question_answer

Four identical electrical lamps are labelled 1.5V, 0.5A which describes the condition necessary for them to operate at normal brightness. A 12V battery of negligible internal resistance is connected to lamps as shown, then [UPSEAT 2001]

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

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

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

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

Correct Answer: B

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 \[=1.5\,V\]  So voltage across the resistance \[=10.5\,V\] Hence for resistance \[V=i\,R\] Þ\[10.5=2\times R\] Þ\[R=\frac{21}{4}\Omega \]