First slide

meter bridge

Question

in the meter bridge shown in figure, initially P/Q = 1. Find the shift in the null point if P is doubled and Q is halved.

Moderate

A

40 cm
B

20 cm
C

15 cm
D

30 cm

Solution

If l be the initial distance of the null point from end A, then $\frac{P}{l} = \frac{Q}{100 - l} \Rightarrow l = \frac{100}{(\frac{Q}{P} + 1)}$
Initially, $l = \frac{100}{(1 + 1)} cm = 50 cm$
Finally $l' = \frac{100}{(\frac{1}{4} + 1)} cm = 80 cm$
$∴ shift = l' - l = 80 cm - 50 cm = 30 cm$

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