Question

In the following circuit, the reading of the voltmeter will be -

Galvanometer shows null deflection.

• A. $7.2\text{V}$
• B. $6.2\text{V}$
• C. $5.2\text{V}$
• D. $4.2\text{V}$

Answer 1

The correct option is A $7.2\text{V}$

Galvanometer is at null point, means the potential drop across $40\text{cm}$ of wire is balanced by the $4.8\text{V}$ battery.

So, $\text{Potential Gradient}\times 40=4.8\text{V}$

$\Rightarrow \text{Potential Gradient}=\frac{4.8}{40}\text{V/cm}$

Reading in voltmeter $=$ Voltage drop across $60\text{cm}$ length

$\therefore V=\text{Potential Gradient}\times 60=\frac{4.8}{40}\times 60=7.2\text{V}$

Hence, option $\left(A\right)$ is the correct answer.

$\text{Alternate Solution :}$

$12=V+4.8$

$\Rightarrow V=7.2\text{V}$