Why does a static magnetic field do no work on a moving charged particle?

The magnetic force is always perpendicular to the velocity

The magnetic force on a moving charge is zero

The magnetic force acts only on stationary charges

The magnetic force is always parallel to the velocity

Given below are two statements: Statement I: A plane current loop in a uniform magnetic field experiences maximum torque when the plane of the loop is parallel to the field. Statement II: The net magnetic force on a closed current loop in a uniform magnetic field is zero. Choose the correct answer.

Both Statement I and Statement II are correct

Statement I is correct but Statement II is incorrect

Both Statement I and Statement II are incorrect

Statement I is incorrect but Statement II is correct

Moving Charges & Magnetism

11 NEET-UG practice questions on Moving Charges & Magnetism, part of Physics. The first 11 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
Why does a static magnetic field do no work on a moving charged particle?
1. AThe magnetic force on a moving charge is zero
2. BThe magnetic force acts only on stationary charges
3. CThe magnetic force is always parallel to the velocity
4. DThe magnetic force is always perpendicular to the velocity
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Answer: D · The magnetic force is always perpendicular to the velocity
The force F = q(v × B) is perpendicular to v, so F·v = 0 and no work is done. Speed and kinetic energy stay constant, though the direction may change.
- A: The force is qvB sinθ, which is not zero in general.
- B: A stationary charge feels no magnetic force at all.
- C: A force along the velocity would do work and change the speed.
Remember this
A magnetic field can change a charge's direction of motion but never its speed or kinetic energy.
Question 2 · Medium
A deuteron and an α-particle, with equal kinetic energy, enter a uniform magnetic field perpendicular to it. What is the ratio of the radius of the deuteron's path to that of the α-particle's path?
1. A1 : 1
2. B2 : 1
3. C1 : √2
4. D√2 : 1
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Answer: D · √2 : 1
Radius r = mv/(qB) = √(2mK)/(qB), so at equal K, r is proportional to √m/q. Deuteron: √2/1 = √2. α-particle: √4/2 = 1. The ratio is √2 : 1.
- A: This holds if both are accelerated through the same potential, not at equal energy.
- B: Only the charge was compared, ignoring the mass.
- C: The ratio has been inverted.
Remember this
At equal kinetic energy r varies as √m/q; at equal accelerating potential it varies as √(m/q).
Question 3 · Medium
Two long parallel wires in air are 10 cm apart and carry currents of 5 A and 8 A in opposite directions. Take μ₀ = 4π × 10⁻⁷ T m/A. Find the force per unit length between them, and its nature.
1. A4 × 10⁻⁵ N/m, repulsive
2. B8 × 10⁻⁷ N/m, repulsive
3. C8 × 10⁻⁵ N/m, repulsive
4. D8 × 10⁻⁵ N/m, attractive
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Answer: C · 8 × 10⁻⁵ N/m, repulsive
Force per unit length = μ₀I₁I₂/(2πd) = (2 × 10⁻⁷)(5)(8)/0.10 = 8 × 10⁻⁵ N/m. Opposite currents repel.
- A: μ₀/4π was used in place of μ₀/2π.
- B: The separation was used as 10 m instead of 0.10 m.
- D: The rule is reversed: only currents in the same direction attract.
Remember this
Parallel currents attract and antiparallel currents repel, with force per unit length μ₀I₁I₂/(2πd).
Question 4 · Medium
Given below are two statements: Statement I: A plane current loop in a uniform magnetic field experiences maximum torque when the plane of the loop is parallel to the field. Statement II: The net magnetic force on a closed current loop in a uniform magnetic field is zero. Choose the correct answer.
1. AStatement I is correct but Statement II is incorrect
2. BBoth Statement I and Statement II are correct
3. CBoth Statement I and Statement II are incorrect
4. DStatement I is incorrect but Statement II is correct
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Answer: B · Both Statement I and Statement II are correct
Torque is mB sinθ, where θ is between the area vector and B. It is largest at θ = 90°, when the loop's plane lies along the field, so I is correct. In a uniform field the forces on opposite parts cancel, so the net force is zero and II is correct.
- A: The net force on a closed loop in a uniform field does vanish.
- C: Both results follow directly from the torque and force on a loop.
- D: The torque is maximum when the normal, not the plane, is perpendicular to the field.
Remember this
A uniform field gives a closed loop zero net force but a torque NIAB sinθ, measured from the normal.
Question 5 · Hard
Singly ionised atoms pass undeviated through crossed fields E = 2 × 10⁴ V/m and B = 0.2 T. They then enter a region with only the same B, move through a semicircle, and strike a plate 10 cm from the entry point. Taking e = 1.6 × 10⁻¹⁹ C, what is the mass of each ion?
1. A1.6 × 10⁻¹⁶ kg
2. B4.0 × 10⁻²⁵ kg
3. C3.2 × 10⁻²⁶ kg
4. D1.6 × 10⁻²⁶ kg
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Answer: D · 1.6 × 10⁻²⁶ kg
No deviation means qE = qvB, so v = E/B = 1 × 10⁵ m/s. The 10 cm is a diameter, so r = 0.05 m. From r = mv/(qB), m = qBr/v = (1.6 × 10⁻¹⁹)(0.2)(0.05)/10⁵ = 1.6 × 10⁻²⁶ kg.
- A: v was inverted to B/E.
- B: v was computed as the product E × B.
- C: The 10 cm was used as the radius instead of the diameter.
Remember this
A velocity selector passes v = E/B; a semicircular path then gives r = half the distance from entry to impact.
Question 6 · Easy
A long solenoid is 50 cm long and has 500 turns. It carries 2 A. Taking μ₀ = 4π × 10⁻⁷ T m/A, what is the magnetic field well inside it?
1. A16π × 10⁻⁴ T
2. B8π × 10⁻⁴ T
3. C4π × 10⁻⁴ T
4. D8π × 10⁻⁷ T
Answer and explanation in LearnFree account
Question 7 · Medium
A galvanometer has a coil of resistance 50 Ω and gives full-scale deflection at 5 mA. How can it be made into a voltmeter of range 0-10 V?
1. AConnect 2000 Ω in series
2. BConnect 1950 Ω in series
3. CConnect 2050 Ω in series
4. DConnect 1950 Ω in parallel
Answer and explanation in LearnFree account
Question 8 · Medium
A galvanometer with the resistance and full-scale current marked is made into a two-range voltmeter, wired as drawn. What must R₂ be?
Figure: MedExams
1. A1400 Ω
2. B1500 Ω
3. C1200 Ω
4. D200 Ω
Answer and explanation in LearnFree account
Question 9 · Easy
At the instant shown, a positive charge moves in the plane of the page through a uniform magnetic field. In which direction does the magnetic force on it act?
Figure: MedExams
1. ATowards the top of the page
2. BInto the page, along B
3. CTowards the bottom of the page
4. DAlong v, to the right of the page
Answer and explanation in LearnFree account
Question 10 · Hard
A rectangular loop lies in the same plane as a long straight wire, with the currents and sizes marked. Find the net magnetic force on the loop. Take μ₀/2π = 2 × 10⁻⁷ T m/A.
Figure: MedExams
1. A1.2 × 10⁻⁴ N, towards the wire
2. B1 × 10⁻⁴ N, towards the wire
3. C8 × 10⁻⁵ N, towards the wire
4. D8 × 10⁻⁵ N, away from the wire
Answer and explanation in LearnFree account
Question 11 · Medium
Two long straight wires P and Q are perpendicular to the page, with the currents marked. What is the magnitude of the magnetic field at M? Take μ₀/2π = 2 × 10⁻⁷ T m/A.
Figure: MedExams
1. A8 × 10⁻⁵ T
2. B4 × 10⁻⁵ T
3. C3 × 10⁻⁵ T
4. D2 × 10⁻⁵ T
Answer and explanation in LearnFree account

Moving Charges & Magnetism

Why does a static magnetic field do no work on a moving charged particle?

A deuteron and an α-particle, with equal kinetic energy, enter a uniform magnetic field perpendicular to it. What is the ratio of the radius of the deuteron's path to that of the α-particle's path?

Two long parallel wires in air are 10 cm apart and carry currents of 5 A and 8 A in opposite directions. Take μ₀ = 4π × 10⁻⁷ T m/A. Find the force per unit length between them, and its nature.

Given below are two statements: Statement I: A plane current loop in a uniform magnetic field experiences maximum torque when the plane of the loop is parallel to the field. Statement II: The net magnetic force on a closed current loop in a uniform magnetic field is zero. Choose the correct answer.

Singly ionised atoms pass undeviated through crossed fields E = 2 × 10⁴ V/m and B = 0.2 T. They then enter a region with only the same B, move through a semicircle, and strike a plate 10 cm from the entry point. Taking e = 1.6 × 10⁻¹⁹ C, what is the mass of each ion?

A long solenoid is 50 cm long and has 500 turns. It carries 2 A. Taking μ₀ = 4π × 10⁻⁷ T m/A, what is the magnetic field well inside it?

A galvanometer has a coil of resistance 50 Ω and gives full-scale deflection at 5 mA. How can it be made into a voltmeter of range 0-10 V?

A galvanometer with the resistance and full-scale current marked is made into a two-range voltmeter, wired as drawn. What must R₂ be?

At the instant shown, a positive charge moves in the plane of the page through a uniform magnetic field. In which direction does the magnetic force on it act?

A rectangular loop lies in the same plane as a long straight wire, with the currents and sizes marked. Find the net magnetic force on the loop. Take μ₀/2π = 2 × 10⁻⁷ T m/A.

Two long straight wires P and Q are perpendicular to the page, with the currents marked. What is the magnitude of the magnetic field at M? Take μ₀/2π = 2 × 10⁻⁷ T m/A.