Four identical electrical lamps are labelled `1.5 V`, `0.5 A` 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

Options

A. The value of `R` for normal brightness of each lamp is `(3//4) Omega`
B. The value of `R` for normal brightness of each lamp is `(21//4) Omega`
C. Total power dissipated in circuit when all lamps are normally bright is `24 W`
D. Power dissipated in `R` is `21 W` when all lamps are normally bright

Answer

B

Explanation

(b) Fro normal brightness of each bulb see following circuit. Current through each bulb `= 0.5 A`
So main current `i= 2 A`
Also, voltage across the combination ` = 1.5 V`
So voltage across the resistance `= 10.5 V`
Hence for resistace `V = iR implies 10.5 = xx R = R = (21)/(4) Omega`