A galvanometer has resistance 80 Ω and it is shunted with resistance 20 Ω. If 20% of the main current flows through galvanometer, then what is the value of main current? (a) 0.2 A (b) 0.8 A (c) 1 A (d) 1.2 A

Answer: (c). Shunt and galvanometer have the same p.d.: Ig G = Is S. Fraction through galvanometer = (S)/(G + S) = 20/100 = 20%, true for any main current. So the data cannot fix the main current; the key's value 1 A needs extra data (e.g. Ig = 0.2 A). Taking Ig = 0.2 A: I = (0.2)/(0.2) = 1 A, answer (c).

A galvanometer has resistance 80 Ω and it is shunted with resistance… | MHT CET 2024 (15 May, Shift 1)

Q67MHT CET 2024 · 15 MAY · SHIFT 1Medium

Kirchhoff's Laws & Measuring Instruments

A galvanometer has resistance $80\ \Omega$ and it is shunted with resistance $20\ \Omega$ . If $20\%$ of the main current flows through galvanometer, then what is the value of main current?

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Answer: (c)

Shunt and galvanometer have the same p.d.: $I_g G = I_s S$ .
Fraction through galvanometer $= \dfrac{S}{G + S} = \dfrac{20}{100} = 20\%$ , true for any main current.
So the data cannot fix the main current; the key's value $1\text{ A}$ needs extra data (e.g. $I_g = 0.2\text{ A}$ ).
Taking $I_g = 0.2\text{ A}$ : $I = \dfrac{0.2}{0.2} = 1\text{ A}$ , answer (c).

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