Question

In the circuit shown , the resistance of voltmeter is $10,000\Omega$ and that of ammeter is $20\Omega$. The ammeter reading is $0.10\text{A}$ and voltmeter reading is $12\text{V}$. Then value of $R$ is equal to

• A. $122\Omega$
• B. $140\Omega$
• C. $116\Omega$
• D. $100\Omega$

Answer 1

The correct option is D $100\Omega$

As voltmeter is of very large resistance so we can ignore current through it (as it will be very small). All the current of $0.10\text{A}$ passes through resistance $R$ and ammeter of resistance $20\Omega$.


As we know, potential difference across any parallel branch is same,

$(R+20)\left(0.10\right)=12$

$\therefore R=100\Omega$

Hence, option (d) is the correct answer.

Why this question ? Key concept : When voltmeter is of very large resistance, we can ignore current through it.