A system undergoes a cyclic process. The internal energy change over one complete cycle is ΔU. Which statement is correct?

The correct answer is A: ΔU is always zero for any cyclic process. Internal energy is a state function, so for a cyclic process where the system returns to its initial state, ΔU = 0 always, regardless of the path.

A system absorbs 8000 J of heat and its internal energy increases by 5000 J. According to the first law of thermodynamics, what work is done by the system?

The correct answer is A: 3000 J. First law: ΔU = Q - W, so W = Q - ΔU = 8000 - 5000 = 3000 J (work done by the system)

Which of the following best explains why semiconductor resistance decreases with increase in temperature?

The correct answer is C: Increase in number of free charge carriers. In semiconductors, increased temperature promotes more electrons from valence to conduction band, increasing carrier concentration and decreasing resistance

The band gap energy of Germanium at room temperature (300K) is approximately:

The correct answer is A: 0.66 eV. Germanium has a band gap of ~0.66 eV at room temperature, making it a narrow band gap semiconductor used in infrared applications.

A wave pulse travels along a rope. When it reaches a fixed end, the reflected pulse is:

The correct answer is B: 180° out of phase with incident pulse. At a fixed end, the reflected wave undergoes a phase change of 180° (or π radians) due to the fixed boundary condition

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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 11–20 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.11 Medium Magnetism

Two coils have self-inductances L₁ and L₂ with coupling coefficient k. Their mutual inductance is:

A M = k√(L₁L₂)

B M = k(L₁ + L₂)

C M = kL₁L₂

D M = k(L₁L₂)²

Correct Answer: A. M = k√(L₁L₂)

EXPLANATION

Mutual inductance M = k√(L₁L₂), where 0 ≤ k ≤ 1 is the coupling coefficient

Test

Q.12 Medium Magnetism

The energy density of a magnetic field is given by:

A B²/(2μ₀)

B B²μ₀/2

C μ₀B²

D B/(2μ₀)

Correct Answer: A. B²/(2μ₀)

EXPLANATION

Magnetic energy density u = B²/(2μ₀), similar to electric field energy density

Test

Q.13 Medium Magnetism

A rectangular loop is placed in a non-uniform magnetic field. The loop will experience:

A A net force and a torque

B Only torque, no net force

C Only force, no torque

D Neither force nor torque

Correct Answer: A. A net force and a torque

EXPLANATION

Non-uniform field causes net force on the loop due to unequal forces on different sides, plus torque due to magnetic moment

Test

Q.14 Medium Magnetism

In a Hall effect experiment with a rectangular conductor in a magnetic field, the Hall voltage is proportional to:

A Magnetic field strength and current, inversely proportional to charge carrier concentration

B Only the magnetic field strength

C Only the current

D The resistance of the conductor

Correct Answer: A. Magnetic field strength and current, inversely proportional to charge carrier concentration

EXPLANATION

Hall voltage V_H = (1/ne) × (IB/t), proportional to B and I, inversely proportional to carrier concentration n

Test

Q.15 Medium Magnetism

A charged particle moves in a helical path in a uniform magnetic field. This happens when the particle's velocity has:

A Components both parallel and perpendicular to B

B Only parallel component to B

C Only perpendicular component to B

D No relationship with B direction

Correct Answer: A. Components both parallel and perpendicular to B

EXPLANATION

Perpendicular component causes circular motion, parallel component causes linear motion, resulting in helical path

Test

Q.16 Medium Magnetism

The self-inductance of a solenoid depends on:

A Number of turns, cross-sectional area, and length

B Current flowing through it

C Resistance of the wire

D External magnetic field

Correct Answer: A. Number of turns, cross-sectional area, and length

EXPLANATION

L = μ₀n²AL, where n = N/l. Self-inductance depends on geometry and material properties, not on current.

Test

Q.17 Medium Magnetism

The energy stored in a magnetic field of an inductor carrying current I is:

A U = ½LI²

B U = LI²

C U = L²I

D U = I²/L

Correct Answer: A. U = ½LI²

EXPLANATION

Energy stored in inductor: U = ½LI², derived from dW = LI dI integrated from 0 to I

Test

Q.18 Medium Magnetism

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

A τ = L/R

B τ = R/L

C τ = LR

D τ = √(L/R)

Correct Answer: A. τ = L/R

EXPLANATION

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

Test

Q.19 Medium Magnetism

A paramagnetic material placed in a magnetic field becomes magnetized. This is because:

A Unpaired electrons align with the field

B Electrons stop orbiting nuclei

C All electrons pair up

D Nuclear spins align

Correct Answer: A. Unpaired electrons align with the field

EXPLANATION

In paramagnetic materials, unpaired electrons have permanent magnetic moments that align with external field

Test

Q.20 Medium Magnetism

When a conductor moves through a magnetic field, an EMF is induced. This is due to the Lorentz force on:

A Free electrons in the conductor

B Fixed ions in the conductor

C Nucleus of atoms

D Lattice vibrations

Correct Answer: A. Free electrons in the conductor

EXPLANATION

Motional EMF arises because free electrons experience Lorentz force F = -e(v × B) as conductor moves through field

Test

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JEE Physics
Magnetism