energy at different rates. A different number of joules per second are carried from the power supply to the lamp.
  • Students may wish to see the markings on the lamps to confirm the power ratings.

This experiment was safety-tested in February 2006

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