Q. A torch bulb rated at 4.5 W, 1.5 V is connected as shown in figure. The emf of the cell needed to make the bulb glow at full intensity if

A) 4.5 V
B) 1.5 V
C) 2.67 V
D) 13.5 V
Explanation

(d) Resistance of the bulb$= \left( \ 1.5 \times \frac{1.5}{4.5} \right) = 0.5 = \frac{1}{2}\Omega$
Resistance of the circuit $R = \frac{1 \times \frac{1}{2}}{1 + \frac{1}{2}} = \frac{1}{3}\ \Omega$
$\ Now\ ,\ \ \ \ \ r = \frac{E - V}{V}R$
$\ \ \frac{8}{3} = \frac{E - 15}{1.5} \times \frac{1}{3}\ or\ E = 13.5$ volt

Dr. Abubacker
Reviewed by Dr. Abubacker

Physics Educator & NEET/JEE Specialist

Ph.D. in Physics with 19+ years of teaching experience.

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