First slide

meter bridge

Question

In the meter bridge shown in figure, distance of the null point N from end A is 40 cm. When a $20 Ω$ resistor is connected in series with P, the null point is displaced to the right by 20 cm. Then value of Q is

Moderate

A

$24 Ω$
B

$18 Ω$
C

$10 Ω$
D

$12 Ω$

Solution

$\begin{array}{l} Initially, \frac{P}{Q} = \frac{40}{100 - 40} \Rightarrow \frac{P}{Q} = \frac{2}{3} . . . (1) \\ Finally, \frac{P + 20}{Q} = \frac{40 + 20}{100 - (40 + 20)} \Rightarrow \frac{P}{Q} + \frac{20}{Q} = \frac{3}{2} . . . (2) \\ Solving, Q = \frac{6}{5} \times 20 Ω = 24 Ω \end{array}$

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