If the charge q is projected from point P in the direction of the current, it is deflected towards the wire (q is assumed 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 wire is 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

  • The current emerges out of the plane of the paper,
  • The current moves into the plane of the paper.
  • The arrangement of iron filings around the wire. The darkened ends of the needle represent north poles. The effect of the earth’s magnetic field is neglected.


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