Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

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.

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. (See Magnetic model of alpha particle scattering). 

A video demonstration of the Van de Graaff generator is available at the National STEM Centre eLibrary.

 

Procedure


a The table tennis ball is coated with Aquadag to make it conducting. 

b 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. 
 
c 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. 
 
d With the ball touching the sphere, start the machine so that the two become charged. 
 
e 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. 
 
f Move the suspension (or the Van de Graaff generator) so that the ball passes by the dome and is deflected only a small amount. 

Aparatus set-up
 


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

 

Related guidance


The great scattering experiments

Evidence for the hollow atom

Developing a model: the nuclear atom

Alpha particles as tools

 

Related experiments


Alpha particle scattering

Display of cloud chamber photographs

Magnetic model of alpha particle scattering

Gavitational model of alpha particle scattering