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Q.

Atypical light dimmer used to dim the stage lights in a theatre consists of a variable induction for L (where inductance is adjustable between zero and Lmax) connected in series with a light bulb B as shown in figure. The mains electrical supply rs 220 V at 50 Hz, the light bulb is rated at 220 V, 1100 W. What  Lmax is required if the rate of energy dissipated in the light bulb is to be varied by a factor of 5 from its upper limit of 1100 W?

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a

0.69 H

b

0.28 H

c

0.38 H

d

0.56 H

answer is B.

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Detailed Solution

Resistance of bulb: R=V02P0=22021100=44Ω

when Z is maximum, power consumed will be minimum which is

P=11005=220 W

Now P=Iv2R220=Iv2×44Iv=5A

Iv=EvZ5=220442+XL2XL=88Ω

2πfLmax=882×227×50 Lmax=88

Lmax=0.28H

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