Electricity and Magnetism

Electric Fields

Coulomb’s Law

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

This law has been verified by experiment.

The constant of proportionality depends on the medium.

The constant e is called the permittivity of the medium.

If the medium is a vacuum the constant is written e_o. 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.

The units of are NC-1 and the sense of is the sense of the force acting on a positive charge.

Electric Field Strength at a distance r from a Point Charge Q

To calculate the field strength at point p, imagine a 1C (positive) charge to be placed at p and then use Coulomb’s law.

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