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CAIE A-Level Physics 20.3.5 Charged Particle Motion in a Magnetic Field

Practise analysing circular or helical motion of charges in a uniform magnetic field, relating force direction to velocity and radius or period.

Syllabus
2028–2030
Course
Physics 9702
Level
A2

Exam points

  • describe the circular or helical motion of a charged particle moving perpendicular to a uniform magnetic field
  • relate magnetic force direction to the changing velocity and circular path
  • interpret how speed, charge, mass and field affect the radius or period of motion

20.3.5—The motion of a charged particle moving in a uniform magnetic field question 1

[Maximum number: 3]

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.

The electric field is now removed so that the positively-charged particle follows a curved path in the magnetic field. This path is an arc of a circle of radius 4.0 cm .

Calculate, for the particle, the ratio qm\frac{q}{m}.

ratio = ................................................ C kg−1\text{ratio = ................................................ } \mathrm{C\,kg}^{-1}
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