First slide

Electrical power

Question

If two bulbs of watt age 25 and 100, respectively, each rated at 220 volt are connected in series with the supply of 440 volt, then which bulbs will fuse?

Moderate

A

100 watt bulb
B

25 watt bulb
C

None of them
D

Both of them

Solution

Resistance of 25 W bulb $= \frac{220 \times 220}{25} = 1936 Ω$
Its safe current $= \frac{220}{1936} = 0 .11 A$
Resistance of 100 W bulb $= \frac{220 \times 220}{100} = 484 Ω$
Its safe current $= \frac{220}{484} = 0 .48 A$
When connected in series to 440 V supply, then the
current $I = \frac{440}{(1936 + 484)} = 0 .18 A$

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