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JEE Main 2017 · Physics · Magnetic Effects of Current · medium

A moving-coil galvanometer of resistance 50 Ω gives full-scale deflection for a current of 2 mA. It is to be used as an ammeter of range 0 to 1 A…

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A moving-coil galvanometer of resistance 50 Ω50\ \Omega gives full-scale deflection for a current of 2 mA2\ \mathrm{mA}. It is to be used as an ammeter of range 00 to 1 A1\ \mathrm{A} and, separately, as a voltmeter of range 00 to 5 V5\ \mathrm{V}. The shunt resistance and the series resistance required are respectively, nearly
  1. A. 0.1 Ω0.1\ \Omega and 2450 Ω2450\ \Omega
  2. B. 0.1 Ω0.1\ \Omega and 2500 Ω2500\ \Omega
  3. C. 0.05 Ω0.05\ \Omega and 2450 Ω2450\ \Omega
  4. D. 25 Ω25\ \Omega and 2450 Ω2450\ \Omega
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Answer: (A) 0.1 Ω0.1\ \Omega and 2450 Ω2450\ \Omega

Ammeter: the shunt SS carries I−IgI - I_g while the galvanometer carries IgI_g at the same voltage, so S=IgGI−Ig=2×10−3×500.998≈0.1 ΩS = \dfrac{I_gG}{I - I_g} = \dfrac{2\times10^{-3}\times50}{0.998} \approx 0.1\ \Omega. Voltmeter: a series resistance RR makes the total resistance V/IgV/I_g, so R=52×10−3−50=2500−50=2450 ΩR = \dfrac{5}{2\times10^{-3}} - 50 = 2500 - 50 = 2450\ \Omega. Forgetting the galvanometer's own resistance in the voltmeter gives 2500 Ω2500\ \Omega. The shunt is very small so that the combined resistance is small, as an ammeter needs.
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Original question written in the JEE Main 2017 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake