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Electric fields and electric currents

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:

An electric current is produced when a static electric charge moves

schoolphysics moving charge and electric current animation

To see an animation of the experiment click on the animation link.

© Keith Gibbs 2020