Magnetic force of a moving charge
Magnetic force is attraction or repulsion that arises between electrically charged particles because of their motion. Electric force exists among stationary electric charges; both electric and magnetic forces exist among moving electric charges.
A charged particle at rest will not interact with a static magnetic field. But if
the charged particle is moving in a magnetic field, the magnetic character of a
charge in motion becomes evident. It experiences a deflecting force. The force is
greatest when the particle moves in a direction perpendicular to the magnetic field
lines. At other angles, the force is less and becomes zero when the particles move
parallel to the field lines. In any case, the direction of the force is always perpendicular
to the magnetic field lines and to the velocity of the charged particle.
So a moving charge is deflected when it crosses through a magnetic field, but when
it travels parallel to the field no deflection occurs. The force that causes this
sideways deflection is very different from the forces that occur in other interactions,
such as the gravitational forces between masses, the electric forces between charges,
and the magnetic forces between magnetic poles.
The force that acts on a moving
electron does not act along the line that joins the sources of interaction, but
instead acts perpendicularly to both the magnetic field and the electron's path. This fact is employed to guide electrons onto the inner surface of a TV tube and provide a picture. More interestingly, charged particles from outer space are deflected by the earth's magnetic field. The intensity of harmful cosmic rays striking the earth's surface would otherwise be greater.