Electric Force

Electric force

The fundamental characteristic of an electric charge is its ability to exert a force on another charge. Unlike charges are attracted to each other and like charges are repelled from each other.

Coulomb’s law states that the magnitude of an electric force between charges Q and q, separated by distance r, is given by

where is a fundamental physics constant called permittivity of vacuum.

Coulomb's law defines the unit of charge. According to this law two charges of 1 coulomb each, separated by a distance of 1 meter, experience a force given by

This is a very large value, so the charge 1 coulomb is extremely large. The charges of usual objects, as a rule, are measured in nanocoulombs (), or microcoulombs ().

Electric f orce is vector defined by its magnitude and direction. Let us direct the distance vector, , from Q to q.

Now the electric force can be expressed in vector form, and the coulomb's law in vector form becomes

where the force is produced on q by Q

Here the positive sign of corresponds to the repulsive force directed as shown in the above diagram. The negative sign of corresponds to attraction when the direction of force is opposite to that shown in the diagram.

It is important to note that the above Coulomb's law is valid for point charges, when the dimensions of charges are much less than the distance between them. We will show below that the Coulomb's law is also valid for uniformly charged spheres.

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