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MCQEasyJEE 2026Cells, EMF & Internal Resistance

JEE Physics 2026 Question with Solution

A moving coil galvanometer of resistance 100 Ω100\,\Omega shows a full scale deflection for a current of 1 mA1\,\text{mA}. The value of resistance required to convert this galvanometer into an ammeter, showing full scale deflection for a current of 5 mA5\,\text{mA}, is _____ Ω\Omega.

  • A

    2525

  • B

    2.52.5

  • C

    1010

  • D

    0.50.5

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: galvanometer resistance G=100 ΩG = 100\,\Omega, galvanometer full scale current Ig=1 mAI_g = 1\,\text{mA}, required full scale current I=5 mAI = 5\,\text{mA}.

Find: the shunt resistance required to convert the galvanometer into an ammeter.

To convert a galvanometer into an ammeter, a low resistance shunt is connected in parallel.

Using the shunt formula,

S=GIgI−IgS = \frac{G I_g}{I - I_g}

Substitute the given values:

S=100×15−1=1004=25 ΩS = \frac{100 \times 1}{5 - 1} = \frac{100}{4} = 25\,\Omega

Therefore, the required resistance is 25 Ω25\,\Omega and the correct option is A.

Common mistakes

  • Using series connection instead of parallel connection for the shunt is incorrect because an ammeter must have very low effective resistance. Always connect the shunt in parallel with the galvanometer.

  • Substituting II in place of I−IgI - I_g is wrong because only the excess current bypasses the galvanometer through the shunt. Use I−IgI - I_g in the denominator.

  • Ignoring consistent current units can lead to errors. Convert or cancel current units carefully so that IgI_g and II are used in the same unit before substitution.

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