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A Level · A2 · multiple choice

A Level extension · Question 5

A Level extension · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
recall
Marks
1
Topics
1
Answer
Complete
An electron entering a uniform magnetic field at right anglesuniform magnetic field into the page−electronv
Fig. 5.1An electron, drawn as a small circle marked with a negative sign, travels horizontally to the right, and an arrow labelled v gives its velocity. Ahead of it lies a large rectangular region with a dashed boundary, filled with a regular array of crosses and labelled as a uniform magnetic field directed into the page. The electron is shown outside that region, at the instant before it crosses the boundary, with its velocity lying in the plane of the page.
multiple choice1 mark

An electron enters a uniform magnetic field with its velocity perpendicular to the field. What is its subsequent path within the field?

  1. A

    a straight line at constant speed

  2. B

    a straight line with increasing speed

  3. C

    a circular arc at constant speed

  4. D

    a parabolic arc with increasing speed

Ready to self-mark?Reveal the detailed answer guide
Answer overviewCorrect answer: C — a circular arc at constant speed
01

Answer and reasoning

1 mark
How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

  1. 1

    The correct choice is C: a circular arc at constant speed

  2. 2

    D is the path in a uniform electric field, and is the answer of a candidate who has not separated the two cases. The magnetic force is always perpendicular to the velocity, so it does no work: the speed cannot change, only the direction.

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