Moving Charges and Magnetism — Class 12 MCQs with Answers

Moving Charges and Magnetism — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Force on charge q moving with v in field B:

   AqvB²Bq/vCqB/vDqv × B
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   Answer: qv × B

   Explanation: Lorentz force law: F = qv × B, a vector (cross) product — force is perpendicular to both velocity and field, unlike a scalar mix like qv/B or qB/v that ignores direction.

2. Q2 of 6Easy

   SI unit of magnetic field B:

   AVoltBTeslaCNewtonDOhm
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   Answer: Tesla

   Explanation: SI unit of B is the tesla, defined via F = qvB so 1 T = 1 N/(A·m). Confusing it with volt or ohm mixes up electric-field and resistance units, which carry different dimensions.

3. Q3 of 6Medium

   The force per unit length on a straight wire carrying current I in a uniform magnetic field B is:

   AI/BBI·B (dot product)CI × B (I along the wire)DB/I
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   Answer: I × B (I along the wire)

   Explanation: Dividing the wire force F = IL × B by length L leaves force per unit length as I × B (I taken along the wire) — a cross product, magnitude IB sinθ, so it depends on the angle between current and field; a dot product would wrongly give a scalar that is largest when they are parallel.

4. Q4 of 6Medium

   Assertion (A): The magnetic field at the centre of a tightly wound circular coil of 100 turns is 100 times the field of a single turn of the same radius carrying the same current. Reason (R): The fields of the individual turns at the centre all point the same way and add up by the principle of superposition.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: B = μ₀NI/(2R). R is true: each turn gives the same field μ₀I/(2R) along the axis, and superposition adds the N equal contributions, which is exactly why the field is N times larger.

5. Q5 of 6Hard

   Cyclotron frequency:

   AqB/mBqB/(2πm)CmB/qDqBm
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   Answer: qB/(2πm)

   Explanation: Setting magnetic force equal to centripetal force and using v = 2πrf gives cyclotron frequency f = qB/(2πm) — independent of the particle's speed or orbit radius.

6. Q6 of 6Hard

   Assertion (A): The magnetic field due to a small current element is zero at every point on the line along which the element lies. Reason (R): The magnitude of the field of a current element is proportional to sin θ, where θ is the angle between the element and the line joining it to the point.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true and R is true. For points on the line of the element, θ is 0° or 180°, so sin θ = 0 and dB = 0. R is exactly the reason for A.

Moving Charges and Magnetism notes: key concepts (Class 12 Physics)

The magnetic effects of moving charge: the Lorentz force and circular motion in a field, the cyclotron and velocity selector, the force on a current-carrying wire and between parallel currents, the Biot-Savart law and Ampere's circuital law, the fields of a wire, loop, solenoid and toroid, and the torque on a current loop.

Magnetic effect of current

Oersted's discovery that a current deflects a compass needle, establishing that electricity and magnetism are two aspects of one phenomenon.

Lorentz force

The total force on a charge from electric and magnetic fields together, F = q(E + v × B) — the starting point for the whole chapter.

Magnetic force on a charge

Proportional to qvB sin θ, so it is zero for a stationary charge and zero for motion parallel to the field. It is always perpendicular to velocity.

Magnetic force does no work

Because the force stays perpendicular to velocity, it can never change the SPEED or kinetic energy of a charged particle — only its direction.

Circular motion in a field

A charge entering perpendicular to a uniform field moves in a circle of radius mv/qB. Entering at an angle gives a helical path.

Cyclotron

Accelerates charged particles using a fixed-frequency alternating voltage across dees. Its frequency is independent of the particle's speed and orbit radius.

Velocity selector

Crossed electric and magnetic fields let through only particles with speed E/B, regardless of charge or mass — the front end of a mass spectrometer.

Force on a current conductor

A wire of length L carrying current I in a field experiences BIL sin θ, the force on a moving charge scaled up to the whole conductor.

Force between parallel currents

Parallel currents attract and antiparallel currents repel. This force is what formally defines the ampere.

Biot-Savart law

Gives the magnetic field of a current element, falling off as the inverse square with a sin θ directional factor — the magnetic analogue of Coulomb's law.

Field of a straight wire

Circles the wire with magnitude μ₀I/2πr, so it falls off as 1/r rather than the inverse square of a point source.

Field at the centre of a loop

Equal to μ₀I/2R, directed along the axis with the sense given by the right-hand grip rule.

Ampere's circuital law

The line integral of B around a closed loop equals μ₀ times the enclosed current — the magnetic counterpart of Gauss's law.

Field of a solenoid

Uniform and equal to μ₀nI inside a long solenoid, and essentially zero outside. It depends on turns per unit LENGTH, not the total number of turns.

Field of a toroid

Confined entirely within the toroid; the field both outside and in the central hole is zero, which is why toroidal cores leak so little.

Torque on a current loop

A current loop behaves as a magnetic dipole of moment NIA, feeling a torque m × B in a field — the operating principle of the moving-coil galvanometer.