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Question

Two bulbs 40W and 60W and rated voltage 240V are connected in series across a potential difference of 420V. Which bulb will work at above its rated voltage?

A
40W bulb
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B
60W bulb
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C
Both will work
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D
None of these
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Solution

The correct option is D 60W bulb
Maximum current flowing through a bulb I=PV
For bulb of 40 W :
Maximum current I1=40240=0.167 A
Resistance of bulb, R1=(240)240=1440Ω
For bulb of 60 W :
Maximum current I2=60240=0.25 A
Resistance of bulb, R2=(240)260=960Ω
Now both the bulbs are connected in series :
Total resistance of the circuit R=960+1440=2400Ω
Thus current flowing through the circuit I=4202400=0.175 A
As current flowing through the circuit is greater than the maximum current that flows through bulb of 40 W, thus this bulb will not work but bulb of 60 W does.

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