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Electromagnetism is the physics of the electromagnetic field; a field encompassing all of space which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles. It is often convenient to understand the electromagnetic field in terms of two separate fields: the electric field and the magnetic field. The electric field is produced by the presence of electrically charged particles, and causes the electric force. Electric force is the force observed as static electricity, and causes the flow of electric charge (electric current) in electrical conductors. The magnetic field is produced by the motion of electric charges, i.e. electric current. The magnetic field causes the magnetic force associated with magnets. The term “electromagnetism” comes from the fact that electrical and magnetic forces are involved simultaneously. A changing magnetic field produces an electric field (this is the phenomenon of electromagnetic induction, which provides for the operation of electrical generators, induction motors, and transformers). Similarly, a changing electric field generates a magnetic field. Because of this interdependence of the electric and magnetic fields, it makes sense to consider them as a single coherent entity — the electromagnetic field. This unification, which was completed by James Clerk Maxwell, is one of the triumphs of 19th century physics. It had far-reaching consequences, one of which was the understanding of the nature of light. As it turns out, what is thought of as “light” is actually a propagating oscillatory disturbance in the electromagnetic field, i.e., an electromagnetic wave. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies. The theoretical implications of electromagnetism led to the development of special relativity by Albert Einstein in 1905.