Question

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

• A. $4.5V$
• B. $1.5V$
• C. $2.67V$
• D. $13.5V$

Answer 1

The correct option is D $13.5V$


Given, $P=4.5W,V=1.5V$
current passing through the bulb= $\frac{4.5}{1.5}=3A$

In the given situation, potential difference across bulb is due to potential difference due to $1\Omega$ resistor because of parallel connection.
Hence, current in $1\Omega$ resistor=$\frac{1.5}{1}=1.5A$

$i_{total}=1.5+3=3.5A$
EMF of the battery =$V=e-i_{total}\times r_{internal}e=1.5+4.5\times 2.67=13.5V$