A convex mirror has a focal length of 30 cm. An object of height 5 cm is placed at 60 cm from the mirror. What is the height of the image?

The correct answer is A: 1.67 cm. For convex mirror, f = 30 cm (positive in sign convention), u = -60 cm. Using 1/f = 1/v + 1/u: 1/30 = 1/v - 1/60, so 1/v = 1/30 + 1/60 = 3/60 = 1/20, v = 20 cm. Magnification m = v/u = 20/(-60) = -1/3. Height of image = |m| × h₀ = (1/3) × 5 = 1.67 cm.

The heat capacity at constant pressure Cp is greater than heat capacity at constant volume Cv for ideal gases because:

The correct answer is A: At constant pressure, some energy goes into work expansion. At constant P: Q = ΔU + PΔV, so more heat is needed to produce same temperature rise. Relation: Cp - Cv = R

An electron transitions from n=3 to n=1 in hydrogen atom. The frequency of emitted photon is (R = 1.097×10⁷ m⁻¹):

The correct answer is A: 2.47×10¹⁵ Hz. ν = Rc(1/n₁² - 1/n₂²) = 1.097×10⁷ × 3×10⁸ × (1 - 1/9) = 1.097×10¹⁵ × 8/9 ≈ 2.47×10¹⁵ Hz

A uniformly charged sphere of radius R and total charge Q has its charge density reduced by half. The electric potential at the surface becomes:

The correct answer is A: Half of original. Surface potential V = kQ/R. If charge becomes Q/2, then V becomes kQ/(2R), which is half the original.

A body of mass 5 kg moves with velocity 4 m/s. A force acts on it for 2 seconds and velocity becomes 10 m/s. What is the impulse?

The correct answer is B: 30 N·s. Impulse = Δ(momentum) = m(v₂ - v₁) = 5(10 - 4) = 30 N·s

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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 21–30 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.21 Medium Magnetism

An electron (charge -e) and a proton (charge +e) move perpendicular to the same uniform magnetic field with the same velocity. The ratio of their radii of curvature is:

A 1:1

B m_e/m_p

C m_p/m_e

D e/(2e) = 1/2

Correct Answer: B. m_e/m_p

EXPLANATION

r = mv/(eB), so r_e/r_p = (m_e/m_p) = 1/1836 approximately

Test

Q.22 Medium Magnetism

The mutual inductance between two coils depends on:

A Only the geometry and relative orientation of coils

B Only the currents in the coils

C Only the resistances of coils

D The distance between coils and their currents

Correct Answer: A. Only the geometry and relative orientation of coils

EXPLANATION

Mutual inductance M₁₂ = k√(L₁L₂) depends on coil geometry, orientation, and proximity, not on current or resistance

Test

Q.23 Medium Magnetism

A bar magnet is cut into two equal halves perpendicular to its length. The magnetic moment of each half is:

A M/2

B M

C 2M

D M/4

Correct Answer: A. M/2

EXPLANATION

When cut perpendicular to length, magnetic moment is halved. Each piece has half the pole strength or length.

Test

Q.24 Medium Magnetism

A conducting rod of mass m slides on two parallel rails separated by distance d in a magnetic field B perpendicular to the plane. If the rod moves with constant velocity v, the magnetic force on it is:

A F = Bvd

B F = B²d²v/R

C F = Bdv

D F = B²vd²

Correct Answer: B. F = B²d²v/R

EXPLANATION

Induced EMF: ε = Bdv, induced current: I = Bdv/R, magnetic force: F = BId = B²d²v/R (opposes motion)

Test

Q.25 Medium Magnetism

A solenoid of length L and cross-sectional area A has N turns. Its self-inductance is:

A L = μ₀N²A/L

B L = μ₀NA²/L

C L = μ₀N/AL

D L = μ₀L/N²A

Correct Answer: A. L = μ₀N²A/L

EXPLANATION

Self-inductance of solenoid: L = μ₀n²V = μ₀(N/L)²AL = μ₀N²A/L

Test

Q.26 Medium Magnetism

A rectangular coil of area A and N turns is rotated in a uniform magnetic field B with angular velocity ω. The maximum induced EMF is:

A ε = NABω

B ε = NBω

C ε = NAB/ω

D ε = AB/Nω

Correct Answer: A. ε = NABω

EXPLANATION

Maximum EMF in rotating coil: ε_max = NABω, where Φ = NBA cos(ωt), so ε = dΦ/dt = NABω sin(ωt)

Test

Q.27 Medium Magnetism

A rectangular conducting loop is partially inside a uniform magnetic field region. If the loop is pulled out with constant velocity v, the induced EMF depends on:

A The length of the side cutting field lines, velocity, and magnetic field strength

B Only the magnetic field strength

C Only the velocity

D The area of the loop inside the field

Correct Answer: A. The length of the side cutting field lines, velocity, and magnetic field strength

EXPLANATION

EMF induced ε = BLv where L is the length of the conductor cutting magnetic field lines and v is the velocity. It depends on all three factors: B, L, and v.

Test

Q.28 Medium Magnetism

In a cyclotron, a charged particle spirals outward as it gains energy. The frequency of revolution is independent of:

A Magnetic field strength

B Particle velocity

C Particle mass

D Particle charge

Correct Answer: B. Particle velocity

EXPLANATION

The cyclotron frequency f = qB/(2πm) is independent of velocity. As velocity increases, radius increases but period remains constant (independent of v).

Test

Q.29 Medium Magnetism

A conducting rod of length L = 0.5 m moves with velocity v = 10 m/s perpendicular to a uniform magnetic field B = 2 T. The motional EMF induced is:

A 5 V

B 10 V

C 15 V

D 20 V

Correct Answer: B. 10 V

EXPLANATION

Motional EMF ε = BLv = 2 × 0.5 × 10 = 10 V.

Test

Q.30 Medium Magnetism

A beam of electrons is accelerated through a potential difference V and then enters a region of perpendicular electric and magnetic fields. For the electrons to move undeflected, which condition must be satisfied?

A qE = qvB

B E = vB

C eV = ½mv²

D qE = ½mv²B

Correct Answer: B. E = vB

EXPLANATION

For undeflected motion, electric force equals magnetic force: qE = qvB, which simplifies to E = vB. This is the velocity selector condition.

Test

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