Fuse yourself!

Electricity and Magnetism Level 2

Two bulbs rated ( 25W - 220V ) and ( 100W - 220V) are connected in series to a 440V line. Which one is likely to fuse?

100W bulb none both bulbs 25W bulb

Rohan Bansal by rohan bansal , 22, India

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2 solutions

Maximus Orilius
Jul 20, 2015

The bulb having more resistance is likely to go down first because of higher power consumption. Clearly first one has more resistance.

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But we still need to check. If 440 V is replaced with 300 V and the 25 W bulb can withstand 30 W of operation, then it will not damage. Nevertheless, a doubled voltage may make you an immediate conclusion provided you thought about that and realize.

Lu Chee Ket - 5 years, 7 months ago
Lu Chee Ket
Oct 30, 2015

To fuse here means to explode or to melt.

R1 = 220^2/ 25 = 1936 Ohms

R2 = 220^2/ 100 = 484 Ohms

I = 440/ (1936 + 484) = (2/ 11) A

P1 = I^2 R1 = (4/ 121) 1936 = 64 W

P2 = I^2 R2 = (4/ 121) 484 = 16 W

P = 440^2/ (1936 + 2420) = 80 W {Confirmed correct. 352 V and 88 V respectively.}

In series, the 25 W bulb cannot withstand 64 W of operation. Therefore the 25 W bulb shall damage.

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