Physics is where JEE ranks are usually won or lost, since a single conceptual slip changes the whole answer. Questions here span mechanics, rotational motion, thermodynamics, waves and oscillations, electrostatics, current electricity, magnetism, optics, and modern physics. Numerical problems carry full derivations so you can see which step you skipped, not just which option was right.
A charged particle moves in a uniform magnetic field perpendicular to its velocity. Which of the following remains constant?
Answer: A
When a charged particle moves perpendicular to a uniform magnetic field, the magnetic force is always perpendicular to velocity, so it does no work. Hence kinetic energy remains constant. However, direction changes continuously.
Q.2Easy
The magnetic field at the center of a circular loop of radius R carrying current I is:
Answer: C
Using Biot-Savart law, the magnetic field at the center of a circular loop of radius R carrying current I is B = μ₀I/2R. This is a fundamental formula in magnetism.
Q.3Easy
A bar magnet is cut into two equal halves along its length. The magnetic moment of each half is:
Answer: A
When a bar magnet is cut along its length, each half has half the number of magnetic dipoles. Since magnetic moment is the sum of individual dipole moments, each half has magnetic moment M/2.
Q.4Easy
The SI unit of magnetic flux density is:
Answer: B
Magnetic flux density (B) is measured in Tesla (T) in SI units. 1 Tesla = 1 Weber/m². Gauss is CGS unit, Weber is unit of flux, and Henry is unit of inductance.
Q.5Easy
A solenoid of length L, cross-sectional area A, and N turns carries current I. The magnetic field inside the solenoid is:
Answer: A
The magnetic field inside a solenoid is uniform and given by B = μ₀nI, where n = N/L is the number of turns per unit length. This is independent of the cross-sectional area A.
Q.6Easy
A moving charge experiences a magnetic force. This force is always:
Answer: B
Magnetic force is given by F = q(v × B), which is the cross product of velocity and field. Cross product is always perpendicular to both vectors.
Q.7Easy
A magnetic field B is applied perpendicular to a current-carrying wire of length L carrying current I. The magnetic force on the wire is F = BIL sin θ. What is the angle θ when the force is maximum?
Answer: A
Force F = BIL sin θ is maximum when sin θ = 1, which occurs at θ = 90°. This means the wire must be perpendicular to the magnetic field direction.
Q.8Easy
The SI unit of magnetic field intensity is Tesla. One Tesla is equivalent to which of the following?
Answer: A
1 Tesla = 1 Weber/m² = 1 kg/(A·s²). This is the correct dimensional formula for magnetic field intensity.
Q.9Easy
An electron moves in a circular path in a perpendicular magnetic field. Which quantity remains constant during its motion?
Answer: A
Magnetic force is always perpendicular to velocity, so it does no work. Kinetic energy (½mv²) remains constant, but direction changes, so velocity changes while speed remains constant.
Q.10Easy
A charged particle with charge q and mass m enters a magnetic field region with velocity v perpendicular to B. The radius of circular motion is given by:
Answer: A
From qvB = mv²/r, we get r = mv/(qB). This is the radius of curvature for a charged particle in a perpendicular magnetic field.
Q.11Easy
The magnetic flux through a surface is Φ = ∫B·dA. For a uniform field B perpendicular to a rectangular surface of area 2 m², with B = 0.5 T, the flux is:
Answer: C
Φ = BA cos θ. Since B is perpendicular to the surface, θ = 0° and cos θ = 1. Φ = 0.5 × 2 = 1 Wb.
Q.12Easy
According to Lenz's law, the induced EMF in a coil opposes the change in magnetic flux. If the flux through a coil increases at a rate of 0.1 Wb/s and the coil has 100 turns, the induced EMF is:
Answer: A
Induced EMF ε = -N × dΦ/dt = -100 × 0.1 = -10 V. The magnitude is 10 V.
Q.13Easy
The magnetic field due to a solenoid inside and outside differs significantly. Which statement is correct?
Answer: A
An ideal solenoid produces a uniform, strong magnetic field inside (B = μ₀nI) and negligible field outside. This is due to cancellation of fields from adjacent turns outside.
Q.14Easy
A magnetic dipole of moment M is placed in a uniform magnetic field B. The torque experienced by the dipole when it makes an angle θ with the field is:
Answer: A
Torque on a magnetic dipole: τ = M × B = MB sin θ, where θ is the angle between M and B.
Q.15Easy
The magnetic field intensity at a distance r from a long straight current-carrying wire is:
Answer: B
Using Ampere's circuital law for an infinite straight wire: B = μ₀I/(2πr), where μ₀ = 4π × 10⁻⁷ T·m/A
Q.16Easy
A proton moving with velocity v perpendicular to a magnetic field B follows a circular path. The radius of this path is:
Answer: A
For circular motion in magnetic field: qvB = mv²/r, therefore r = mv/(qB)
Q.17Easy
Two parallel wires carrying currents I₁ and I₂ in the same direction, separated by distance d. The force per unit length between them is:
Answer: C
Magnetic force per unit length between parallel wires: F/L = μ₀I₁I₂/(2πd), attractive for same direction currents
Q.18Easy
A charged particle with charge q and mass m moves in a magnetic field. The cyclotron frequency is:
Answer: A
Cyclotron frequency: f = qB/(2πm) = ω/(2π), where ω = qB/m is the angular frequency
Q.19Easy
The magnetic moment of a current loop carrying current I with area A is:
Answer: A
Magnetic moment of a current loop: μ = IA, where I is current and A is area of the loop
Q.20Easy
A transformer has primary coil with N₁ turns and secondary coil with N₂ turns. The voltage ratio is:
Answer: A
Transformer equation: V₂/V₁ = N₂/N₁ (ideal transformer with no losses)
