NationMaster - Encyclopedia: Electromagnet

An electromagnet is a type of magnet in which the magnetic field is produced by the flow of an electric current. The magnetic field disappears when the current ceases. For other uses, see Magnet (disambiguation). ... For the indie-pop band, see The Magnetic Fields. ... In electricity, current refers to electric current, which is the flow of electric charge. ...

1 Invention and history
2 Introduction
3 Electromagnets and permanent magnets
4 Force on ferromagnetic materials
5 Force between Electromagnets
6 Use of electromagnets
7 Patents
8 See also
9 References

Invention and history

British electrician William Sturgeon invented the electromagnet in 1825. The first electromagnet was a horseshoe-shaped piece of iron that was wrapped with a loosely wound coil consisting of several turns of wire. When a current was passed through the coil, the electromagnet became magnetized and when the current was stopped, the coil was de-magnetized. Sturgeon displayed its power by lifting nine pounds with a seven-ounce piece of iron wrapped with wires through which the current of a single cell battery was sent. William Sturgeon (May 22, 1783 - December 4, 1850) was an English physicist and inventor who made the first electromagnets. ... Year 1825 (MDCCCXXV) was a common year starting on Saturday (link will display the full calendar) of the Gregorian Calendar (or a common year starting on Thursday of the 12-day slower Julian calendar). ...

Sturgeon could regulate his electromagnet; this was the beginning of using electrical energy for making useful and controllable machines and laid the foundations for large-scale electronic communications.

Introduction

The most fundamental type of electromagnet is a simple segment of wire (see figure). The amount of magnetic field generated depends upon the amount of electrical current that flows through the wire. In order to increase the effective current available to generate magnetic field, the wire is commonly configured as a coil, where many segments of wire sit side by side. 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 soft iron) is placed inside the coil. The core concentrates the magnetic field that can then be much stronger than that of the coil itself. A wire is a single, usually cylindrical, elongated strand of drawn metal. ... A coil is a series of loops. ... A basic corkscrew A waiters corkscrew A corkscrew is a tool for drawing stopping corks from bottles. ... For other uses, see Solenoid (disambiguation). ... A magnetic core is the core of an electromagnet or inductor. ... Simple Illustration of a paramagnetic probe made up from miniature magnets. ... Ferromagnetism is the phenomenon by which materials, such as iron, in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. ... General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ...

Current (I) flowing through a wire produces a magnetic field (B) around the wire. The field is oriented according to the left-hand rule.

Magnetic fields caused by coils of wire follow a form of the right-hand rule (for conventional current or left hand rule for electron current) [1]. If the fingers of the left hand are curled in the direction of electron 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. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... In mathematics and physics, the right-hand rule is a convention for determining relative directions of certain vectors. ... The left-handed orientation is shown on the left, and the right-handed on the right. ... This box: Electric current is the flow (movement) of electric charge. ...

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 amount of electric current. However, a continuous supply of electrical energy is required to maintain the field. Magnetic lines of force of a bar magnet shown by iron filings on paper A magnet is an object that has a magnetic field. ...

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. Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. ... For magnetic materials, saturation is the state when the material can not absorb a stronger magnetic field, such that an increase of magnetization produces no significant change in magnetic flux density. ... Remanence is the magnetization left behind in a medium after an external magnetic field is removed. ... A system with hysteresis can be summarised as a system that may be in any number of states, independent of the inputs to the system. ... City lights viewed in a motion blurred exposure. ... The Curie point is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic material. ...

In applications where a variable magnetic field is not required, permanent magnets are generally superior. Additionally, permanent magnets can be manufactured to produce stronger fields than electromagnets of similar size.

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: Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. ... Visualization of how a car deforms in an asymmetrical crash using finite element analysis. ...

$F = frac{B^2 A}{2 mu_o}$

Where:

F is the force in newtons
B is the magnetic field in teslas
A is the area of the pole faces in square meters
$μ o$ is the permeability of free space

See energy in a magnetic field for more details on the derivation. In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. ... For the indie-pop band, see The Magnetic Fields. ...

In the case of free space (air), $mu_o = 4 pi cdot 10^{-7},mbox{H}cdot mbox{m}^{-1}$ , the force per unit area (pressure) is:

$P approx 398 , mathrm{kPa}$ or $57.7 , mbox{lbf}cdotmbox{in}^{-2}$ @ B = 1 tesla

$P approx 1592 , mathrm{kPa}$ or $230.8 , mbox{lbf}cdotmbox{in}^{-2}$ @ B = 2 teslas

In a closed magnetic circuit: A magnetic circuit is a closed path containing a magnetic flux. ...

$B = frac{mu N I}{L}$

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,

$F = frac{mu^2 N^2 I^2 A}{2mu_0 L^2}$

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:

$Happrox 787 mbox{ampere.turns/meter or} 20 mbox{ampere.turns/inch}$ .

For this reason, there is no reason to build 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. For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ...

Force between Electromagnets

Force between two electromagnets can be found from

 F = μ₀*m₁*m₂/(4*π*r²)

 where μ₀=Permeability of free space =4π.10^-7 m₁,m₂ = pole strength of electromagnets 1 and 2 resp. r = distance between electromagnets

Magnetic pole strenth of electromagnets can be found from

 m = N.i.A/L

 where N = no of turns in electromagnet coil. i = Current through electromagnet coil A = cross sectional area of electromagnet ( in flux path direction) L = length of electromagnet

Use of electromagnets

Electromagnets are widely used in many applications, including:

relays, including reed relays originally used in telephone exchanges
loudspeakers
Magnetic tape sound recording using a tape recorder
particle accelerators

Automotive style miniature relay A relay is an electrical switch that opens and closes under the control of another electrical circuit. ... It has been suggested that this article or section be merged into Reed switch. ... A telephone operator manually connecting calls with patch cables at a telephone switchboard. ... For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ... Magnetic tape has been used for sound recording for more than 75 years. ... Sony reel-to-reel tape recorder. ... For the DC Comics Superhero also called Atom Smasher, see Albert Rothstein. ...

Patents

U.S. Patent 427,606 -- Francis Patton's patent of the electromagnet from 1890

Year 1890 (MDCCCXC) was a common year starting on Wednesday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Monday of the Julian calendar). ...

References

1. Magnetic Fields and Forces[2] 2. Fundamental Relationships[3]

Categories: Types of magnets

Results from FactBites:

Electromagnetism (913 words)
	Thus, an electromagnet consists of a long coil of insulated copper wire wound on a soft iron core.
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