thumb|A simple electromagnet consisting of a coil of wire wrapped around an iron core. A core of ferromagnetic material like iron serves to increase the magnetic field created. His first electromagnet was a horseshoe-shaped piece of iron that was wrapped with about 18 turns of bare copper wire. ([[Insulator (electricity)|Insulated wire did not then exist.) The iron was varnished to insulate it from the windings. When a current was passed through the coil, the iron became magnetized and attracted other pieces of iron; when the current was stopped, it lost magnetization. Sturgeon displayed its power by showing that although it only weighed seven ounces (roughly 200 grams), it could lift nine pounds (roughly 4 kilos) when the current of a single-cell power supply was applied. However, Sturgeon's magnets were weak because the uninsulated wire he used could only be wrapped in a single spaced-out layer around the core, limiting the number of turns.
Beginning in 1830, US scientist Joseph Henry systematically improved and popularised the electromagnet. By using wire insulated by silk thread and inspired by Schweigger's use of multiple turns of wire to make a galvanometer, he was able to wind multiple layers of wire onto cores, creating powerful magnets with thousands of turns of wire, including one that could support . The first major use for electromagnets was in telegraph sounders.
The magnetic domain theory of how ferromagnetic cores work was first proposed in 1906 by French physicist Pierre-Ernest Weiss, and the detailed modern quantum mechanical theory of ferromagnetism was worked out in the 1920s by Werner Heisenberg, Lev Landau, Felix Bloch, and others.
Applications of electromagnets
thumb|upright|Industrial electromagnet lifting scrap iron, 1914
Electromagnets are very widely used in electric and electromechanical devices, including:
- Motors and generators
- Transformers
- Relays
- Electric bells and buzzers
- Loudspeakers and headphones
- Actuators such as valves
- Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks
- MRI machines
- Scientific equipment such as mass spectrometers
- Particle accelerators
- Magnetic locks
- Magnetic separation equipment used for separating magnetic from nonmagnetic material; for example, separating ferrous metal in scrap
- Industrial lifting magnets
- Magnetic levitation, used in maglev trains
- Induction heating for cooking, manufacturing, and hyperthermia therapy
A portative electromagnet is one designed to just hold material in place; an example is a lifting magnet. A tractive electromagnet applies a force and moves something.
Simple solenoid
thumb|upright=1.5|Magnetic field produced by a [[solenoid (coil of wire). This drawing shows a cross-section through the center of the coil. The crosses are wires in which current is moving into the page; the dots are wires in which current is moving up out of the page.]]
A common tractive electromagnet is a uniformly wound solenoid and plunger. The solenoid is a coil of wire, and the plunger is made of a material such as soft iron. Applying a current to the solenoid applies a force to the plunger and may make it move. The plunger stops moving when the forces upon it are balanced. For example, the forces are balanced when the plunger is centered in the solenoid.
The maximum uniform pull happens when one end of the plunger is at the middle of the solenoid. An approximation for the force is For example, a 12-inch-long coil () with a long plunger with a cross section of one inch square () and 11,200 ampere-turns () had a maximum pull of 8.75 pounds (corresponding to ).
The maximum pull is increased when a magnetic stop is inserted into the solenoid. The stop becomes a magnet that will attract the plunger; it adds little to the solenoid pull when the plunger is far away but dramatically increases the pull when the plunger is close. An approximation for the pull is
:<math>P = A N I \left[\frac{N I}{(\ell_\mathrm{a})^2 (C_1)^2} + \frac C \ell\right] = \frac{A N^2 I^2}{(\ell_\mathrm{a})^2 (C_1)^2} + \frac{C A N I}{\ell}</math>
Here is the distance between the end of the stop and the end of the plunger. The additional constant for units of inches, pounds, and amperes with slender solenoids is about 2660<!-- units? -->. The first term inside the bracket represents the attraction between the stop and the plunger; the second term represents the same force as the solenoid without a stop ().
Some improvements can be made on this basic design. The ends of the stop and plunger are often conical. For example, the plunger may have a pointed end that fits into a matching recess in the stop. The shape makes the solenoid's pull more uniform as a function of separation. Another improvement is to add a magnetic return path around the outside of the solenoid (an "iron-clad solenoid"). The magnetic return path, just as the stop, has little impact until the air gap is small.
Physics
thumb|Current (<math>I</math>) through a wire produces a magnetic field (<math>B</math>). The field is oriented according to the [[Right-hand rule#Electromagnetism|right-hand rule.]]
thumb|The magnetic field lines of a current-carrying loop of wire pass through the center of the loop, concentrating the field there.
thumb|Magnetic field generated by passing a current through a coil
An electric current flowing in a wire creates a magnetic field around the wire, due to Ampere's law (see drawing of wire with magnetic field). To concentrate the magnetic field in an electromagnet, the wire is wound into a coil with many turns of wire lying side-by-side. 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.
Magnetic core
For definitions of the variables below, see box at end of article.
Much stronger magnetic fields can be produced if a magnetic core, made of a soft ferromagnetic (or ferrimagnetic) material such as iron, is placed inside the coil. A core can increase the magnetic field to thousands of times the strength of the field of the coil alone, due to the high magnetic permeability <math>\mu</math> of the material. This is why the very strongest electromagnets, such as superconducting and very high current electromagnets, cannot use cores.
When the current in the coil is turned off, most of the domains in the core material lose alignment and return to a random state, and the electromagnetic field disappears. However, some of the alignment persists because the domains resist turning their direction of magnetization, which leaves the core magnetized as a weak permanent magnet. This phenomenon is called hysteresis and the remaining magnetic field is called remanent magnetism. The residual magnetization of the core can be removed by degaussing. In alternating current electromagnets, such as those used in motors, the core's magnetization is constantly reversed, and the remanence contributes to the motor's losses.
Ampere's law
The magnetic field of electromagnets in the general case is given by Ampere's law:
:<math>\int \mathbf{J}\cdot d\mathbf{A} = \oint \mathbf{H}\cdot d\boldsymbol{\ell}</math>
which says that the integral of the magnetizing field <math>\mathbf{H}</math> around any closed loop is equal to the sum of the current flowing through the loop. A related equation is the Biot–Savart law, which gives the magnetic field due to each small segment of current.
Force exerted by magnetic field
Likewise, on the solenoid, the force exerted by an electromagnet on a conductor located at a section of core material is:
This equation can be derived from the energy stored in a magnetic field. Energy is force times distance. Rearranging terms yields the equation above.
The 1.6 T limit on the field
:<math>NI = H_{\mathrm{core L_{\mathrm{core + H_{\mathrm{gap L_{\mathrm{gap</math>
}{\mu} + \frac{L_{\mathrm{gap}{\mu_0} \right) </math>|
This is a nonlinear equation, because the permeability of the core <math>\mu</math> varies with <math>B</math>. For an exact solution, <math>\mu(B)</math> must be obtained from the core material hysteresis curve. The necessary refrigeration equipment and cryostat make them much more expensive than ordinary electromagnets. However, in high-power applications this can be offset by lower operating costs, since after startup no power is required for the windings, since no energy is lost to ohmic heating. They are used in particle accelerators and MRI machines.
Bitter electromagnets
Both iron-core and superconducting electromagnets have limits to the field they can produce. Therefore, the most powerful man-made magnetic fields have been generated by air-core non-superconducting electromagnets of a design invented by Francis Bitter in 1933, called Bitter electromagnets. Instead of wire windings, a Bitter magnet consists of a solenoid made of a stack of conducting disks, arranged so that the current moves in a helical path through them, with a hole through the center where the maximum field is created. This design has the mechanical strength to withstand the extreme Lorentz forces of the field, which increase with <math>B^2</math>. The disks are pierced with holes through which cooling water passes to carry away the heat caused by the high current. The strongest continuous field achieved solely with a resistive magnet is 41.5 T , produced by a Bitter electromagnet at the National High Magnetic Field Laboratory in Tallahassee, Florida. The previous record was 37.5 T. The strongest continuous magnetic field overall, 45 T, have been created by using explosives to compress the magnetic field inside an electromagnet as it is pulsed; these are called explosively pumped flux compression generators. The implosion compresses the magnetic field to values of around 1,000 T They are used in physics and materials science research to study the properties of materials at high magnetic fields.
Definition of terms
{| class="wikitable"
!Term
!Significance
!Unit
|-
| width="40" |<math>A\,</math>||Cross sectional area of core
|square meter
|-
|<math>B\,</math>||Magnetic field (magnetic flux density)
|tesla
|-
|<math>F\,</math>||Force exerted by magnetic field
|newton
|-
|<math>H\,</math>||Magnetizing field
|ampere per meter
|-
|<math>I\,</math>||Current in the winding wire
|ampere
|-
|<math>L\,</math>||Total length of the magnetic field path <math>L_{\mathrm{core+L_{\mathrm{gap\,</math>
|meter
|-
|<math>L_{\mathrm{core\,</math>||Length of the magnetic field path in the core material
|meter
|-
|<math>L_{\mathrm{gap\,</math>||Length of the magnetic field path in air gaps
|meter
|-
|<math>m_1, m_2\,</math>||Pole strength of the electromagnet
|ampere meter
|-
|<math>\mu\,</math>||Permeability of the electromagnet core material
|newton per square ampere
|-
|<math>\mu_0\,</math>||Permeability of free space (or air) = <math>4 \pi (10^{-7}) \ \mathrm{N} \cdot \mathrm{A}^{-2}</math>
|newton per square ampere
|-
|<math>\mu_r\,</math>||Relative permeability of the electromagnet core material
| dimensionless
|-
|<math>N\,</math>||Number of turns of wire on the electromagnet
| dimensionless
|-
|<math>r\,</math>||Distance between the poles of two electromagnets
|meter
|}
See also
- Dipole magnet – the most basic form of magnet
- Electromagnetism
- Electropermanent magnet – a magnetically hard electromagnet arrangement
- Field coil
- Magnetic bearing
- Pulsed field magnet
- Quadrupole magnet – a combination of magnets and electromagnets used mainly to affect the motion of charged particles
References
External links
- Electromagnets - The Feynman Lectures on Physics