A galvanometer is used to detect or/and measure small currents in an electrical circuit. It essentially works on the fact that a current-carrying coil experiences a deflecting torque when placed in a magnetic field. This deflection in the coil can be measured and it is related to the current flowing in the coil, the number of turns in the coil, area of the coil and the magnetic field. A hair spring attached to the coil provides a counter torque and helps in measuring the deflection. A galvanometer can be converted to an ammeter or a voltmeter of desired range by using suitable resistances. (I) The torque on the coil remains constant irrespective of the coil’s orientation during rotation due to (II) The best way to increase current sensitivity of a galvanometer is by (III) A moving coil galvanometer has a coil with area of cross-section 4.0 × 10^-3 m² and number of turns 50. The coil is rotating in a magnetic field of 0.25 T. The torque acting on the coil when a current of 5 A passes through it is (III) A galvanometer coil has a resistance of 15 Ω and the meter shows full scale deflection for a current of 3 mA. The value of resistance required to convert it into a voltmeter of range (0 – 12 V) is (IV) A galvanometer with coil of resistance 20 Ω shows full scale deflection for a current of 5 mA. To convert it into an ammeter of range (0 – 10 A), a resistance of

PhysicsCBSE Class 12 · Board exam 2026, set 55/1/1 · 4 marks · Case-based

CBSE Class 12 Physics 2026: question 29

Set in the CBSE Class 12 board exam of 2026, paper 55/1/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedMoving Charges and Magnetism

A galvanometer is used to detect or/and measure small currents in an electrical circuit. It essentially works on the fact that a current-carrying coil experiences a deflecting torque when placed in a magnetic field. This deflection in the coil can be measured and it is related to the current flowing in the coil, the number of turns in the coil, area of the coil and the magnetic field. A hair spring attached to the coil provides a counter torque and helps in measuring the deflection. A galvanometer can be converted to an ammeter or a voltmeter of desired range by using suitable resistances.

(I)
The torque on the coil remains constant irrespective of the coil’s orientation during rotation due to
1. (A)use of soft iron core which increases the magnetic field.
2. (B)radial magnetic field
3. (C)hair spring which provides the counter torque
4. (D)eddy current in the iron core which causes damping.
1 mark
(II)
The best way to increase current sensitivity of a galvanometer is by
1. (A)increasing number of turns of the coil
2. (B)increasing area of coil and magnitic field strength
3. (C)decreasing area of coil and magnetic field strength
4. (D)increasing torsional constant of the hair spring
1 mark
(III)
A moving coil galvanometer has a coil with area of cross-section $4.0 \times 1 0^{- 3}$ m $^{2}$ and number of turns 50. The coil is rotating in a magnetic field of 0.25 T. The torque acting on the coil when a current of 5 A passes through it is
1. (A)1.0 N m
2. (B)2.0 N m
3. (C)0.50 N m
4. (D)0.25 N m
1 mark
OR
(III)
A galvanometer coil has a resistance of 15 $Ω$ and the meter shows full scale deflection for a current of 3 mA. The value of resistance required to convert it into a voltmeter of range (0 – 12 V) is
1. (A)4015 $Ω$
2. (B)3985 $Ω$
3. (C)415 $Ω$
4. (D)385 $Ω$
1 mark
(IV)
A galvanometer with coil of resistance 20 $Ω$ shows full scale deflection for a current of 5 mA. To convert it into an ammeter of range (0 – 10 A), a resistance of
1. (A)0.05 $Ω$ should be connected in series with it.
2. (B)0.05 $Ω$ should be connected in parallel with it.
3. (C)0.01 $Ω$ should be connected in parallel with it.
4. (D)0.01 $Ω$ should be connected in series with it.
1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Recall the formula for current sensitivity, φ/I = NAB/k.

Hint 2

Coil area and magnetic field are limited by the instrument's size and magnet; which factor can a manufacturer change most easily?

Hint 3

NCERT states that the convenient way to increase current sensitivity is to increase the number of turns N.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(I) A radial magnetic field keeps the plane of the coil always parallel to $B$ , so $τ = N I A B$ at every orientation. Option (B).
1 mark
(II) Current sensitivity is $\frac{ϕ}{I} = \frac{N A B}{k}$ . Coil area and magnet strength are limited in practice, so the convenient (best) way to raise it is to increase the number of turns $N$ . Option (A).
1 mark
(III) $τ = N I A B = 50 \times 5 \times 4.0 \times 1 0^{- 3} \times 0.25 = 0.25$ N m. Option (D).
OR: Series resistance $R = \frac{V}{I _{g}} - G = \frac{12}{3 \times 1 0 ^{- 3}} - 15 = 3985 Ω$ . Option (B).
1 mark
(IV) Shunt $S = \frac{I _{g} G}{I - I _{g}} = \frac{5 \times 1 0 ^{- 3} \times 20}{10 - 0.005} \approx 0.01 Ω$ , connected in parallel. Option (C).
1 mark

Answer: (I) B; (II) A; (III) D [or alternative: B]; (IV) C

Where the marks are

(I) Radial magnetic field, B (1 mark)
(II) Increasing number of turns, A (1 mark)
(III) 0.25 N m, D; or alternative 3985 Ω, B (1 mark)
(IV) 0.01 Ω in parallel, C (1 mark)

Revise the chapter

Chapter: Moving Charges and Magnetism (notes, formulas, where marks are lost and every past question)
Topic: Moving Coil Galvanometer

Practise this chapterBoard-style questions from Moving Charges and Magnetism, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 55/1/1: every question, by section.

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CBSE Class 12 Physics 2026: question 29

Hints

Step-by-step solution

Revise the chapter

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