Comparing the powers of lamps
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.
a Switch the lamps on and compare the brightness of their illumination.
b 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.
1 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.
2 Students may wish to see the markings on the lamps to confirm the power ratings.
This experiment was safety-checked in February 2006