Ammeter: galvanometer + low-resistance shunt in parallel. Voltmeter: galvanometer + high resistance in series.
-- NCERT Class 12 Physics, Ch. 4, p. 130Moving Coil Galvanometer
8 MCQs2 revision cards9-step worked example
Source: NCERT Magnetic Effects of Current and MagnetismOfficial key: NTA-verifiedLast updated: 25 Sep 2026
Moving Coil Galvanometer, explained for NEET
The galvanometer question that costs marks is rarely the formula — it is the word sensitivity. Two different sensitivities exist, and a stem that says "increase the sensitivity" is ambiguous until you read which one.
A moving coil galvanometer holds a pivoted coil of N turns and area A between the poles of a permanent magnet. NCERT Class 12 Physics Chapter 4 (page 130) makes the design point: the pole pieces are cut cylindrically concave and a soft-iron core sits inside the coil. Together these make the field radial — the field lines always lie in the plane of the coil, so the angle between the coil's normal and the field stays at 90° no matter how far the coil turns. Deflecting torque is therefore NIAB, with no sin θ riding on it.
The restoring torque from the spring is kφ. At equilibrium NIAB = kφ, so
φ = (NAB/k) I
Deflection is proportional to current — that linear scale is the whole purpose of the radial field.
Now the two sensitivities. Current sensitivity is φ/I = NAB/k. Voltage sensitivity is φ/V = NAB/(kR), where R is the galvanometer's resistance. They are not interchangeable. Increase N by winding more turns and current sensitivity rises — but the extra wire raises R too, so voltage sensitivity need not rise at all. A stem that doubles N and asks about voltage sensitivity is testing exactly this.
For NEET, the reliably examined facts are: the radial field's role, φ ∝ I, the two sensitivity expressions, and why a galvanometer cannot be used directly as an ammeter (its resistance is comparable to circuit resistance, and it takes very small currents).
Watch-out: k is the torsional constant of the spring, in N·m/rad — not a spring constant in N/m, and not the same as the number of turns.
Can you answer these Moving Coil Galvanometer MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
In a moving coil galvanometer, the magnetic field in which the coil turns is made radial. This is achieved by
Show answer and why every option is right or wrong
Answer: B. B is correct. NCERT Class 12 Physics Chapter 4 (page 130) describes the concave cylindrical pole pieces plus the soft-iron cylinder that together shape the field so it is always along the coil's plane.
Why A is wrong: A is wrong because flat pole pieces give a uniform field, not a radial one, and the deflecting torque would then carry a sin θ factor that destroys the linear scale.
Why C is wrong: C is wrong because the former's material affects damping of the coil's motion, not the geometry of the field lines.
Why D is wrong: D is wrong because a long solenoid's interior field is uniform and axial, which is exactly the field shape the radial design is built to avoid.
MCQ 2Easy RecallPractice
In the moving coil galvanometer relation NIAB = kφ, the constant k is
Show answer and why every option is right or wrong
Answer: A. A is correct. The restoring torque of a twisted spring is kφ, so k must have units of torque per unit angle — N·m/rad. See NCERT Class 12 Physics Chapter 4 (page 130).
Why B is wrong: B is wrong because a N/m constant multiplies a displacement to give a force; here k multiplies an angle to give a torque, so the units cannot be N/m.
Why C is wrong: C is wrong because turns-per-length is a solenoid quantity; the galvanometer expression uses the total number of turns N separately, and k is unrelated to winding density.
Why D is wrong: D is wrong because the core's permeability shapes the field but does not appear as k in the torque balance; k is a mechanical property of the spring.
MCQ 3Concept TrapPractice
Because the field inside a moving coil galvanometer is radial, the deflecting torque on the coil is NIAB rather than NIAB sin θ. The consequence is that
Show answer and why every option is right or wrong
Answer: C. C is correct. With the sin θ factor removed, NIAB = kφ gives φ = (NAB/k)I, a straight proportionality, so equal current steps produce equal angular steps on the dial. NCERT Class 12 Physics Chapter 4 (page 130).
Why A is wrong: A is wrong because the torque balance is linear in I on the left and linear in φ on the right; no squaring enters anywhere.
Why B is wrong: B is wrong because the coil rests wherever deflecting and restoring torques are equal, which for any non-zero current is a non-zero angle.
Why D is wrong: D is wrong because the restoring torque kφ is by definition proportional to the deflection; it is the deflecting torque that the radial field makes angle-independent.
MCQ 4Direct ApplicationPractice
A moving coil galvanometer has a coil of 50 turns (exact), area 2.00 × 10⁻⁴ m², placed in a radial field of 0.400 T. The torsional constant of the suspension is 2.00 × 10⁻⁵ N·m/rad. The current sensitivity of the galvanometer is
Show answer and why every option is right or wrong
Answer: B. B is correct. Current sensitivity is NAB/k = (50 × 2.00 × 10⁻⁴ × 0.400)/(2.00 × 10⁻⁵) = 4.00 × 10⁻³/2.00 × 10⁻⁵ = 2.00 × 10² rad/A. The relation φ = (NAB/k)I is on NCERT Class 12 Physics Chapter 4, page 130.
Why A is wrong: A is wrong because it leaves out the number of turns: AB/k = 8.00 × 10⁻⁵/2.00 × 10⁻⁵ = 4.00 rad/A, fifty times too small.
Why C is wrong: C is wrong because it inverts the ratio, k/(NAB) = 2.00 × 10⁻⁵/4.00 × 10⁻³ = 5.00 × 10⁻³; deflection per unit current is NAB/k.
Why D is wrong: D is wrong because it multiplies by the torsional constant instead of dividing: NAB × k = 4.00 × 10⁻³ × 2.00 × 10⁻⁵ = 8.00 × 10⁻⁸.
MCQ 5Concept TrapPractice
A galvanometer is rewound with twice the original number of turns using the same wire, so its resistance also doubles. Everything else is unchanged. Which statement is correct?
Show answer and why every option is right or wrong
Answer: D. D is correct. Current sensitivity NAB/k scales directly with N, so it doubles; voltage sensitivity NAB/(kR) has N and R both doubling, so the ratio is unchanged. NCERT Class 12 Physics Chapter 4 (page 130) separates the two sensitivities for exactly this reason.
Why A is wrong: A is wrong because it treats voltage sensitivity as if R were fixed; the added winding raises R in step with N, cancelling the gain.
Why B is wrong: B is wrong because current sensitivity contains N explicitly and cannot be unchanged when N doubles.
Why C is wrong: C is wrong because it inverts the dependence: sensitivity rises with N, it does not fall.
MCQ 6Direct ApplicationPractice
A galvanometer of resistance 40.0 Ω has a current sensitivity of 5.00 × 10² rad/A. Its voltage sensitivity is
Show answer and why every option is right or wrong
Answer: C. C is correct. Voltage sensitivity = current sensitivity ÷ R = 5.00 × 10²/40.0 = 1.25 × 10¹ rad/V, which follows from φ/V = NAB/(kR). NCERT Class 12 Physics Chapter 4, page 130.
Why A is wrong: A is wrong because it multiplies by R instead of dividing; voltage sensitivity is always the smaller of the two for R greater than 1 Ω.
Why B is wrong: B is wrong because it divides R by the current sensitivity, inverting the required ratio.
Why D is wrong: D is wrong because it subtracts 40.0 from 5.00 × 10² — treating the two sensitivities as differing by an additive resistance term rather than a factor of R.
MCQ 7CalculationPractice
Two moving coil galvanometers P and Q use identical springs and identical radial fields. Q's coil has three times the number of turns of P and one-third the area of P's coil, and Q's resistance is three times P's. The ratio of Q's voltage sensitivity to P's voltage sensitivity is
Show answer and why every option is right or wrong
Answer: D. D is correct. Voltage sensitivity is NAB/(kR). For Q the product NA is (3N)(A/3) = NA, unchanged, while R is tripled — so Q's voltage sensitivity is one-third of P's. NCERT Class 12 Physics Chapter 4, page 130.
Why A is wrong: A is wrong because it multiplies the resistance factor of 3 by an unearned second factor of 3; the N and A changes cancel exactly and contribute nothing.
Why B is wrong: B is wrong because it tracks the tripling of N while ignoring both the one-third area and the tripled resistance.
Why C is wrong: C is wrong because it stops after noting that NA is unchanged and forgets that voltage sensitivity also carries R in the denominator.
MCQ 8CalculationPractice
A moving coil galvanometer has 30 turns (exact) of coil area 1.50 × 10⁻⁴ m² in a radial field of 0.200 T, with a torsional constant of 3.00 × 10⁻⁶ N·m/rad. The current needed to produce a deflection of 0.150 rad is
Show answer and why every option is right or wrong
Answer: A. A is correct. From NIAB = kφ, I = kφ/(NAB) = (3.00 × 10⁻⁶ × 0.150)/(30 × 1.50 × 10⁻⁴ × 0.200) = 4.50 × 10⁻⁷/9.00 × 10⁻⁴ = 5.00 × 10⁻⁴ A. NCERT Class 12 Physics Chapter 4, page 130.
Why B is wrong: B is wrong because it computes NABφ = 9.00 × 10⁻⁴ × 0.150, multiplying where the torque balance divides and leaving out k.
Why C is wrong: C is wrong because it divides NAB by φ, 9.00 × 10⁻⁴/0.150 = 6.00 × 10⁻³, instead of dividing kφ by NAB.
Why D is wrong: D is wrong because it omits the number of turns from the denominator, using AB alone in place of NAB: 4.50 × 10⁻⁷/3.00 × 10⁻⁵ = 1.50 × 10⁻².
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Moving Coil Galvanometer: quick recall before you leave
How do you solve a Moving Coil Galvanometer question? A worked example
- 1
Given
Coil turns N = 60 (exact, a counting number)
Coil area A = 2.50 × 10⁻⁴ m²
Radial field B = 0.300 T
Torsional constant k = 4.50 × 10⁻⁶ N·m/rad
Galvanometer resistance R = 25.0 Ω - 2
Required
(a) The current sensitivity. (b) The voltage sensitivity. (c) The deflection produced by a current of 2.00 × 10⁻⁴ A.
- 3
Concept
The pole pieces are concave and a soft-iron core sits inside the coil, so the field is radial: the coil's plane always contains B, and the angle between the coil normal and B stays at 90°. Deflecting torque is NIAB with no sin θ. At rest this balances the spring's restoring torque kφ. Voltage sensitivity is just current sensitivity divided by the galvanometer's own resistance, because V = IR across the instrument.
- 4
Formula
NIAB = kφ
Current sensitivity: φ/I = NAB/k
Voltage sensitivity: φ/V = NAB/(kR) - 5
Substitution
NAB = 60 × 2.50 × 10⁻⁴ m² × 0.300 T
φ/I = (60 × 2.50 × 10⁻⁴ × 0.300)/(4.50 × 10⁻⁶)
φ/V = (φ/I)/25.0
φ = (φ/I) × 2.00 × 10⁻⁴ A - 6
Calculation
NAB = 60 × 2.50 × 10⁻⁴ = 1.50 × 10⁻² ; × 0.300 = 4.50 × 10⁻³ N·m/A
φ/I = 4.50 × 10⁻³/4.50 × 10⁻⁶ = 1.00 × 10³ rad/A
φ/V = 1.00 × 10³/25.0 = 4.00 × 10¹ rad/V
φ = 1.00 × 10³ × 2.00 × 10⁻⁴ = 2.00 × 10⁻¹ rad
The number of turns, 60, is a counting integer and is exact — it does not limit the significant figures. The measured quantities (2.50 × 10⁻⁴ m², 0.300 T, 4.50 × 10⁻⁶ N·m/rad, 25.0 Ω, 2.00 × 10⁻⁴ A) each carry three significant figures, so the answers are quoted to three. - 7
Final answer
(a) Current sensitivity = 1.00 × 10³ rad/A
(b) Voltage sensitivity = 4.00 × 10¹ rad/V
(c) Deflection = 2.00 × 10⁻¹ rad - 8
Common trap
Reporting the current sensitivity when the question asked for voltage sensitivity, or the reverse. The two differ by the factor R, which is 25.0 here — a full order of magnitude, and every distractor set in this topic exploits it. Read whether the stem gives you a current or a potential difference before you pick the expression. The second trap is inserting a sin θ: with a radial field there is no angle factor, and writing NIAB sin φ turns a one-line balance into an unsolvable transcendental equation.
- 9
Similar NEET-style question
A moving coil galvanometer with 80 turns (exact) of area 1.20 × 10⁻⁴ m² sits in a radial field of 0.250 T, with a torsional constant of 2.40 × 10⁻⁶ N·m/rad and a resistance of 50.0 Ω. Find its voltage sensitivity, and state what happens to that value if the coil is rewound with 160 turns of the same wire (resistance doubling to 100. Ω).
What to remember before solving Moving Coil Galvanometer questions
More in Magnetic Effects of Current and Magnetism: 3 exam traps and mistakes · 11 formulas · 5 question patterns from its other lessons.
Moving Coil Galvanometer questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
All 17 past-paper questions from Magnetic Effects of Current and Magnetism →
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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