Question

A battery of emf 1.4 V and internal resistance $2\Omega$ is connected to a resistor of $100\Omega$ through an ammeter. The resistance of the ammeter is $4/3\Omega$. A voltmeter has also been connected to find the potential differences across the resistor.
If the ammeter reads 0.02 A, then resistance of the voltmeter is

• A. $100\Omega$
• B. $200\Omega$
• C. $150\Omega$
• D. $300\Omega$

Answer 1

The correct option is B $200\Omega$

The effective resistance in the circuit is $1.4/0.02=70\Omega$.

Let the resistance of voltmeter be $R$, so total resistance is $2+4/3+100R(100+R)=70\Rightarrow 100R/(100+R)=66.66=200/3$

$\Rightarrow 300R=20000+200R\Rightarrow R=200\Omega$