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Four identical electrical lamps are labelled 1.5V, 0.5A which describes the condition necessary for them to operate at normal brightness. A 12 V battery of negligible internal resistance is connected to lamps as shown, then

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The value of R for normal brightness of each lamp is (3/4) Ω

The value of R for normal brightness of each lamp is (21/4) Ω

Total power dissipated in circuit when all lamps are normally bright is 24W

Power dissipated in R is 21W when all lamps are normally bright

answer is B.

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Detailed Solution

For normal brightness of each bulb see following circuit. Current through each bulb = 0.5 A

So main current i = 2A
Also, voltage across the combination =1.5V
So voltage across the resistance = 10.5V
Hence for resistance $V = iR \Rightarrow 10 .5 = 2 \times R \Rightarrow R = \frac{21}{4} Ω$

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