JEE Main Physics: Force on a moving charge in a uniform magnetic field Practice Questions
12 free multiple-choice questions on Force on a moving charge in a uniform magnetic field (Electricity and Magnetism) for JEE Main Physics, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.
Practice thousands more JEE Main questions in the app →Q1. A positively charged particle moves with a velocity v⃗ of 5 m/s perpendicular to a uniform magnetic field B⃗ of 0.4 T. What is the magnitude of the magnetic force acting on the particle?
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Correct answer: C — 1 N
The magnetic force is given by F = qvB sin θ. Since the velocity is perpendicular to the magnetic field (θ= 90°, sin 90° = 1), the force is F = qvB. Assuming the charge q=1 C, F = (1) × (5) × (0.4) = 2.0 N. (If we assume q=1 for simplicity, or treat it as a relative question, the setup yields 2 N for standard units).
Q2. The direction of the force exerted by a magnetic field on a moving charge is determined by:
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Correct answer: A — the direction of motion and the direction of the magnetic field
The direction of the force is determined by the cross product v⃗ × B⃗. This direction is inherently determined by the relative orientation of the velocity vector and the magnetic field vector (using the Right-Hand Rule).
Q3. If a charged particle moves such that its velocity vector is parallel to the magnetic field, the force experienced by the particle is:
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Correct answer: D — Zero
Since the force is proportional to sin(θ), when θ=0 (parallel motion), sin(0)=0, resulting in zero force.
Q4. A proton moves in a uniform magnetic field. If the proton's velocity is exactly parallel to the magnetic field lines, the magnitude of the magnetic force acting on the proton will be:
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Correct answer: B — zero
As established, the force is F = qvB sin θ. If the velocity is parallel to the magnetic field, θ= 0°, and sin 0° = 0. Thus, the magnetic force is zero.
Q5. If a charged particle moves parallel to a uniform magnetic field, the magnetic force acting on it will be:
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Correct answer: A — zero
The magnitude of the magnetic force is F = qvB sin θ. If the velocity v⃗ is parallel to the magnetic field B⃗, the angle θ between them is 0°, and sin 0° = 0. Therefore, the force experienced by the particle is zero.
Q6. The magnetic force experienced by a charged particle moving in a magnetic field is always directed:
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Correct answer: B — perpendicular to the velocity
The magnetic force is given by F⃗ = q(v⃗ × B⃗). Since the cross product v⃗ × B⃗ results in a vector perpendicular to both v⃗ and B⃗, the magnetic force is always perpendicular to the velocity of the charge.
Q7. A particle moves with velocity v in a magnetic field B. If the magnitude of the velocity is doubled while the direction remains the same, how does the magnetic force change?
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Correct answer: A — It is doubled
The magnitude of the force is F = qvB. If v is doubled, the force is doubled.
Q8. A particle of charge q moves in a magnetic field B. If the velocity vector is perpendicular to the magnetic field, the direction of the magnetic force is:
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Correct answer: A — Perpendicular to B
The direction of the force is given by F⃗ = q(v⃗ × B⃗). Since v⃗ and B⃗ are perpendicular, the resulting force F⃗ must be perpendicular to both, hence perpendicular to B⃗.
Q9. A charged particle moves in a uniform magnetic field. If the velocity vector of the particle is exactly parallel to the magnetic field vector, the magnetic force acting on the particle is:
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Correct answer: B — Zero
The magnetic force is given by F⃗ = q (v⃗ × B⃗). If v⃗ is parallel to B⃗, the angle between them is zero, so sin(θ) = 0. Thus, the force is zero.
Q10. A particle of charge q moves in a region of uniform magnetic field B⃗. If the particle moves parallel to the magnetic field lines, the magnetic force acting on it is:
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Correct answer: C — Zero
The magnetic force is F⃗ = q(v⃗ × B⃗). If v⃗ is parallel to B⃗, the angle θ= 0°, so sin θ= 0. Therefore, the magnetic force experienced by the particle is zero.
Q11. A moving charge experiences a magnetic force F⃗. If F⃗ is directed entirely opposite to the direction of the velocity v⃗, the particle must be moving in a direction that is:
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Correct answer: C — Perpendicular to B⃗
The magnetic force F⃗ = q(v⃗ × B⃗) is always perpendicular to both v⃗ and B⃗. If F⃗ is opposite to v⃗, it implies that the component of v⃗ perpendicular to B⃗ is balanced by the force component, which is only possible if the velocity component perpendicular to B⃗ is involved in the rotation. The force direction implies that the motion is circular or helical, meaning v⃗ must have a component perpendicular to B⃗.
Q12. A proton moves in a magnetic field B⃗. If the proton moves such that its velocity is entirely perpendicular to the magnetic field lines, the resulting motion will be:
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Correct answer: D — Circular motion
When the velocity of a charged particle is perpendicular to the magnetic field, the magnetic force provides the necessary centripetal force, causing the particle to move in a circular path.
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