Static Electricity

Charging by friction

Rubbing plastic comb or pen against fabric attracts hair, dust specs or piece of paper.

There are two type of charge

i) negative charge (acquired by rubbing polythene (plastics))

ii) positive charge (acquired by rubbing glass and perspex)

It is found that

- like charges repel each other

- unlike charges attract each other

- neutral objects will always experience an attraction from a charged object therefore repulsion is the only sure way to know that an object is charged.

Charges are never created or destroyed but it can be transferred from one material to another.

So how does an object get charged?

An atom is made of protons that contain a positive charge, electrons that are negatively charged and neutrons that have no charge at all. The protons and neutrons are found at the center of the atom in the nucleus while the electrons circle the nucleus in orbits. Atoms are thought to be neutral since they have equal number of protons and electrons. In some materials, 1 or 2 of the electrons can break away from the atom and move about freely between atoms as mobile electrons. When the material is rubbed, the loosely attached electrons will become totally detached from one material and attached to the other material. This is charging by friction. The result is that the material that lost the electron essentially lost some negative charge and has a resulting positive charge while the material gaining the electron has resulting negative charge.

Electric Fields

The region around an electric charge where a force is exerted on another charge is called an electric field. The field is represented by lines of forces or field line. The direction of the field at any point is taken as the direction of the force on a positive charge at that point. The direction of this field is given by the direction of the force felt on a positively charged object.

Electric Field of Negative and Positive Point Charge in Isolation

Electric field around a positive and negative point charge that are close to each other

Electric field around two negative point charges that close

Draw the electric field around parallel plates that are i) the same charge ii) opposite charges

ii) opposite charges

Advantages/disadvantages

Static electricity can be hazardous as in cases of lightning strikes or can be used for an advantage in the case of dust extraction, xerography (a type of photocopier).

Two videos describing how the photocopier works.

Conductors and Insulators

Electrical Conductors are able to conduct electricity well, generally because of their mobile electrons, (the ones that break away from their atom)

Examples of good conductors are metals: silver, copper, aluminum

Insulators have no mobile electrons and so are unable to conduct electricity

Examples of good conductors are rubber, wood, quartz