4.Main Content
experiments
Moving charges are an electric current
Demonstration
Both the Van de Graaff and the EHT supply cause the ball to shuttle between the plates. They both affect an ammeter in the same way.
Apparatus and materials
- Van de Graaff generator
- Metal plates with insulating handles, 2
- Table-tennis (ping-pong) ball, coated with Aquadag
- Nylon thread, e.g. fishing line, 1 reel
- Power supply, EHT, 0-5 kV (with internal safety resistor)
- Retort stands, bosses and clamps, 3
Technical notes
As an alternative to the metal plates, a pair of copper plates bent as shown and placed upright on a dry bench can be used.
Safety
Read this comprehensive safety note.
Read our standard health & safety guidance
Procedure
a Fix the handles of the plates in clamps, so that the two plates are set up parallel to each other with their planes vertical. They should be approximately 10 cm apart.
b Connect one of the plates to the dome of the generator; earth the other, together with the base of the Van de Graaff machine. (The plates recommended have a small peg at the back to which a crocodile clip can be attached. This avoids having to connect the clip to the plate itself.)
c Hang the table-tennis ball, coated with Aquadag, from a suitable length of nylon thread between the two plates. When the generator is switched on, the ball transfers charges between the plates.
d If a sensitive galvanometer is available, insert it into this circuit between the second plate and the earth connection. Each transfer of charge from plate to plate will cause a deflection of the meter. For this to be significant, show that the galvanometer is capable of indicating the passage of a tiny momentary current derived from a dry cell. Connect a dry cell to the instrument through a student who completes the circuit momentarily by a brief touch on one terminal.
e Disconnect the Van de Graaff generator and temporarily earth all the equipment. Then connect the positive terminal of the EHT power pack (with the 50 megohm safety resistor in circuit) to the first plate and repeat the experiment, again showing charge transferred.
f If one is available, connect the sensitive galvanometer between the second plate and the earth connection. Observe the current.
Teaching notes
1 Try out this demonstration in advance to determine the optimum separation of the plates and the length of the thread. You may have to give the ball a gentle nudge to start it moving.
2 The ball gains charge of one sign when it touches one plate. It is attracted to the other plate, where it gives up its charge and gains charge of the opposite sign. It is then attracted in the opposite direction, and so on. We can picture positive charge being carried in one direction, and negative charge in the opposite direction. Both contribute to the current (which flows in one direction).
This experiment was safety-checked in February 2005