Lorentz force simulation

A magnetic field pushes a moving charge sideways, at right angles to both its velocity and the field. The push never speeds the particle up or slows it down; it only bends its path, which is why charges spiral around field lines.

Interactive, free and runs in your browser. Part of the Electromagnetism simulations.

What you will see

Charged particles move through a uniform magnetic field. Motion across the field bends into circles; any motion along the field carries on unchanged, so the combination is a helix. The force arrow always points to the centre of the circle.

Try this

  1. Double the field strength and check the radius halves.
  2. Look for helices: they come from the part of the velocity along the field.
  3. Watch the speed: it never changes, only the direction.

Common misconception

A magnetic field does no work on a moving charge. Because the force is always perpendicular to the velocity, kinetic energy stays constant; only an electric field can change the speed.

Exam-style question

A proton (m = 1.67 × 10⁻²⁷ kg, q = 1.6 × 10⁻¹⁹ C) moves at 2.0 × 10⁶ m/s perpendicular to a 0.50 T field. What is the radius of its path?

r = mv/qB = (1.67 × 10⁻²⁷ × 2.0 × 10⁶) / (1.6 × 10⁻¹⁹ × 0.50) ≈ 0.042 m, about 4.2 cm.

Key equations

  • F = qv × B
  • r = mv / qB
  • ω = qB / m

Curriculum

AQA A-level physics 3.7.5, IB Physics D.3.

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