question_answer

\[30\Omega \]and \[12\Omega \] resistance are connected with a 84V ideal battery in series. A voltmeter of \[20\Omega \] resistance is used to measure the potential difference across \[30\Omega \] resistance. The reading of the voltmeter is:

A)  60V    

B)                                     34V

C)  42 V       

D)                                     56 V

Correct Answer: C

Solution :

\[20\,\Omega \] voltmeter is used in parallel with \[30\,\Omega \].             Hence their effective resistance \[=\frac{30\,\times 20}{30+20}\,=12\Omega \] Now  Current through battery \[=\frac{84}{12+12}\,=\frac{84}{24}A\] Hence potential difference across \[12\,\Omega \] resistance \[=iR=\frac{84}{24}\times 12\,=42\] Volt Hence reading of volt meter = 42 volt