Question

A constant $60V$ dc supply is connected across two resistors of resistance $400k\Omega$ and $200k\Omega$. What is the reading of the voltmeter when a resistance of $200k\Omega$ is connected across the second resistor as shown in the figure?

• A. 12 V
• B. 15 V
• C. 20 V
• D. 30 V

Answer 1

Answer: (A)

12 V

The voltmeter is arranged in parallel combination with the $200$ k$\Omega$ resistance. Hence the equivalent resistance $R$ of the combination is
$\frac{1}{R}=\frac{1}{200}+\frac{1}{200}$
$R=\frac{200}{2}=100$ k$\Omega$
Now, the total resistance across DC supply $R_{dc}$(say) is
$R_{dc}=400+100=500k\Omega$
Hence the voltage across voltmeter $V_v$ is
$V_v=\frac{100}{500}\times 60=12$ V