An electron having charge –q and mass m is accelerated from rest in a vacuum through a potential difference V. The electron then enters a region of uniform magnetic field of magnetic flux density B, as shown in Fig. 7.1.
[Figure 7.1]
The direction of the uniform magnetic field is into the plane of the paper. The velocity of the electron as it enters the magnetic field is normal to the magnetic field. The radius of the circular path of the electron in the magnetic field is r.

Q: What topic does this question test?

This question tests Magnetic fields in A-Level Physics (syllabus code 9702). It is worth 10 marks.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Physics May/June 2017 examination, Paper 4 Variant 1 (9702_s17_qp_41.pdf).

Question

An electron having charge –q and mass m is accelerated from rest in a vacuum through a potential difference V. The electron then enters a region of uniform magnetic field of magnetic flux density B, as shown in Fig. 7.1.
[Figure 7.1]
The direction of the uniform magnetic field is into the plane of the paper. The velocity of the electron as it enters the magnetic field is normal to the magnetic field. The radius of the circular path of the electron in the magnetic field is r.

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