Demos: 6B-02 Force on a Moving Charge

A discharge tube is built with a fluorescent screen along the length of the tube (inside) and placed in such a way that the ribbon beam of electrons from the gun grazes the screen. The fluorescing screen shows a green glow, indicating the path of the electrons. A horseshoe magnet is brought near the tube and moved around in various ways, causing the beam to deflect up and down and sideways, depending on the orientation of the magnet. The movement of the beam is consistent with



Directions: (A slightly darkened room will increase the visibility of the demonstration.) Attach the beam tube leads to the terminals of the induction coil. Turn on the induction coil. Pick up the horseshoe magnet and bring it close to the tube. Experiment with placing the magnet in different configurations to get the beam to move up and down.

Suggestions for Presentation: After showing that an electron beam interacts with a magnetic field, discuss the force equation above and focus on the cross product. Have the students predict the required orientation of the magnetic that would cause the beam to move, say, vertically. Which orientation causes the beam to move upward? Downward? If the predictions are incorrect, it is likely because the right-hand-rule is based on a positive charge and the electron is a negative charge.

Applications: The cathode ray tube in television and computer monitors uses this force extensively to guide the electron beam.

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