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Current Electricity question

2019 · 10 Apr · Shift 1 · Q46
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Current Electricity question

2019 · 10 Apr · Shift 1 · Q46

JEE MainPhysicsCurrent ElectricityMCQ+4 / −1
A moving coil galvanometer allows a full scale current of 10–4 A. A series resistance of 2 M Ω\OmegaΩ is required to convert the above galvanometer into a voltmeter of range 0-5 V. Therefore the value of shunt resistance required to convert the above galvanometer into an ammeter of range 0.10 mA is :
  1. A
    200 Ω\OmegaΩ
  2. B
    500 Ω\OmegaΩ
  3. C
    100 Ω\OmegaΩ
  4. D
    None of the options are correct
View written solutionFree

Correct answer: D

  1. Given data
  • Full scale current of galvanometer: Ig=10−4 AI_g = 10^{-4}\,\text{A}Ig​=10−4A
  • Series resistance required to make voltmeter of range 000 to 5 V5\,\text{V}5V: Rs=2 MΩ=2×106 ΩR_s = 2\,\text{M}\Omega = 2\times 10^6\,\OmegaRs​=2MΩ=2×106Ω

We first find the resistance of the galvanometer, GGG.


  1. Find galvanometer resistance

For a voltmeter made from a galvanometer: V=Ig(G+Rs)V = I_g(G + R_s)V=Ig​(G+Rs​)

Substitute the values: 5=10−4(G+2×106)5 = 10^{-4}(G + 2\times 10^6)5=10−4(G+2×106)

So, G+2×106=510−4=5×104G + 2\times 10^6 = \frac{5}{10^{-4}} = 5\times 10^4G+2×106=10−45​=5×104

Thus, G=5×104−2×106=−1.95×106 ΩG = 5\times 10^4 - 2\times 10^6 = -1.95\times 10^6\,\OmegaG=5×104−2×106=−1.95×106Ω

This is physically impossible because resistance cannot be negative.

So the given data are inconsistent.


  1. Check the ammeter conversion condition

Desired ammeter range: I=0.10 mA=10−4 AI = 0.10\,\text{mA} = 10^{-4}\,\text{A}I=0.10mA=10−4A

But this is exactly equal to the galvanometer full scale current: I=IgI = I_gI=Ig​

Hence, to convert the galvanometer into an ammeter of range 0.10 mA0.10\,\text{mA}0.10mA, no shunt is needed at all.

That means: Rsh=∞R_{sh} = \inftyRsh​=∞

because the galvanometer itself already measures up to 0.10 mA0.10\,\text{mA}0.10mA.


  1. Evaluate options
  • A: 200 Ω200\,\Omega200Ω — incorrect
  • B: 500 Ω500\,\Omega500Ω — incorrect
  • C: 100 Ω100\,\Omega100Ω — incorrect
  • D: None of the options are correct — correct

  1. Final answer

The required shunt resistance is not any finite value from the options; effectively no shunt is required.

So the correct option is: D\boxed{\text{D}}D​

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