JEE Main Physics: Electricity and Magnetism 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 — Square of the distance between them

Coulomb's law is mathematically defined as F = k (|q₁ q₂|)/r². The force is directly proportional to the product of the charges (q₁ q₂) and inversely proportional to the square of the distance (r²).

Correct answer: B — 0

The field due to +σ is E₁ = σ/2ε₀ (pointing away from the positive sheet). The field due to -σ is E₂ = (-σ)/2ε₀ (pointing towards the negative sheet). Since the sheets are identical and parallel, the fields cancel out: Eₙₑₜ = E₁ + E₂ = 0.

Correct answer: B — Conservation of energy

Lenz's Law ensures that the process of changing magnetic flux involves an energy exchange, keeping the total energy conserved.

Correct answer: B — Repulsive

Like charges attract each other, and unlike charges repel each other. Since the charges are the same (like charges), the force exerted between them is repulsive.

Correct answer: C — The distance

As established by Ampere's Law and field principles, the electric field magnitude is inversely proportional to the radial distance (r) from the wire.

Correct answer: B — They are always parallel to the direction of the electric field.

Electric field lines originate from positive charges and terminate on negative charges. Electric field lines are always tangent to the direction of the field, not perpendicular to surfaces (unless dealing with surface charge density effects).

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: C — Zero EMF

Faraday's law states that EMF is induced only when there is a change in magnetic flux. If the flux is constant (Δφ_B = 0), the induced EMF is zero.

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: A — λ/(2 πε₀ r)

The formula for the electric field magnitude E due to an infinite wire is E = λ/(2 πε₀ r).

Correct answer: B — It is the proportionality constant relating force to the product of charges and distance

Coulomb's constant (k = 1/4πε₀) is the proportionality constant that relates the electrostatic force between two charges to the product of the charges and the inverse square of the distance.

Correct answer: B — Volt (V)

Electric potential (Voltage) is defined as the potential energy per unit charge, and its SI unit is the Volt (V).

Correct answer: C — 1/r

The electric field (E) due to an infinite straight wire is inversely proportional to the distance (r) from the wire. Therefore, E ∝ 1/r.

Correct answer: B — V = IR

Ohm's Law states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends and inversely proportional to the resistance (R). Mathematically, this is expressed as V = IR.