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Forty electric bulbs are connected in series across a 220 V supply. After one bulb is fused the remaining 39 are connected again in series across the same supply. The illumination will be

1. more with 40 bulbs than with 39
2. more with 39 bulbs than with 40
3. equal in both cases
4. in the ratio 40² : 39²

Correct answer: in the ratio 40² : 39²

Solution

The power of a bulb is proportional to the square of the voltage across it. When one bulb fuses, the total resistance decreases, increasing the current and voltage across each remaining bulb. The illumination ratio is proportional to the square of the number of bulbs, i.e., 40²:39².

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