Collisions with electrostatic forces
An illustration of a collision with 'invisible' forces.
Apparatus and materials
Ping-pong ball coated with Aquadag, 2
Fine nylon suspension
Health & Safety and Technical
Do not add extra capacitance to the Van de Graaff generator.
Coat the two ping-pong balls with Aquadag to make their surfaces conduct. Using the fine nylon thread, suspend them one diameter apart, preferably from the ceiling but in any case on threads as long as possible.
a Switch on the Van de Graaff generator and charge each of the balls by allowing them successively to touch its sphere.
b Study collision processes between the charged balls by drawing back one of the threads and allowing the balls to swing together. The 90° angle between paths after collision may be seen.
c You could also demonstrate collisions of the charged ping-pong ball with the Van de Graaff sphere.
Step c provides a useful introduction on the scattering of alpha particles in gold foil.