A charged particle moves through a magnetic field B with a velocity v. Which one of the following statements is true for the force (F) experienced by the particle?
- A.F is maximum when v and B are parallel to each other.
- B.F is maximum when v and B are anti-parallel to each other.
- C.F is maximum when v and B are perpendicular to each other.
- D.The force F is independent of the angle between v and B.
Correct answer
Explanation
The correct answer is C, F is maximum when v and B are perpendicular to each other. The magnetic force depends on the sine of the angle between velocity and field, and sin 90° is the largest value.
The force on a charge q moving with velocity v in a magnetic field B is the Lorentz magnetic force, F = qvB sin θ, where θ is the angle between v and B. At θ = 90° the force is qvB, its maximum, and it acts at right angles to both v and B, which is why such a particle moves in a circle. The direction is given by Fleming's left-hand rule. Because the force is always perpendicular to the motion, it changes the particle's direction but not its speed.
A and B are wrong because for parallel (θ = 0°) or anti-parallel (θ = 180°) motion sin θ is zero, so the force is zero and the particle moves straight on. D is wrong because the force clearly changes with the angle through sin θ.
Exam tip: F = qvB sin θ; maximum at 90°, zero along the field; magnetic force does no work.
Practice Questions
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2. Containers made of it can be used to store any alcoholic beverage.
3. Bottles made of it can be recycled into other products.
4. Articles made of it can be easily disposed of by incineration without causing greenhouse gas emissions.
Which of the statements given above are correct?
- A.1 and 3
- B.2 and 4
- C.1 and 4
- D.2 and 3
Show answer
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Show answer
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