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:
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.
