An inductor with inductance L and a resistor with resistance R are connected in series with a DC source. The time constant is:

The correct answer is A: τ = L/R. For LR circuit, time constant τ = L/R. Current rises as I = (V/R)(1 - e^(-t/τ))

When a wave reflects from a denser medium, the phase change is:

The correct answer is C: 180°. Upon reflection from a denser medium (fixed end), there is a phase change of π radians or 180°

In an intrinsic semiconductor at room temperature, what is the relationship between electron concentration (n_e) and hole concentration (n_h)?

The correct answer is B: n_e = n_h = n_i. In an intrinsic semiconductor, every electron-hole pair is generated together, so the concentration of free electrons equals the concentration of holes, both equal to the intrinsic carrier concentration n_i.

A wave pulse on a rope travels from a denser to a lighter medium (from thin to thick rope). The reflected pulse:

The correct answer is B: Is not inverted and has reduced amplitude. At boundary from denser to less dense medium, reflected pulse is NOT inverted but has smaller amplitude due to partial reflection

Two conducting spheres of radii r₁ and r₂ (r₁ > r₂) have charges Q₁ and Q₂. They are connected by a wire. The final charge distribution will be such that:

The correct answer is D: Both A and C. Connected spheres have same potential. V = kQ/r, so Q ∝ r. Both statements are equivalent and correct.

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JEE Physics
Magnetism

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

44 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 41–44 of 44

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.41 Medium Magnetism

The magnetic moment of an electron orbiting in the first Bohr orbit is approximately:

A 1 Bohr magneton

B 0.5 Bohr magneton

C 2 Bohr magneton

D 0.25 Bohr magneton

Correct Answer: A. 1 Bohr magneton

EXPLANATION

The magnetic moment of an electron in the first Bohr orbit equals 1 Bohr magneton (μ_B = eℏ/2m_e ≈ 9.27 × 10⁻²⁴ J/T). This is a fundamental quantum result.

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Q.42 Medium Magnetism

A rectangular loop of dimensions a × b carrying current I is placed in a uniform magnetic field B. The maximum torque on the loop is:

A τ = BIab

B τ = BIa/b

C τ = BI/ab

D τ = BIa²b

Correct Answer: A. τ = BIab

EXPLANATION

Torque on a current loop in a magnetic field is τ = NIAB sin(θ), where A is the area and θ is the angle. Maximum torque occurs when sin(θ) = 1, giving τ_max = BIab for N=1.

Test

Q.43 Medium Magnetism

Two parallel wires carrying currents I₁ and I₂ in opposite directions are separated by distance d. The force between them is:

A Repulsive with magnitude F = μ₀I₁I₂d/2πℓ

B Attractive with magnitude F = μ₀I₁I₂/2πdℓ

C Repulsive with magnitude F = μ₀I₁I₂/2πdℓ

D Attractive with magnitude F = μ₀I₁I₂d/2πℓ

Correct Answer: C. Repulsive with magnitude F = μ₀I₁I₂/2πdℓ

EXPLANATION

Currents in opposite directions repel each other. The force per unit length is F/ℓ = μ₀I₁I₂/2πd, making total force F = μ₀I₁I₂ℓ/2πd (repulsive).

Test

Q.44 Medium Magnetism

An electron enters a region of uniform magnetic field with velocity v perpendicular to the field. If the magnetic field strength is B, the radius of curvature is:

A r = mv/eB

B r = eB/mv

C r = m/eBv

D r = eBv/m

Correct Answer: A. r = mv/eB

EXPLANATION

For a charged particle in a magnetic field, centripetal force equals magnetic force: mv²/r = evB, giving r = mv/eB. This is the radius of the circular path.

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