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Question:

Two electric bulbs marked 25W-220V and 100W-220V are connected in series to a 440V supply. Which of the bulbs will fuse?

neither

25W

Both

100W

Solution:

R1 = (220²)/25, R2 = (220²)/100
R = R1 + R2
R = 220² * (1/25 + 1/100)
R1 > R2
When connected in series, the voltages divide in them in the ratio of their resistances. The voltage of 440V divides in such a way that voltage across the 25W bulb will be more than 220V. Hence the 25W bulb will fuse.