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Question

A galvanometer of resistance 30 Ω is connected to battery of emf 2V with 1970 Ω resistance in series. A full scale deflection of 20 divisions is obtained in the galvanometer. To reduce the deflection to 10 divisions, the resistance in series required is

A
4030 Ω
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B
4000 Ω
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C
3970 Ω
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D
2000 Ω
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Solution

The correct option is B 3970 Ω
Case 1 : V=2 volts Rg=30Ω Rs=1970
A full scale deflection of 20 divisions (Ig) is obtained.
V=Ig(Rg+Rs)
Or 2=Ig(30+1970)
We get Ig=0.001 A
Case 2 : V=2 volts Rg=30Ω
A full scale deflection of 10 divisions is obtained i.e. Ig2 is obtained.
2=Ig2(30+Rs)
Or 2=0.0012(30+Rs)
Or Rs+30=4000
Thus we get Rs=3970Ω

675578_633646_ans_367c3d0b392d46e5be3b6a47999f3e52.png

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