JEE Main · 2020 · 08 Jan 2020 · Shift 2 · Current Electricity

A galvanometer having a coil resistance

100Ω

gives a full scale deflection when a current of 1 mA is passed through it. What is the value of the resistance which can convert this galvanometer into a voltmeter giving full scale deflection for a potential difference of 10 V.

(A) $9.9 k Ω$ ✓ Correct
(B) $7.9 k Ω$
(C) $10 k Ω$
(D) $8.9 k Ω$

Answer: (A)

Solution

To read voltage, a galvanometer needs a large resistance in series so that full-scale current flows only at the chosen voltage. Full-scale current is

i_{g} = 1

mA through a coil of

R_{g} = 100 Ω

, and for full-scale at

10

V the total resistance must be

\frac{V}{i _{g}} = \frac{10}{1 0 ^{- 3}} = 10000 Ω

. The series resistance is therefore

R = 10000 - 100 = 9900 Ω = 9.9 kΩ

JEE Main · 2020 · 08 Jan 2020 · Shift 2 · Single correct option

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