Two metal plates are held in
clamps by insulating handles, a very high voltage supply (up to 5000 V, or a Van de Graaff
generator) is connected to one and the other is connected to earth through a very sensitive
meter.
An electric current is simply just a flow of electric charges and we can
see this very clearly using the apparatus in the following
diagram.
The power supply is switched on and the table tennis ball pushed so that it touches one of the plates. When the ball is released it starts swinging backwards and forwards touching first one plate and then the other.
As it
does so it transfers charge from one plate to the other and the meter shows a small
deflection. This means that a current is flowing and this current is obviously the flow of the
electric charge between the plates. When the ball is touching the negative plate electrons
flow onto the ball from the plate – the ball is now negatively charged and so is repelled by the
negative plate. It swings over and touches the positive plate. Electrons now flow off the ball
onto the plate, the ball now becomes slightly positively charged, is repelled by the positive
plate, and so swings back to touch the negative plate and the process repeat itself over and
over again.
If the plates are brought closer to each other the strength of the electric
field between them is increased and the ball swings faster. The meter also shows an
increase in current - the more charge that is transferred per second the bigger the
current.
The experiment is very good evidence that:
