Q 12-03-247JEE MainJEE Main 2019 (10 Jan, Shift 2)Medium
The actual value of a resistance $R$ is $30\ \Omega$. It is measured using the standard formula $R = \dfrac VI$, where $V$ and $I$ are the readings of a voltmeter connected directly across $R$ and an ammeter connected in series with this combination and a cell. If the measured value of $R$ is $5\%$ less, then the internal resistance of the voltmeter is:
Answer: (C) $570\ \Omega$
The ammeter reads the total current through $R$ and the voltmeter, so the measured value is their parallel combination:
$$\frac{R R_V}{R + R_V} = 0.95R \;\Rightarrow\; R_V = 0.95R + 0.95R_V \;\Rightarrow\; R_V = 19R = 570\ \Omega$$
Solution by Sreeraj P, M.Sc Physics