Charged Particles Moving in a Magnetic Field · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Steering charges with magnets

Science · Forces & Motion · ages 17-18
Name ______________________   Date ____________

  1. A magnetic field increases the speed of a charge moving across it.

    Circle one:   True   False

  2. A positive and a negative particle enter the same uniform magnetic field with velocities in the same direction, at right angles to the field. Which statement describes their initial magnetic forces?

    • Their forces point in the same direction
    • Their forces point in opposite directions
    • Neither particle experiences a magnetic force

  3. A particle with charge magnitude Q moves at speed v at right angles to a magnetic field of strength B. What is the magnetic force?

    • B Q v, sideways
    • Q times v over B, forward
    • B over Q times v, backward

  4. Two particles move at right angles to the same uniform field with the same mass and speed. One has a larger charge magnitude. Which follows the smaller circle?

    • The particle with the smaller charge magnitude
    • The particle with the larger charge magnitude
    • Both particles follow circles of the same radius

  5. A particle moves at right angles to a uniform field. In matching units, m = 2, v = 3, B = 1 and charge magnitude Q = 2. What is its circle radius?

    Answer: ______________

  6. Why does kinetic energy stay constant in the field?

    • The charge has no mass
    • The field points forward
    • The field does no work on the charge

  7. Kai changes a particle's charge from +Q to -Q, keeping its mass, speed and the uniform field fixed. Motion is perpendicular to the field. He says the radius changes sign too. What happens?

    • Radius reverses sign and the bending stays the same
    • Radius stays the same and the bending reverses
    • Radius becomes zero and the particle goes straight

  8. In one field, a particle moves in a circle of radius 3 cm. A second particle has double the mass but the same speed and charge, in the same field. What is its radius in cm?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Steering charges with magnets W1-mt_evuhZ6cSfI-s1

  1. False · The sideways push does no work, so speed and kinetic energy stay fixed.
  2. Their forces point in opposite directions · For the same velocity direction and magnetic field, reversing the charge sign reverses the magnetic force direction.
  3. B Q v, sideways · The force size is B Q v. Its direction is perpendicular to both velocity and field.
  4. The particle with the larger charge magnitude · r = (m v)/(B Q). With mass, speed and field fixed, a larger charge magnitude Q gives a smaller radius.
  5. 3 · r = (m v)/(B Q) = (2 times 3)/(1 times 2) = 6/2 = 3.
  6. The field does no work on the charge · A purely sideways force cannot add energy, so the energy stays unchanged.
  7. Radius stays the same and the bending reverses · Changing +Q to -Q leaves charge magnitude Q unchanged, so r = (m v)/(B Q) stays the same. The reversed charge sign reverses the magnetic force and bending direction.
  8. 6 · Radius is m v over B Q, so it grows in step with mass. Doubling the mass doubles 3 cm to 6 cm.
Worksheet · LightMySky