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Static electricity

When you take off a pullover over a nylon shirt there is a crackling sound
A pen rubbed with a piece of cloth will pick up small pieces of paper
A television screen easily collects dust
You sometimes get a small electric shock when getting out of a car
If you roll over in bed you can sometimes see small flashes of light between the sheets
You can sometimes get a small electric shock from a cat that has rolled on a synthetic carpet
In a thunderstorm there are huge flashes of lightning
Book covering film can be difficult to work with

All these things are due to the same thing - STATIC ELECTRICITY.
Static electricity is exactly what you would think it is - electricity that is not moving. You can usually make it by friction.

There are two types of electric charge POSITIVE and NEGATIVE

If you take a piece of polythene and rub it with a duster, the polythene becomes charged negatively. The duster becomes positively charged. If a rod of cellulose acetate is rubbed the charge on it is positive.



The original material used by scientists to study charging by friction were amber, fur, glass and silk.

When ebonite is rubbed with fur the ebonite becomes negatively charged.
When glass is rubbed with silk the glass becomes positively charged.
When amber is rubbed with fur the amber becomes negatively charged.

All these things were neutral to start with, in other words they had equal numbers of positive and negative charges.

The diagrams below explain how they become charged by negative charges (electrons) being rubbed off one thing onto another. There is the same amount of positive and negative charge after the rubbing as there was before the rubbing, it is just that it has been distributed differently. These charges can be tested using an electroscope.

If a negatively charged polythene rod is hung up and another negatively charged polythene rod is brought towards it, the two rods repel. A positively charged cellulose acetate rod will attract a negatively charged polythene rod. This simple experiment shows that:

 

Two charges of the same type repel each other
Two charges of different types attract each other

The photo shows the hairs of a Barbie doll repelling each other when she is charged on a Van de Graaff generator!

The closer the charges get to each other, the bigger the attraction or repulsion becomes. This rule holds right down to the forces between particles inside atoms.

When an object is charged the charges do not always spread equally over its surface. With a round shape they are evenly spread but with a pointed shape the charges are always concentrated around the point.




 

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB
 
 
 
© Keith Gibbs 2020