Electric Field experiments

Shuttling ball experiment:

The shuttling ball experiment shows that an electric current is a flow of charge.

A conducting ball is suspended by an insulating thread between two vertical plates. When a high voltage is applied across the two plates, the ball bounces back and forth between the two plates. Each time it touches the negative plate, the ball gains some electrons and becomes negatively charged so that it repels away from the negative plate and is attracted to the positive plate. When it touches the positive plate electrons on the ball transfer to the positive plate and the ball becomes positively charged and is repelled from the positive plate and attracted to the negative plate. This process repeats itself continually as long as a potential difference is applied across the plates.

The ball is carrying charge across between the plates and completes the circuit causing a current to pass across the circuit. The average current carried around the circuit is the charge Q multiplied by the frequency of oscillations of the ball f: I = Qf



The gold leaf electroscope:

The gold leaf electroscope is used to detect charge. If a charged object is in contact with the metal cap of the electroscope, some of the charge on the object transfers to the electroscope. As a result, the gold leaf and the metal stem which is attached to the cap gain the same type of charge and the leaf rises because it is repelled by the stem.If another object with the same type of charge is brought near the electroscope, the leaf rises further because the object forces some charge on the cap to transfer to the leaf and stem.

The Van de Graaff generator: A Van de Graaff can easily produce sparks in the air several centimetres in length. Charge created when the rubber belt rubs against a pad is carried by the belt up to the metal dome of the generator. As charge gathers on the dome, the potential difference between the dome and the Earth increases until sparking occurs.

A spark suddenly transfers energy from the dome. Work must be done to charge the dome because a force is needed to move the charge on the belt up to the dome. So the electric potential energy of the dome increases as it charges up. Some or all of this energy is transferred from the dome when a spark is created.