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Q.

The length of a potentiometer wire is 1200cm and it carries a current of 60mA. For a cell of emf 5V and internal resistance of 20Ω, the null point on it is found to be at 1000cm. the resistance of whole wire is:

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a

100Ω

b

60Ω

c

80Ω

d

120Ω

answer is D.

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Detailed Solution

 

Question Image

Null point at 1000 cm means , potential across 1000 cm is 5V.

Let say potential across 1200 cm is Vp

Thus, potential gradient = 51000=Vp1200   ⇒VP=6V

Now, use Ohm's Law across the whole wire, 

  RP=VPI=660×10−3=100Ω

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