# 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

#### Procedure

a Hang up two inflated balloons by long nylon threads. The balloons must be far from any metal supports.

b Charge them with like charges by rubbing them against clothing. Then let them hang freely to show repulsion.

c 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