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IB · D · multiple choice

Fields · Question 3

Fields · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
routine
Marks
1
Topics
1
Answer
Complete
A proton on a circular path in a uniform magnetic field into the pageuniform magnetic field of magnitude B, into the pagerprotonv
Figure 1A rectangular region bounded by a dashed line is filled with an evenly spaced grid of crosses and labelled 'uniform magnetic field of magnitude B, into the page'. Inside the region a complete circle is drawn: the path followed by the proton. A line from the centre of the circle out to the circle is marked with a tick at each end and labelled r. A solid dot on the circle at its lowest point is labelled 'proton', and from that dot an arrow labelled v points horizontally to the right, along the tangent to the circle.
multiple choice1 mark

A proton moves in a circular path of radius r in a uniform magnetic field of magnitude B, perpendicular to its velocity. If the speed of the proton is doubled, what is the new radius?

  1. A

    r/2

  2. B

    r

  3. C

    2r

  4. D

    4r

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Answer overviewCorrect answer: C — 2r
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: 2r

  2. 2

    B is the answer of a candidate who remembers that the period of circular motion in a magnetic field is independent of speed, which is true but is not what was asked. From r = mv/(qB), the radius is proportional to the speed.

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