Forces in an electrostatic field

Demonstration

A shuttling ping-pong ball serves as a model of ions moving in an electric field.

Apparatus and Materials

• Power supply, EHT, 0–5 kV
• Metal plates with insulating handles, 2
• Table-tennis (ping-pong) ball, coated with Aquadag
• Nylon thread, e.g. fishing line, 1 reel
• Clamp
• Retort stands and bosses, 3

Health & Safety and Technical Notes

Read our standard health & safety guidance

Although educational EHT supplies are limited to safe output currents, the shock obtained by touching the live plate in this demonstration can make the demonstrator jump. Switch the supply off before making adjustments.

Procedure

1. Attach the two metal plates to retort stands using bosses, and set them up parallel to each other 10 cm apart.
2. Connect the plates directly to the positive and negative terminals of the EHT power supply using crocodile clips attached to the special lugs on the back of the plates.
3. Suspend the table tennis-ball, coated with Aquadag, by nylon thread from a further retort stand so that it hangs freely between the two plates.
4. Apply a potential difference of 3,000 to 4,000 volts to the plates. The ball is charged by contact with one of the plates. It will then move rapidly backwards and forwards between the two plates.
5. Vary the potential difference and the separation of the plates.

Teaching Notes

• This demonstration shows the forces on a charged object in an electric field. It also serves as a model of an ion moving in an electric field. As the ball moves to and fro between the plates, it represents a positive ion moving one way and then a negative ion moving the other.
• The electric field between parallel charged plates, E = V/d, where V is the potential difference between the plates and d is the separation of the plates.

This experiment was safety-tested in June 2007