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Question: Answered & Verified by Expert
In a circuit for finding the resistance of a galvanometer by half deflection method, a 6 V battery and a high resistance of 11  are used. The figure of merit of the galvanometer is 60 μA division-1. In the absence of shunt resistance, the galvanometer produces a deflection of θ=9 divisions when current flows in the circuit. The value of the shunt resistance that can cause the deflection of θ2, is closest to:
PhysicsCurrent ElectricityJEE MainJEE Main 2018 (16 Apr Online)
Options:
  • A 550 Ω
  • B 220 Ω
  • C 55 Ω
  • D 110 Ω
Solution:
2690 Upvotes Verified Answer
The correct answer is: 110 Ω


We are given a galvanometer connected in the first circuit in the above figure, where E=6 VR = 11  and the deflection θ = 9 divisions. The galvanometer has a resistance of G. The current in the circuit,

I=ER+G  ...(1)

Also, since the figure of merit is 60 μA/division

I = 60×9 μA = 540 μA  ...(2)

Substituting equation (2) in (1),

540×10-6 = 611,000+GG = 6540×10-6-11,000G = 111 Ω  ...(3)

In the second circuit, it is given that the deflection is half that of the previous circuit. So, current flowing through the galvanometer half the first circuit. Hence,

I2= ER+GSG+S ×SS+G          I2=ESRS+G+GS

S= RG ×I2E-(R+G)I2

Substituting the values from equations (1), (2) and (3),

S= 11 ×103 ×111 ×5402 ×10-66-62=110 Ω

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