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CAIE A-Level Physics 20.3.6 Velocity Selection

Practise explaining velocity selection with perpendicular electric and magnetic fields, deriving the force-balance condition and identifying required directions.

Syllabus
2028–2030
Course
Physics 9702
Level
A2

Exam points

  • explain how perpendicular electric and magnetic fields can select particles of a particular velocity
  • derive or use the balance condition between electric and magnetic forces
  • identify the direction and conditions required for velocity selection

20.3.6—How electric and magnetic fields can be used in velocity selection question 1

[Maximum number: 5]

A particle of charge +q and mass m is travelling with a constant speed of 1.6×105 ms−11.6 \times 10^{5} \mathrm{~ms}^{-1} in a vacuum. The particle enters a uniform magnetic field of flux density 9.7×10−2 T9.7 \times 10^{-2} \mathrm{~T}, as shown in Fig. 9.1.

Fig. 9.1

Fig. 9.1

The magnetic field direction is perpendicular to the initial velocity of the particle and perpendicular to, and out of, the plane of the paper.

A uniform electric field is applied in the same region as the magnetic field so that the particle passes undeviated through the fields.

Question (a)

(a)

State and explain the direction of the electric field.

[ 2 ]

Question (b)

(b)

Calculate the magnitude of the electric field strength.

Explain your working.
electric field strength = Vm−1\mathrm{Vm}^{-1}

[ 3 ]
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