Charged Particle in B

This simulation shows how a magnetic field bends a moving charge. Students choose a proton, electron or alpha particle, set the speed across the field, the speed along it and the field strength, and flip the field direction. The path is a circle, or a helix when there is a component along the field. Readouts give the radius, period, cyclotron frequency and magnetic force in real units. It is useful for lessons on the force on a moving charge, the radius formula and the idea behind cyclotrons and mass spectrometers.

• Switch Particle between Proton and Electron and watch the direction of turning reverse.
• Raise B strength from 150 to 400 mT and read how the Radius r = mv/qB shrinks.
• Increase Speed perpendicular to B and check that the radius grows but the Period T stays the same.
• Set Speed parallel to B above zero to turn the circle into a helix.
• Toggle B out of screen to reverse the field and predict the new turning direction before you look.

1. 1Explain why a magnetic force bends a charge without changing its speed.
2. 2Use r = mv/qB to predict how radius changes with speed, field and particle type.
3. 3Recognise that the period of circular motion does not depend on speed.
4. 4Describe why a velocity component along the field produces a helix.

Questions teachers ask