Physics

Electricity and Magnetism

HEAT AND MAGNETISMStatic Electric Charge

• All matter is made of atoms.

• All atoms contain particles which possess electric charge.

protons possess positive charge electrons possess negative charge

• The unit of measurement of electric charge is the Coulomb.

• If a body has equal quantities of positive and negative charges, it is said to be electrically neutral.

• If a material allows electrons to move through it easily, it is a conductor. All metals are conductors because they have free electrons (that is, electrons which are not bound to a particular atom).

• A material through which electrons cannot easily flow is an insulator.

Forces between charges

similar charges repel opposite charges attract

Also, a positively or negatively charged object will weakly attract an object which is neutral. This occurs because the charged object has the effect of "rearranging" the charges inside the originally neutral object.

When two different insulators are rubbed together, electrons can be transferred from one insulator to the other. The body which has gained electrons has a negative charge and the one which lost electrons has a positive charge of equal magnitude.

This process is called charging by friction.

A body can also be charged by simply placing it near a charged body. This process is called charging by induction. This is the process of "rearranging" the charges mentioned above.

The diagram below represents the situation when a piece of metal is charged by induction.

The situation is different when an insulator is charged by induction, as shown by the following diagram.

Electric charges which remain still are called static charges.The only charges which can move freely through metals are negative charges carried by electrons.

A flow of electric charges is called an electric current

Electric Fields

Coulomb’s Law

The force between two charges is directεly proportional to their magnitudes and inversely proportional to the square of the distance between them.

F α Q1Q2

F α 1/r²

This law has been verified by experiment.

The constant of proportionality depends on the medium.

F = (1/πε)(Q1Q2/r2)

The constant ε is called the permittivity of the medium.

If the medium is a vacuum the constant is written eo. The

units of e are N-1C2m-2, (this is usually written as Farads per metre, Fm-1).

Electric Field Strength (E)

The electric field strength at a point is the force acting on a unit positive charge placed at that point.