Magnetic fields and forces

Sample questions

Original practice questions written in the style of the exam.

1. Question 13 marks

   An electron travelling at $4.0 \times 10^{6}\,\text{m s}^{-1}$ enters a uniform magnetic field of flux density $2.5\,\text{mT}$ at right angles to the field.

   Calculate the radius of the electron's circular path.

   Mark scheme

   • Magnetic force provides centripetal force: $Bqv = \dfrac{mv^2}{r}$ (1)
   • $r={\displaystyle }$ (1)
2. Question 23 marks

   A charged particle moves at right angles to a uniform magnetic field. Explain why it moves in a circle at constant speed.

   Mark scheme

   • The magnetic force is always perpendicular to the particle's velocity (1)
   • So no work is done on the particle and its kinetic energy/speed stays constant (1)
   • A force of constant magnitude perpendicular to the velocity provides a centripetal force, so the path is a circle (1)