JEE Main Physics: Effect of uniform magnetic field on a current loop Practice Questions

Correct answer: D — BIL

The magnetic force on a current loop in a uniform magnetic field is given by F = I (l⃗ × B⃗) = I l B sin θ. For a rectangular loop placed in a uniform field, the net force is F = I (l × B) sin(90°) = I l B. If l is the side length, and assuming the loop is oriented optimally, the magnitude is proportional to IB times some dimension, but the simplest conceptual choice often focuses on the fundamental force structure. For a rectangular loop where sides are parallel/perpendicular to B, the force calculation depends on orientation. If the sides are perpendicular to B, the force is F=2(IlB) or simply related to the area and B. In the context of simple conceptual application, the fundamental force is F=ILB (if l is the length perpendicular to B).

Correct answer: A — Parallel to the magnetic field

The induced EMF is proportional to the rate of change of magnetic flux (EMF = -dΦ_B/dt). If the loop is oriented parallel to the magnetic field, the magnetic flux passing through the loop does not change as the loop moves or if the field is static, resulting in zero induced EMF.

Correct answer: B — Perpendicular to the magnetic field line

The torque experienced by a current loop is always perpendicular to both the magnetic field (B⃗) and the magnetic dipole moment (M⃗), meaning the torque is perpendicular to the magnetic field direction.

Correct answer: D — The force is always directed perpendicular to the magnetic field.

The force experienced by a current loop is determined by the cross product F⃗ = I (l⃗ × B⃗). This force is always directed perpendicular to both the direction of the current (or the loop area vector) and the magnetic field direction. If the loop is oriented such that it experiences a net force, the force is perpendicular to B⃗.

Correct answer: D — The property of uniform fields

Magnetic flux (Φ_B) through a loop is Φ_B = B⃗ · A⃗ = BAcos θ. Since B is uniform and for a simple case where θ is constant (or the problem implies the flux dependence based on area), the flux is directly proportional to the area (A).

Correct answer: A — BL

The magnetic force on a straight current-carrying wire in a uniform magnetic field is given by F = I L B sin θ. If the wire is perpendicular to the magnetic field (θ= 90°), sin(90°) = 1. Thus, the magnitude of the force is F = I L B.

Correct answer: D — 0

The magnetic force experienced by a current loop is directly proportional to the magnetic field (F ∝ B). If the magnetic field B is zero, the net magnetic force acting on the loop is zero, regardless of the current or the geometry of the loop.

Correct answer: D — 0

The torque experienced by a magnetic dipole moment (μ⃗) in a magnetic field (B⃗) is given by τ⃗ = μ⃗ × B⃗. If the plane of the loop is parallel to B⃗ (θ= 0°), then sin(0°) = 0. Therefore, the torque experienced is τ= μB sin(0°) = 0.

Correct answer: D — Double

The force is proportional to B (F ∝ B). If B is doubled, the force will also double, assuming θ is constant.

Correct answer: D — 90°

The torque is τ= I A B sin θ. Since sin(90°) = 1, the torque is maximum when θ= 90°.

Correct answer: A — I B A

The magnitude of the force experienced by a current loop in a uniform magnetic field is given by F = I (μB) sin θ. For a simple rectangular loop where the sides are aligned with the field or perpendicular to it, the force depends directly on the area (A) and the magnetic field (B) interaction, scaled by the current I (when considering the resulting magnetic moment interaction).

Correct answer: D — (3) Blw

The magnetic dipole moment of the loop is M = NIA = (l × w) I. When the loop is perpendicular to the field (θ= 90°), the torque is τ= MBsin(90°) = MB. Substituting M, we get τ= (l w I) B.

Correct answer: C — A 0.25 N

When the sides of a rectangular loop are parallel to the magnetic field (B⃗), the area vector (A⃗) is parallel to B⃗, so the force is zero (F⃗ = I (A⃗ × B⃗) = 0). Since the sides are parallel to B⃗, the force is zero.