The actual value of resistance R, shown in the figure is 30 Ω. This… | JEE Main 2019 (10 Jan, Shift 2)

Q164JEE Main 2019 · 10 JAN · SHIFT 2Medium

Kirchhoff's Laws & Measuring Instruments

The actual value of resistance R, shown in the figure is 30 $\Omega$ . This is measuered in an experiment as shown using the standard formula R = $\frac{V}{I}$ , where V and I are the readings of the voltmeter and ammeter, respectively. If the measured value of R is 5% less, then the internal resistance of the voltmeter is -

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Answer: (a)

The voltmeter is across R, so the ammeter reads the current through $R \| R_V$ ; measured value $= \dfrac{R R_V}{R + R_V}$ .
Measured $= 0.95 \times 30 = 28.5\ \Omega$
$\dfrac{30 R_V}{30 + R_V} = 28.5 \Rightarrow 1.5 R_V = 855$
$R_V = 570\ \Omega$ . So the answer is (a).

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