Electrostatic model of alpha particle scattering

Demonstration

This is another model for the scattering of alpha particles; this time using the electrostatic repulsion between the dome of a Van de Graaff generator and a table tennis ball.

Apparatus and Materials

• Van de Graaff generator

• Table-tennis (ping-pong) ball, coated with Aquadag
• Fine nylon thread
• Sellotape

Health & Safety and Technical Notes

If the table-tennis ball suspension is attached to the ceiling, ensure that an adult is available to hold the step-ladder while another adult works at a height.

Read our standard health & safety guidance

This electrostatic model gives a fairer illustration of the path of an alpha particle under nuclear repulsion. The magnetic model is the one that Rutherford used in lectures.

Magnetic model of alpha particle scattering

Procedure

1. The table tennis ball is coated with Aquadag to make it conducting.
2. Attach a long nylon thread to the ball with Sellotape. Suspend the thread so that the suspension is as long as possible - preferably from the ceiling.
3. Set up the Van de Graaff generator below the suspension so that the ball can swing freely near the sphere and at the same height.
4. With the ball touching the sphere, start the machine so that the two become charged.
5. Pull the charged ball to one side with an insulating rod. Let it fall back towards the sphere of the generator and watch its motion.
6. Move the suspension (or the Van de Graaff generator) so that the ball passes by the dome and is deflected only a small amount.

Teaching Notes

• The suspended ball represents the alpha particle which is repelled by the Van de Graaff sphere when both have charges of the same sign.

This experiment was safety-tested in April 2006

• A video showing how to use a Van de Graaff generator: