Question

The voltmeter shown in the figure reads 10 V across 40Ω. The resistance of voltmeter is:-

Answer 1

Correct option is (a) 40Ω

\(R' =\frac{40R}{40+R}\)

I = current in circuit

\(I=\frac{v}{R'+20}=\frac{20}{20+\frac{40R}{40+R}}\)

\(v=I\,(\frac{40R}{40+R})\)

\(10=\frac{20}{20+\frac{40R}{40+R}}\times\frac{40R}{40+R}\)

\(10=\frac{800R}{800+60R}\)

80R = 800 + 60 R

20R = 800

R = 40Ω