Throughout most of history, there has been much confusion about the relationship between electric and magnetic phenomena. In 1600, the English physician William Gilbert made a clear distinction between the attractive power achieved by rubbing a non-conducting substance, an electric phenomenon, and the lasting ability of the mineral lodestone to attract iron, a magnetic one. The distinction was not to last for in 1820, the Danish scientist Hans Oersted showed that the motion of electric charge is always accompanied by magnetic effects. Oersted’s observation of electromagnetic effects was greatly enlarged by the subsequent work of the British physicist Michael Faraday in the first part of the 19th century.
One common manifestation of electromagnetism occurs when a current flowing in a wire produces a magnetic field—the operating principle of an electromagnet, and this effect can be harnessed to produce motion in electric motors through the attractive and repulsive forces of magnetic fields. When a magnet (either a permanent magnet or electromagnet) is moved near an electrical conductor, turbulent eddy currents are induced in the conductor, and it experiences a "dragging" force. This dragging force can be used to produce motion, and conversely, the eddy currents can be harnessed to produce a useful electric current (such as in alternators and dynamos). This is an example of a moving magnetic field producing an electric current.
A more complex example of electromagnetism is found in devices such as transformers, where a changing magnetic field produces a current. When two coils of wire are placed close together, a changing current (that is, changing in amplitude and/or direction) flows through one coil producing a changing magnetic field, which induces a voltage in the second coil. If this second coil is included in any kind of electric circuit, a current flows.
Understanding by analogy
A complete description of electromagnetic phenomena is provided by the set of four equations developed in the 19th century by the Scottish physicist James Clerk Maxwell. Solving Maxwell’s equations requires advanced mathematics, however, and most people who work with electromagnetic devices make use of simpler mental models or analogs to understand how machines operate and to design new ones.
For electric circuits, it is helpful to compare the flow of electrons through a wire to the flow of water through a pipe. The rate of water flow depends on both the water pressure and the diameter of the pipe. The rate at which electrons move around a simple electrical circuit likewise depends on two quantities, the voltage, or electromotive force (emf) of the power supply, which is the analog of pressure, and the resistance, where a larger resistance corresponds to a lower diameter pipe. By another analogy, we sometimes think of a magnetic circuit in which the driving pressure is called the magnetomotive force (mmf) and the quantity analogous to the water flow is the magnetic flux.
Many authors and teachers declare that, despite its name, flux does not flow. The fact is that flux does not exist, except as a human concept, and the only right or wrong about its flow is to be judged by whether the concept is useful to a particular individual. For some, it is more profitable to think of flux as merely being set up because it represents only stored energy and not a continuous loss of power, as is the case when electric current flows in a wire. For others, the analog is more profitable if flux is considered to be a more precise analog of electric current so that a magnetic circuit can be given the properties appropriate to those of inductance and capacitance in an electric circuit.
