Device consisting of a core of magnetic material such as iron, surrounded by a coil through which an electric current is passed to magnetize the core. When the current is stopped, the core is no longer magnetized. Electromagnets are particularly useful wherever controllable magnets are required, as in devices in which the magnetic field is to be varied, reversed, or switched on and off. Suitably designed magnets can lift many times their own weight and are used in steelworks and scrap yards to lift loads of metal. Other devices that utilize electromagnets include particle accelerators, telephone receivers, loudspeakers, and televisions.
Learn more about electromagnet with a free trial on Britannica.com.
A wire with an electric current passing through it generates a magnetic field around it(see figure), this is a simple electromagnet. The strength of magnetic field generated is proportional to the amount of current.
In order to concentrate the magnetic field generated by a wire, it is commonly wound into a coil, where many turns of wire sit side by side. The magnetic field of all the turns of wire passes through the center of the coil. A coil forming the shape of a straight tube, a helix (similar to a corkscrew) is called a solenoid; a solenoid that is bent into a donut shape so that the ends meet is a toroid. Much stronger magnetic fields can be produced if a "core" of ferromagnetic material, such as soft iron, is placed inside the coil. The core magnifies the magnetic field to thousands of times the strength of the field of the coil alone. This is called a ferromagnetic-core or iron-core electromagnet.
The direction of the magnetic field through a coil of wire can be found from a form of the right-hand rule. If the fingers of the right hand are curled around the coil in the direction of current flow (conventional current, flow of positive charge) through the windings, the thumb points in the direction of the field inside the coil. The side of the magnet that the field lines emerge from is defined to be the north pole.
As a current is passed through the coil, small magnetic regions within the material, called magnetic domains, align with the applied field, causing the magnetic field strength to increase. As the current is increased, all of the domains eventually become aligned, a condition called saturation. Once the core becomes saturated, a further increase in current will only cause a relatively minor increase in the magnetic field. In some materials, some of the domains may realign themselves. In this case, part of the original magnetic field will persist even after power is removed, causing the core to behave as a permanent magnet. This phenomenon, called remanent magnetism, is due to the hysteresis of the material. Applying a decreasing AC current to the coil, removing the core and hitting it, or heating it above its Curie point will reorient the domains, causing the residual field to weaken or disappear.
The magnetic field of electromagnets in the general case is given by Ampere's Law:
which says that the integral of the magnetizing field H around any closed loop of the field is equal to the sum of the current flowing through the loop. Computing the magnetic field and force exerted by ferromagnetic materials is difficult for two reasons. First, because the geometry of the field is complicated, particularly outside the core and in air gaps, where fringing fields and leakage flux must be considered. Second, because the magnetic field B and force are nonlinear functions of the current, depending on the nonlinear relation between B and H for the particular core material used. For precise calculations the finite element method is used.
However, in designing a DC electromagnet, in which the current is either on or off, the relations can be simplified. The main feature of ferromagnetic materials is that the B field saturates at a certain value, which is around 1.6 T for most high permeability core steels. The B field increases quickly with increasing current up to that value, but above that value the field levels off and increases at the much smaller paramagnetic value, regardless of how much current is sent through the windings. So the strength of the magnetic field possible from an iron core electromagnet is limited to 1.6-2 T. Inside the core, the magnetic field is approximately uniform. If the magnetic circuit is only broken by air gaps small compared to the cross sectional area of the core, the B field in the gap is approximately the same as in the core.
The magnetic field created by an electromagnet is proportional to both the number of turns in the winding, , and the current in the wire, , so this product, , in Ampere-turns, is given the name magnetomotive force. For an electromagnet with a single magnetic circuit, of which length is in the core material and length is in air gaps, Ampere's Law reduces to:
This is a nonlinear equation, because the permeability of the core, , is a function of the magnetic field. For an exact solution, the value of at the B value used must be obtained from the core material hysteresis curve. If B is unknown, the equation must be solved by numerical methods. However, if the magnetomotive force is well above saturation, so the core material is in saturation, the magnetic field won't vary much with changes in anyway. For a closed magnetic circuit (no air gap) this occurs at a magnetomotive force of approximately 787 Ampere-turns per meter of flux path.
For most core materials, . So in equation (1) above, the second term dominates. Therefore, in magnetic circuits with an air gap, the behavior of the magnet depends strongly on the length of the air gap, and the length of the flux path in the core doesn't matter much.
Where Newton Ampere-2. The 1.6 T limit on the field mentioned above sets a limit on the maximum force per unit core area, or pressure, an iron-core electromagnet can exert; roughly:
Given a core geometry, the B field needed for a given force can be calculated from (2); if it comes out to much more than 1.6 T, a larger core must be used.
Substituting into (2), the force is:
It can be seen that to maximise the force, a short flux path with a wide cross sectional area is preferred. To achieve this, in applications like lifting magnets (see photo above) and loudspeakers a flat cylindrical design is often used. The winding is wrapped around a short wide cylindrical core that forms one pole, and a thick metal housing that wraps around the outside of the windings forms the other part of the magnetic circuit, bringing the magnetic field to the front to form the other pole.
Magnetic pole strength of electromagnets can be found from
|meters2||cross section area of core|
|Newton||Force exerted by magnetic field|
|Ampere||Current in the winding wire|
|meter||Total length of the magnetic field path|
|meter||Length of the magnetic field path in the core material|
|meter||Length of the magnetic field path air gap|
|Ampere meter||Pole strength of the electromagnets|
|Newtons/Ampere2||Permeability of the electromagnet core material|
|Newtons/Ampere2||Permeability of free space (or air) = 4π(10-7)|
|-||Relative permeability of the electromagnet core material|
|-||Number of turns of wire on the electromagnet|
|meters||Distance between the two electromagnets|
Researchers Submit Patent Application, "Superconductive Electromagnet Apparatus and Cooling Apparatus and Method Thereof", for Approval
May 08, 2013; By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a...
WIPO ASSIGNS PATENT TO MITSUBISHI ELECTRIC FOR "ELECTROMAGNET DEVICE AND SWITCHING DEVICE USING ELECTROMAGNET DEVICE" (JAPANESE INVENTORS)
May 07, 2011; GENEVA, May 7 -- Publication No. WO/2011/052011 was published on May 05. Title of the invention: "Electromagnet DEVICE AND...