Question

A part of a circuit is shown in figure. Here reading of ammeter is $5\text{A}$ and voltmeter is $96\text{V}$ & voltmeter resistance is $480\Omega$. Then find the resistance $R$

• A. $20\Omega$
• B. $24\Omega$
• C. $36\Omega$
• D. $48\Omega$

Answer 1

The correct option is A $20\Omega$

Given, resistance and reading of the voltmeter is $480\Omega$ and $96\text{V}$ respectively.

Current through voltmeter is,

$i=\frac{V}{R}=\frac{96}{480}=\frac{1}{5}\text{A}$

The remaining current flows through resistance $R$.

$\therefore$ Current through the resistance $R$ is ,

$I_R=5-\frac{1}{5}=\frac{24}{5}\text{A}$

Voltage across resistor $R$ is,

$V=IR$

$96=\frac{24}{5}\left(R\right)$

$\therefore R=20\Omega$

Hence, option (a) is the correct answer.

Why this question ? Key concept : potential difference across the voltmeter = Resistance of voltmeter $\times$ current through voltmeter