Introduction - Magnetic Field

If a charge q is placed at rest at a point P near a metallic wire carrying a current I, it experience almost no force. We conclude that there s no appreciable electric field at the point P. This is expected because in any volume of wire (which contains several thousand atoms) there are equal amount of positive and negative charges. The wire is electrically neutral and does not produce an electric field.

(In fact, there is small charge density on the surface of the wire which does produce an electric field near the wire. This field is very small and can be neglected.)

However, if the charge q is projected from point P in the direction of current, it is deflected towards the wire (q is assume to be positive). There must be a field which exerts a force on charge when it is projected, but not when it is kept at rest. This field is called Magnetic Field.

The branch of physics which deals with the magnetism due to electric current is called electromagnetism.

Some result of experiments for the magnetic field due to a straight long current-carrying wires are shown below. The wire is perpendicular to the plane of the paper. A ring of compass needles surrounds the wire. The orientation of the needles is shown when