Introduction to electric forces

Demonstration

Electrostatic forces are an example, like gravitational and magnetic forces, of "action at a distance".

Apparatus and Materials

• Balloons, 4
• Nylon thread, e.g. fishing line, 1 reel

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Hang up two inflated balloons by long nylon threads. The balloons must be far from any metal supports.
2. Charge them with like charges by rubbing them against clothing. Then let them hang freely to show repulsion.
3. Take two more balloons and rub them together. This produces unlike charges on the two balloons. They show attraction.

Teaching Notes

• Two similarly charged balloons repel each other. Two differently charged balloons attract each other. This observation is sufficient for us to conclude that there are two types of electric charge. You could invite students to suggest titles for the two types. Then point out that the established titles are positive and negative. These are good names because they suggest oppositeness.
Another demonstration
• Charge a balloon by rubbing it on clothing and 'stick' it to a glass fronted cupboard. Repeat with a second balloon. If the balloons are close enough they will repel each other but remain attracted to the glass.
Alternative method of charging balloons
• The balloons can be made to conduct by various means. Paint them with graphite. Spray them with some aluminium paint. Dip them into a strong detergent solution, which is allowed to dry.
• Hang the two balloons up by insulating threads. Charge one of them. Bring the other balloon, uncharged, near to the charged balloon. Interpose a very thin sheet of plastic (such as polythene) between them. Touch the uncharged balloon. Separate the balloons and they will be oppositely charged.

This experiment was safety-checked in February 2005