History of the Study of Electromagnetism
Electric and magnetic effects were known and even used by ancient civilizations, for example the Chinese used the compass, which is based in the magnetic field of the Earth at least from the year 1000 BC. Magnetism was considered strange and miraculous by the ancients, and would not begin to be understood until the works of Michael Faraday in the 19th century,
James Maxwell would, in 1873, help complete the definitions of electricity and magnetism which would lead to the successful unification of the two forces. His theory of electromagnetism is still used today for everything but particle physics, and significantly contributed to Einsteins Theory of Relativity.
Interestingly Einstein's Theory of Special Relativity shows the there is no such thing as a magnetic field or a magnetic force, but just how the electric field and electric force is observed.
The Electromagnetic Force
An electric charge A produces an electric field around it. The electric field is a somewhat abstract entity, it can be better understood as something that a second charge B “feels” when placed on it. So, when charge B is placed in the electric field generated by A, it will feel a force directed towards or away A. The force is attractive if both forces are of the same sign, and repulsive if they are of opposite sign. In any case, the electric field lines, and thus the force felt by B, is in a radial direction around A. (A is in the centre, and the field lines look like rays coming out of it.)
A continuous flux of electric charge is called an electric current. An electric current (indeed any moving electric charge), produces a magnetic field around it. Again, a magnetic field is an abstract concept; it can be intuitively understood as something that is felt by a compass. So, if a compass is placed near an electric current, its needle with point approximately in the direction of the magnetic field produced by the current (If it is strong enough so that the Earth magnetic field can be neglected). The direction of the magnetic field is at ninety degrees with respect to the direction of motion of the charge or current. For example, in the case of a straight conducting wire, the magnetic field lines are circles around the wire.
The Maxwell Equations
The principle discussed above can be stated,
- There are isolated electric charges, which produce electric fields.
The classical theory of electrodynamics is completed with three more fundamental principles:
- There are not isolated magnetic charges, also called magnetic monopoles. All magnets must have a north and a south pole. A magnetic monopole would be the magnetic equivalent of an electric charge.
- A changing electric field produces a magnetic field.
- A changing magnetic fields produces an electric field.
There four principles can be expressed in four mathematical statements, called Maxwell's Equations.