First slide

voltmeter

Question

A 100 V voltmeter of internal resistance $20 k Ω$ in series with a high resistance R is connected to a 110 V line. If the voltmeter reads 5 V the value of R is

Moderate

A

$210 k Ω$
B

$315 k Ω$
C

$420 k Ω$
D

$440 k Ω$

Solution

$\begin{array}{l} Here i = \frac{110}{20 \times 10^{3} + R} \\ ∵ V = iR \Rightarrow 5 = (\frac{110}{20 \times 10^{3} + R}) \times 20 \times 10^{3} \\ \Rightarrow 10^{5} + 5 R = 22 \times 10^{5} \Rightarrow R = 21 \times \frac{10^{5}}{5} = 420 kΩ \end{array}$

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