Moving Charges and MagnetismNEET MCQs with solutions
Moving Charges and Magnetism covers the magnetic force on moving charges and current-carrying conductors, Biot-Savart law, Ampere's circuital law, cyclotron, solenoid and toroid. NEET tests Lorentz force, radius of circular motion in magnetic field, Biot-Savart law application and force between parallel currents.
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- 12 Physics
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Practise 24 questions
Tap an option to check it. Questions from every NCERT topic in this chapter, from easy to hard.
Q1Mixed Revision
A proton enters a uniform magnetic field perpendicular to it with speed v. If the speed becomes 3v while mass, charge, and field remain unchanged, the new radius of the circular path is:
Not quite — the answer is C.
Using r = mv/qB, radius is directly proportional to speed. Tripling speed gives new radius = 3r. Option A reverses the proportionality; Option D squares the ratio, a common student error.
Q2Magnetic Force on Moving Charges
A proton enters a uniform magnetic field of magnitude B with velocity v making an angle θ with the field. The magnitude of magnetic force acting on it is:
Not quite — the answer is C.
Magnetic force F=qvBsinθ. Force is maximum when θ=90° and zero when θ=0° or 180°. Option qvBtanθ has no physical basis in electromagnetic theory.
Q3Ampere's Circuital Law
According to Ampere's Circuital Law, the line integral of the magnetic field around any closed Amperian path is equal to:
Not quite — the answer is B.
∮B·dl = μ₀I_enclosed; only net current through the bounded surface counts. Option A is dimensionally wrong. Option C is a classic misconception — external currents cancel in the integral. Option D confuses the law with magnetic flux.
Q4Force Between Parallel Current-Carrying Conductors & Definition of Ampere
The force per unit length between two parallel conductors separated by distance d is directly proportional to:
Not quite — the answer is B.
F/L = μ₀I₁I₂/2πd shows direct proportionality to I₁I₂ and inverse proportionality to d. Options A and D reverse this relationship. Option C inverts the current dependence entirely.
Q5Grand Test
A proton enters a uniform magnetic field perpendicularly with speed 2×10⁶ m/s. If the magnetic field is doubled while the speed remains unchanged, the radius of the circular path becomes:
Not quite — the answer is B.
r = mv/qB; radius is inversely proportional to B. Doubling B gives r_new = r/2. Mass, charge, and speed are all unchanged. Option C (Double) is the trap — students confuse direct and inverse proportion.
Q6Motion of Charged Particle in Magnetic Field
A charged particle enters a uniform magnetic field perpendicular to its velocity. The path followed by the particle is:
Not quite — the answer is C.
When v⊥B (θ=90°), magnetic force F=qvB acts perpendicular to velocity at every instant, providing centripetal force for uniform circular motion. Speed is unchanged since magnetic force does no work.
Q7Torque on Current Loop and Magnetic Dipole
A rectangular current-carrying loop of n turns, area A, carrying current I is placed in a uniform magnetic field B. The angle between the magnetic moment and the magnetic field is θ. The torque acting on the loop is:
Not quite — the answer is C.
τ=nIABsinθ where M=nIA is the magnetic moment. Torque is maximum at θ=90° and zero at θ=0°. Option D is dimensionally wrong: nIB/A has units A·T/m², not N·m.
Q8Biot–Savart Law
According to the Biot–Savart law, the magnetic field dB due to a small current element Idl at a point P is directly proportional to:
Not quite — the answer is D.
dB=(μ₀/4π)(Idl sinθ/r²): field is proportional to I, dl, sinθ, and inversely to r². Option A uses cosθ — wrong trigonometric factor. Options B and C are dimensionally incorrect, missing the sinθ dependence.
Q9Solenoid and Toroid
A solenoid has a total of N turns wound over length L and carries current I. Which expression correctly gives the magnetic field inside it?
Not quite — the answer is A.
B = μ₀nI where n = N/L. Option B (μ₀NI/2πr) is the toroid formula — uses total N and mean radius, not length. Option C is the long straight wire formula. Option D is the field at the centre of a single circular loop — different geometry entirely.
Q10Moving Coil Galvanometer
The working principle of a moving coil galvanometer is based on:
Not quite — the answer is A.
A current-carrying coil in a uniform magnetic field experiences deflecting torque τ = nBIA. This torque causes the coil to deflect against a restoring spring until equilibrium. Option B is the trap — EMI involves changing flux; galvanometer responds to steady current.
Q11Conversion of Galvanometer into Ammeter and Voltmeter
A galvanometer is converted into an ammeter by connecting:
Not quite — the answer is A.
An ammeter needs very low resistance. A low-resistance shunt in parallel allows most current to bypass the galvanometer. Series connection (Option C) would increase resistance, opposite of requirement.
Q12NCERT Miscellaneous Facts, Graphs, Exceptions & PYQ Concepts
Which physical quantity is always perpendicular to both the velocity of a charged particle and the magnetic field?
Not quite — the answer is A.
F = q(v × B). The cross product yields a vector perpendicular to both v and B simultaneously. Electric field and current are not defined by this cross product relationship.
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Get RankUp on Google PlayQ13Force on Current-Carrying Conductor
A straight conductor of length l carrying current I is placed perpendicular to a uniform magnetic field B. The magnetic force acting on it is:
Not quite — the answer is A.
F=BIlsinθ. At θ=90°, sin90°=1 giving F=BIl. Options B and C have incorrect dimensions (not newtons). Option D introduces an extra length factor with no physical basis.
Q14Magnetic Force on Moving Charges
A charged particle moves parallel to a uniform magnetic field. The magnetic force on the particle is:
Not quite — the answer is B.
When velocity is parallel to B, θ=0°, so sin0°=0 and F=qvBsin0°=0. Magnetic force vanishes for parallel motion regardless of charge or speed.
Q15Magnetic Force on Moving Charges
A charged particle moves perpendicular to a uniform magnetic field. Which of the following correctly describes the effect of the magnetic force on the particle?
Not quite — the answer is C.
Magnetic force F=q(v×B) is always perpendicular to velocity, so work done is zero and kinetic energy is unchanged. Only direction changes, not speed or energy.
Q16Magnetic Force on Moving Charges
An electron enters a magnetic field normally with speed v. If both magnetic field and speed are doubled, the magnetic force becomes:
Not quite — the answer is D.
F=qvB. Substituting 2v and 2B gives F'=q(2v)(2B)=4qvB=4F. Force scales with the product of speed and field, not their sum.
Q17Magnetic Force on Moving Charges
Which of the following particles experiences the maximum magnetic force when moving with the same speed perpendicular to the same magnetic field?
Not quite — the answer is C.
F=qvB depends on charge magnitude. Alpha particle carries +2e, proton and deuteron carry +e, neutron carries zero charge. Alpha particle experiences double the force of a proton.
Q18Magnetic Force on Moving Charges
A charged particle moves in a region with both electric and magnetic fields. Which of the following statements about magnetic force is correct?
Not quite — the answer is A.
Magnetic force F=q(v×B) is perpendicular to v by definition of the cross product. It cannot do work, has no component along B, and is entirely independent of the particle's mass.
Q19Magnetic Force on Moving Charges
Which one of the following statements is INCORRECT regarding magnetic force on a moving charge?
Not quite — the answer is B.
Magnetic force F=qvBsinθ is always perpendicular to both velocity and magnetic field. It never acts along the velocity direction. All other options correctly describe F=qvBsinθ.
Q20Magnetic Force on Moving Charges
A particle of charge 2 μC moves with speed 5×10⁶ m/s perpendicular to a magnetic field of 0.4 T. The magnetic force is:
Not quite — the answer is D.
F=qvB=(2×10⁻⁶)(5×10⁶)(0.4)=4 N. Common error: forgetting that 10⁻⁶×10⁶=10⁰=1, leading to answers of 2 N (dropping the 0.4) or 6 N (arithmetic slip).
Q21Magnetic Force on Moving Charges
Which formula correctly represents the vector form of magnetic force on a moving charge?
Not quite — the answer is A.
Lorentz magnetic force is the cross product F=q(v×B). The dot product in Option B yields a scalar, not a force. Options C and D have no physical meaning in this context.
Q22Magnetic Force on Moving Charges
A positively charged particle moves towards the east while the magnetic field acts vertically downward. The magnetic force acts towards:
Not quite — the answer is C.
v=+î (east), B=−k̂ (downward). F=q(v×B)=q[î×(−k̂)]=q(ĵ)=north. Common error: applying right-hand rule without accounting for the negative sign on B.
Q23Magnetic Force on Moving Charges
A charged particle enters a uniform magnetic field with velocity making an angle 30° with the field. If the magnetic force is F, then the force when the angle becomes 90° is:
Not quite — the answer is D.
F₁=qvBsin30°=qvB/2. At θ=90°, F₂=qvBsin90°=qvB=2F₁. Option √3F is the trap for students who use sin60° instead of the ratio sin90°/sin30°.
Q24Magnetic Force on Moving Charges
A magnetic field cannot do work on a moving charged particle because:
Not quite — the answer is A.
W=F·ds·cosθ. Since magnetic force is perpendicular to velocity (θ=90°), cos90°=0 and W=0. Speed and kinetic energy remain constant; only direction changes.
ELITE question · AIR under 50 level
This chapter has 213 ELITE questions for students aiming at the very top. They are only in the app.
Unlock ELITE questions in the appKey Magnetic Force Concepts
Quick revision: most questions in this chapter test these facts.
| Concept | Key Formula |
|---|---|
| Lorentz force | F = qv × B = qvB sinθ; perpendicular to both v and B |
| Circular motion in B | r = mv/qB; T = 2πm/qB (independent of velocity) |
| Biot-Savart law | dB = (μ₀/4π)(Idl × r̂)/r²; field at centre of loop: B = μ₀I/2R |
| Ampere's law | ∮B·dl = μ₀I_enc; useful for solenoid (B = μ₀nI) and toroid |
| Force on conductor | F = BIL sinθ; direction by Fleming's left-hand rule |
| Parallel currents | Same direction → attract; opposite → repel; F/L = μ₀I₁I₂/2πd |
What the app covers in this chapter
573 questions in total, each with a detailed explanation.
| Mixed Revision | 102 |
| Magnetic Force on Moving Charges | 40 |
| Ampere's Circuital Law | 40 |
| Force Between Parallel Current-Carrying Conductors & Definition of Ampere | 40 |
| Grand Test | 40 |
| Motion of Charged Particle in Magnetic Field | 39 |
| Torque on Current Loop and Magnetic Dipole | 39 |
| Biot–Savart Law | 39 |
| Solenoid and Toroid | 39 |
| Moving Coil Galvanometer | 39 |
| Conversion of Galvanometer into Ammeter and Voltmeter | 39 |
| NCERT Miscellaneous Facts, Graphs, Exceptions & PYQ Concepts | 39 |
| Force on Current-Carrying Conductor | 38 |
Questions students ask
Is Moving Charges and Magnetism important for NEET?
Yes — Lorentz force, circular motion radius, Biot-Savart law and force between parallel currents are tested every year.
Which topics should I revise first?
Focus on Lorentz force and circular motion radius formula, Biot-Savart law for circular loop, Ampere's law for solenoid/toroid, force between parallel currents, and cyclotron working.
How many questions from this chapter are on RankUp?
The RankUp app has 573 questions on Moving Charges and Magnetism, including 213 ELITE questions. Every question has a detailed explanation.
