Chapter Exam

Unit 13: Magnetic Effects of Current and Magnetism — Online MCQ Test

Q: A long solenoid of 50 cm length having 100 turns carries a current of 2.5 A. The magnetic field at the centre of the solenoid is (μ₀ = 4π x 10⁻⁷ T m A⁻¹):

Answer: D. 6.28 x 10⁻⁴ T — B = μ₀ n I, where n = N/L = 100 / 0.5 = 200 turns/m. B = (4π x 10⁻⁷) * 200 * 2.5 = 4π x 10⁻⁷ * 500 = 2000π x 10⁻⁷ = 2π x 10⁻⁴ = 6.28 x 10⁻⁴ T.

Q: A 2 amp current is flowing through two different small circular copper coils having radii ratio 1:2. The ratio of their respective magnetic moments will be

Answer: A. 1:4 — Magnetic moment M = I * Area = I * pi * r^2. Since I is same, M is proportional to r^2. Ratio = (1)^2 : (2)^2 = 1:4.

Q: The magnetic susceptibility is negative for:

Answer: D. diamagnetic material only — Magnetic susceptibility (chi) is negative only for diamagnetic materials, as they are weakly repelled by magnetic fields.

Q: A square loop ABCD carrying a current i is placed near and coplanar with a long straight conductor XY carrying a current I, the net force on the loop will be:

Answer: D. (2 * mu_0 * i * I) / (3 * pi) — The forces on the segments parallel to the wire are calculated using F = (mu_0 * I1 * I2 * L) / (2 * pi * d). The closer side experiences an attractive force and the farther side experiences a repulsive force. The net force is the difference between these two forces.

Q: A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B' at radial distance a/2 and 2a respectively, from the axis of the wire is:

Answer: B. 1 — Inside the wire, B = (μ₀Ir)/(2πa²). At r = a/2, B = μ₀I/(4πa). Outside the wire, B' = (μ₀I)/(2πr). At r = 2a, B' = μ₀I/(4πa). Thus, the ratio B/B' = 1:1.

Q: In which of the following devices, the eddy current effect is not used?

Answer: A. Electric heater — Electric heaters operate on the principle of Joule heating (I²Rt), not eddy currents.

Q: The net magnetic flux through any closed surface is:

Answer: A. Zero — According to Gauss's Law for magnetism, the magnetic monopoles do not exist, so net magnetic flux through any closed surface is always zero.

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Practice Unit 13: Magnetic Effects of Current and Magnetism with a free chapter-wise online MCQ test for NEET NEET (All) PHYSICS. This chapter covers: Biot - Savart law and its application to current carrying circular loop. Ampere's law and its applications to infinitely long current carrying straight wire and solenoid. Force on.... AI-generated questions from basic to board-exam level, with instant results and explanations.

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Unit 13: Magnetic Effects of Current and Magnetism — Important Questions & Answers (FAQ)

Frequently asked questions from NEET NEET (All) PHYSICS — Unit 13: Magnetic Effects of Current and Magnetism, with answers and explanations. These are sample questions; the exam has its own separate question set.

A long solenoid of 50 cm length having 100 turns carries a current of 2.5 A. The magnetic field at the centre of the solenoid is (μ₀ = 4π x 10⁻⁷ T m A⁻¹):

A. 3.14 x 10⁻⁴ T
B. 6.28 x 10⁻⁵ T
C. 3.14 x 10⁻⁵ T
D. 6.28 x 10⁻⁴ T ✓

Answer: D. 6.28 x 10⁻⁴ T
B = μ₀ n I, where n = N/L = 100 / 0.5 = 200 turns/m. B = (4π x 10⁻⁷) * 200 * 2.5 = 4π x 10⁻⁷ * 500 = 2000π x 10⁻⁷ = 2π x 10⁻⁴ = 6.28 x 10⁻⁴ T.

An electron (mass 9 × 10^-31 kg and charge 1.6 × 10^-19 C) moving with speed c/100 (c = speed of light) is injected into a magnetic field B of magnitude 9 × 10^-4 T perpendicular to its direction of motion. We wish to apply an uniform electric E together with the magnetic field so that the electron does not deflect from its path. Then (speed of light c = 3 × 10^8 ms^-1)

A. E is perpendicular to B and its magnitude is 27 × 10^4 V m^-1
B. E is perpendicular to B and its magnitude is 27 × 10^2 V m^-1 ✓
C. E is parallel to B and its magnitude is 27 × 10^2 V m^-1
D. E is parallel to B and its magnitude is 27 × 10^4 V m^-1

Answer: B. E is perpendicular to B and its magnitude is 27 × 10^2 V m^-1
For zero deflection, F_net = qE - qvB = 0, so E = vB. v = c/100 = 3*10^6 m/s. E = 3*10^6 * 9*10^-4 = 27*10^2 V/m. E must be perpendicular to B to balance the magnetic force.

A 2 amp current is flowing through two different small circular copper coils having radii ratio 1:2. The ratio of their respective magnetic moments will be

A. 1:4 ✓
B. 1:2
C. 2:1
D. 4:1

Answer: A. 1:4
Magnetic moment M = I * Area = I * pi * r^2. Since I is same, M is proportional to r^2. Ratio = (1)^2 : (2)^2 = 1:4.

A parallel plate capacitor is being charged such that the surface charge density is increasing at a constant rate. The magnetic field arising due to displacement current is:

A. zero at all places
B. constant between the plates and zero outside
C. non-zero everywhere with maximum at the imaginary cylindrical surface connecting peripheries of the plates. ✓
D. zero between the plates and non-zero outside.

Answer: C. non-zero everywhere with maximum at the imaginary cylindrical surface connecting peripheries of the plates.
Displacement current exists between the plates. By Ampere-Maxwell law, magnetic field is generated. It varies linearly with radial distance r inside the capacitor and as 1/r outside, making it non-zero everywhere with a maximum at the rim.

A parallel plate capacitor made of circular plates is being charged such that the surface charge density on its plates is increasing at a constant rate with time. The magnetic field arising due to displacement current is:

A. zero at all places
B. constant between the plates and zero outside the plates.
C. non-zero everywhere with maximum at the imaginary cylindrical surface connecting peripheries of the plates. ✓
D. zero between the plates and non-zero outside.

Answer: C. non-zero everywhere with maximum at the imaginary cylindrical surface connecting peripheries of the plates.
For a charging parallel plate capacitor, the displacement current exists between the plates. By Ampere-Maxwell law, the magnetic field is symmetric and non-zero inside (between the plates) and zero outside due to circular symmetry.

The magnetic susceptibility is negative for:

A. paramagnetic material only
B. ferromagnetic material only
C. paramagnetic and ferromagnetic materials
D. diamagnetic material only ✓

Answer: D. diamagnetic material only
Magnetic susceptibility (chi) is negative only for diamagnetic materials, as they are weakly repelled by magnetic fields.

A square loop ABCD carrying a current i is placed near and coplanar with a long straight conductor XY carrying a current I, the net force on the loop will be:

A. (mu_0 * i * I) / (2 * pi)
B. (2 * mu_0 * i * I * L) / (3 * pi)
C. (mu_0 * i * I * L) / (2 * pi)
D. (2 * mu_0 * i * I) / (3 * pi) ✓

Answer: D. (2 * mu_0 * i * I) / (3 * pi)
The forces on the segments parallel to the wire are calculated using F = (mu_0 * I1 * I2 * L) / (2 * pi * d). The closer side experiences an attractive force and the farther side experiences a repulsive force. The net force is the difference between these two forces.

A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B' at radial distance a/2 and 2a respectively, from the axis of the wire is:

A. 1/2
B. 1 ✓
C. 4
D. 1/4

Answer: B. 1
Inside the wire, B = (μ₀Ir)/(2πa²). At r = a/2, B = μ₀I/(4πa). Outside the wire, B' = (μ₀I)/(2πr). At r = 2a, B' = μ₀I/(4πa). Thus, the ratio B/B' = 1:1.

In which of the following devices, the eddy current effect is not used?

A. Electric heater ✓
B. Induction furnace
C. Magnetic braking in train
D. Electromagnet

Answer: A. Electric heater
Electric heaters operate on the principle of Joule heating (I²Rt), not eddy currents.

The net magnetic flux through any closed surface is:

A. Zero ✓
B. Positive
C. Infinity
D. Negative

Answer: A. Zero
According to Gauss's Law for magnetism, the magnetic monopoles do not exist, so net magnetic flux through any closed surface is always zero.

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