Electromagnet
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An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current. The magnetic field disappears when the current ceases.
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Introduction
The simplest type of electromagnet is a coiled piece of wire. A coil forming the shape of a straight tube (similar to a corkscrew) is called a solenoid; a solenoid that is bent so that the ends meet is a toroid. Much stronger magnetic fields can be produced if a "core" of paramagnetic or ferromagnetic material (commonly iron) is placed inside the coil. The field produced by the coil causes the iron to magnetize and generate a field of its own. This field can be hundreds or thousands of times stronger than that of the coil itself.
Magnetic fields caused by coils of wire follow a form of the right-hand rule. If the fingers of the right hand are curled in the direction of current flow through the coil, 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.owner owned a dog ...
Electromagnets and permanent magnets
The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated over a wide range by controlling the electric current. A disadvantage is that if an electromagnet with a ferromagnetic core is turned on and off again, the core retains some residual magnetization due to hysteresis. This magnetic field can persist indefinitely. As more electricity is passed through the electromagnet, more domains align, causing the magnetic field strength to increase.
In applications where a variable magnetic field is not required, permanent magnets are generally superior. Since an electromagnet requires a constant flow of electricity, it consumes electrical power. Additionally, permanent magnets can be manufactured to produce stronger fields than any electromagnet of similar size.
Devices that use electromagnets
Electromagnets are used in many situations where a rapidly or easily variable magnetic field is desired. Many of these applications involve deflection of charged particle beams; the cathode ray tube and mass spectrometer fall into this category.
Other devices cause electromagnetic fields to interact with fields from permanent magnets and produce forces. Electromagnetic actuators take advantage of the fact that, if the core of a solenoid is displaced toward one end of the coil, a force will occur tending to push the core farther in that directional tendencies. Typical uses include relays, electromagnetic door locks, and solenoid valves. Doorbells and similar devices are commonly made by causing the moving core to strike a bell. Image:Quadrupole magnet.gif Electromagnets are the essential components of many circuit-breakers, they are used in cars in electromagnet brakes and clutches. In some trams, electromagnetic brakes grip directly on to the rails, very high powered electromagnets are even used to lift very heavy scraps of metal. Magnetic levitation trains use powerful electromagnets to hover without touching the track. Some trains use attractive forces, while others use repulsive forces.
Electromagnets are used in a rotary electric motor to produce a rotating magnetic field that turns the rotor, or in a linear motor to produce a travelling magnetic field that propels the projectile. Copper is the most oftenly used conductor due to its inexpensiveness, but silver is the best conductor of electricity.
Electric guitars also use electro-magnetic pickups, which sense the motions of the strings, and the energy is converted into sound.
Electromagnets are also used at junkyards to sort out iron and other magnetic metals.
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Force on ferromagnetic materials
Computing the force on ferromagnetic materials is, in general, quite complex. This is due to fringing field lines and complex geometries. It can be simulated using finite element analysis. However, it is possible to estimate the maximum force under specific conditions. If the magnetic field is confined within a high permeability material, such as certain steel alloys, the maximum force is given by:
<math>F = \frac{B^2 A}{2 \mu_o}</math>
Where:jgdhjdhjgh
- F is the force in newtons
- B is the magnetic field in teslas
- A is the area of the pole faces in square meters
- μ is the permeability of free space
See energy in a magnetic field for more details on the derivation.
In the case of free space (air), <math>\mu_o = 4 \pi \cdot 10^{-7}\,\mbox{H}\cdot \mbox{m}^{-1}</math>, the force per unit area (pressure) is:
<math>P \approx 398 \, \mathrm{kPa}</math> or <math>57.7 \, \mbox{lbf}\cdot\mbox{in}^{-2}</math> @ B = 1 tesla
<math>P \approx 1592 \, \mathrm{kPa}</math> or <math>230.8 \, \mbox{lbf}\cdot\mbox{in}^{-2}</math> @ B = 2 teslas
In a closed magnetic circuit:
<math>B = \frac{\mu N I}{L}</math>
Where:
- N is the number of turns of wire around the electromagnet
- I is the current in amperes
- L is the length of the magnetic circuit
Substituting above,
<math>F = \frac{\mu N^2 I^2 A}{2 L^2}</math>
In order to build a strong electromagnet, a short magnetic circuit with large area is preferred. Most ferromagnetic materials saturate around 1 to 2 teslas. This occurs at a field intensity of:
<math>H\approx 787\ \mbox{ampere.turns/meter or}\ 20\ \mbox{ampere.turns/inch}</math>.
For this reason, there is no point in building an electromagnet with a higher field intensity. Industrial lifting electromagnets are designed with both pole faces at one side (the bottom). This confines the field lines to maximize the magnetic field. It's like a cylinder within a cylinder. Many loudspeaker magnets use a similar geometry, although the field lines are radial from the inner cylinder rather than perpendicular to the face. Notice the large surface area compared with the height. With pole faces of one square foot or more, thousands of pounds can be lifted with drive currents of just a few amperes.
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See also
- Dipole magnet - Electromagnet used in particle accelerators
- Electromagnetism
- Quadrupole magnet - Electromagnet used in particle acceleratorsde:Elektromagnet
fr:Électroaimant it:Elettromagnete nl:Elektromagneet ja:電磁石 no:Elektromagnet pl:Elektromagnes sk:Elektromagnet sl:Elektromagnet sv:Elektromagnet uk:Електромагніт