Meaning of ELECTROMAGNET in English
device consisting of a core of magnetic material surrounded by a coil through which an electric current is passed to magnetize the core. An electromagnet is used wherever controllable magnets are required, as in contrivances in which the magnetic flux is to be varied, reversed, or switched on and off. The engineering design of electromagnets is systematized by means of the concept of the magnetic circuit. In the magnetic circuit a magnetomotive force F, or Fm, is defined as the ampere-turns of the coil that generates the magnetic field to produce the magnetic flux in the circuit. Thus, if a coil of n turns per metre carries a current i amperes, the field inside the coil is ni amperes per metre and the magnetomotive force that it generates is nil ampere-turns, where l is the length of the coil. More conveniently, the magnetomotive force is Ni, where N is the total number of turns in the coil. The magnetic flux density B is the equivalent, in the magnetic circuit, of the current density in an electric circuit. In the magnetic circuit the magnetic equivalent to current is the total flux symbolized by the Greek letter phi, f, given by BA, where A is the cross-sectional area of the magnetic circuit. In an electric circuit the electromotive force (E) is related to the current, i, in the circuit by E = Ri, where R is the resistance of the circuit. In the magnetic circuit F = rf, where r is the reluctance of the magnetic circuit and is equivalent to resistance in the electric circuit. Reluctance is obtained by dividing the length of the magnetic path l by the permeability times the cross-sectional area A; thus r = l/mA, the Greek letter mu, m, symbolizing the permeability of the medium forming the magnetic circuit. The units of reluctance are ampere-turns per weber. These concepts can be employed to calculate the reluctance of a magnetic circuit and thus the current required through a coil to force the desired flux through this circuit. Several assumptions involved in this type of calculation, however, make it at best only an approximate guide to design. The effect of a permeable medium on a magnetic field can be visualized as being to crowd the magnetic lines of force into itself. Conversely, the lines of force passing from a region of high to one of low permeability tend to spread out, and this occurrence will take place at an air gap. Thus the flux density, which is proportional to the number of lines of force per unit area, will be reduced in the air gap by the lines bulging out, or fringing, at the sides of the gap. This effect will increase for longer gaps; rough corrections can be made for taking the fringing effect into account. It has also been assumed that the magnetic field is entirely confined within the coil. In fact, there is always a certain amount of leakage flux, represented by magnetic lines of force around the outside of the coil, which does not contribute to the magnetization of the core. The leakage flux is generally small if the permeability of the magnetic core is relatively high. In practice, the permeability of a magnetic material is a function of the flux density in it. Thus, the calculation can only be done for a real material if the actual magnetization curve, or, more usefully, a graph of m against B, is available. Finally, the design assumes that the magnetic core is not magnetized to saturation. If it were, the flux density could not be increased in the air gap in this design, no matter how much current were passed through the coil. These concepts are expanded further in following sections on specific devices.
Britannica English vocabulary. Английский словарь Британика. 2012