Two light bulbs
Electricity and Magnetism
Level
1
Two incandescent light bulbs are connected in series to a 120V power outlet. The first light bulb is rated 60W, 120V; the second one is 100W, 120V. Which one will be brighter?
The will be equally bright
The 60W bulb
The 100W bulb
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1 solution
When connected in series, the same current will flow through the light bulbs and the dissipated power will be P ′ = I 2 R . The light bulb with higher resistance will be brighter.
The light bulb with the lower power rating will have higher resistance. That is because the resistance of the light bulbs are expressed from their original rating as R = V 2 / P .
Note: The resistance of the light bulb depends on the temperature of the filament (increases if the filament is warmer). This will not change our conclusion, it will only amplify the effect.