Question

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

• A. 4.5 V
• B. 1.5 V
• C. 2.6 V
• D. 13.5 V

Answer 1

The correct option is D

13.5 V

Current in the bulb $=\frac{P}{V}=\frac{4.5}{1.5}=3A$

Current in $1\Omega$ resistance =$\frac{1.5}{1}=1.5A$

Hence total current from the cell $i=3+1.5=4.5A$

By using $E=V+ir\Rightarrow E=1.5+4.5\times \left(2.67\right)=13.5V$