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Question

In a meter bridge experiment, resistances are connected as shown in figure. The balancing length l1 is 55 cm. Now an unknown resistance x is connected in series with P and the new balancing length is found to be 75 cm. The value of x is

A
5413 Ω
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B
2011 Ω
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C
4811 Ω
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D
1148 Ω
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Solution

The correct option is C 4811 Ω
For the meter bridge shown in figure the balancing length is l1 = 55 cm.

Applying balanced condition for meter bridge

RPRQ=l1100l1

3RQ=5545

RQ=45×355=2711 Ω

Now an unknown resistance is connected in series with P and the balancing length is found to be l2=75 cm

Applying the balancing condition for 2nd case,

3+xRQ=l2100l2

(RQ=27/11 Ω & l2=75 cm)

(3+x)1127=7525

(3+x)11=81

x=4811 Ω

Ans : (c)
Why this question ?
Tip: In such problems of meter bridge whenever an unknown resistance is connected in series or parallel with any of the arm resistances, first find out net resistance for that arm and then apply balanced bridge condition for new null point.

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