Electromagnetism

Electromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally(相互地) affected by the presence (存在)and motion of such particles.
A changing magnetic field （磁场）produces an electric field (this is the phenomenon(現象event )of electromagnetic induction, the basis of operation(操作) for electrical generators(發電機), induction motors, and transformers). Similarly, a changing electric field generates a magnetic field.
The magnetic field is produced by the motion of electric charges, i.e., electric current. The magnetic field causes the magnetic force associated (聯合的)with magnets.
The theoretical(ideal) implications(meaning) of electromagnetism led to the development of special relativity (相對論)by Albert Einstein in 1905; and from this it was shown that magnetic fields and electric fields are convertible(transformable) with relative motion as a four vector.