Misplaced Pages

#23976

89-475: (Redirected from Magnetic Pole ) [REDACTED] Look up magnetic pole in Wiktionary, the free dictionary. Magnetic pole may refer to: One of the two ends of a magnet Magnetic monopole , a hypothetical elementary particle The magnetic poles of astronomical bodies , a special case of magnets, especially: The North magnetic pole of planet Earth,

178-471: A pacemaker has been embedded in a patient's chest (usually for the purpose of monitoring and regulating the heart for steady electrically induced beats ), care should be taken to keep it away from magnetic fields. It is for this reason that a patient with the device installed cannot be tested with the use of a magnetic resonance imaging device. Children sometimes swallow small magnets from toys, and this can be hazardous if two or more magnets are swallowed, as

267-401: A torque tending to orient the magnetic moment parallel to the field. The amount of this torque is proportional both to the magnetic moment and the external field. A magnet may also be subject to a force driving it in one direction or another, according to the positions and orientations of the magnet and source. If the field is uniform in space, the magnet is subject to no net force, although it

356-415: A combination of aluminium , nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as

445-767: A common ground state in the manner of a Bose–Einstein condensate . The United States Department of Energy has identified a need to find substitutes for rare-earth metals in permanent-magnet technology, and has begun funding such research. The Advanced Research Projects Agency-Energy (ARPA-E) has sponsored a Rare Earth Alternatives in Critical Technologies (REACT) program to develop alternative materials. In 2011, ARPA-E awarded 31.6 million dollars to fund Rare-Earth Substitute projects. Iron nitrides are promising materials for rare-earth free magnets. The current cheapest permanent magnets, allowing for field strengths, are flexible and ceramic magnets, but these are also among

534-416: A different issue, however; correlations between electromagnetic radiation and cancer rates have been postulated due to demographic correlations (see Electromagnetic radiation and health ). If a ferromagnetic foreign body is present in human tissue, an external magnetic field interacting with it can pose a serious safety risk. A different type of indirect magnetic health risk exists involving pacemakers. If

623-427: A high- coercivity ferromagnetic compound (usually ferric oxide ) mixed with a resinous polymer binder. This is extruded as a sheet and passed over a line of powerful cylindrical permanent magnets. These magnets are arranged in a stack with alternating magnetic poles facing up (N, S, N, S...) on a rotating shaft. This impresses the plastic sheet with the magnetic poles in an alternating line format. No electromagnetism

712-432: A larger core must be used. However, computing the magnetic field and force exerted by ferromagnetic materials in general is difficult for two reasons. First, because the strength of the field varies from point to point in a complicated way, 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

801-429: A limit on the maximum force per unit core area, or magnetic pressure , an iron-core electromagnet can exert; roughly: for saturation limit of the core, B sat . In more intuitive units it is useful to remember that at 1 T the magnetic pressure is approximately 4 atmospheres, or kg/cm . Given a core geometry, the B field needed for a given force can be calculated from (1); if it comes out to much more than 1.6 T,

890-401: A loop or magnetic circuit , possibly broken by a few narrow air gaps. Iron presents much less "resistance" ( reluctance ) to the magnetic field than air, so a stronger field can be obtained if most of the magnetic field's path is within the core. Since the magnetic field lines are closed loops, the core is usually made in the form of a loop. Since most of the magnetic field is confined within

979-571: A magnet can be magnetized with different directions and strengths (for example, because of domains, see below). A good bar magnet may have a magnetic moment of magnitude 0.1 A·m and a volume of 1 cm , or 1×10  m , and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter. Such a large value explains why iron magnets are so effective at producing magnetic fields. Two different models exist for magnets: magnetic poles and atomic currents. Although for many purposes it

SECTION 10

#1732855473024

1068-412: 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. The magnetic field of all the turns of wire passes through the center of the coil, creating a strong magnetic field there. A coil forming the shape of a straight tube (a helix )

1157-412: A maximum pull of 8.75 pounds (corresponding to C = 0.0094 psi ). 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 they are close. An approximation for the pull P is Here ℓ a is the distance between

1246-492: A metal. Trade names for alloys in this family include: Alni, Alcomax, Hycomax, Columax , and Ticonal . Injection-molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties. Flexible magnets are composed of

1335-402: A net contribution; shaving off the outer layer of a magnet will not destroy its magnetic field, but will leave a new surface of uncancelled currents from the circular currents throughout the material. The right-hand rule tells which direction positively-charged current flows. However, current due to negatively-charged electricity is far more prevalent in practice. The north pole of a magnet

1424-410: A north and south pole. However, a version of the magnetic-pole approach is used by professional magneticians to design permanent magnets. In this approach, the divergence of the magnetization ∇· M inside a magnet is treated as a distribution of magnetic monopoles . This is a mathematical convenience and does not imply that there are actually monopoles in the magnet. If the magnetic-pole distribution

1513-399: A partially occupied f electron shell (which can accommodate up to 14 electrons). The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore, these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare-earth magnets are samarium–cobalt and neodymium–iron–boron (NIB) magnets. In

1602-551: A permanent magnet has a large influence on its magnetic properties. When a magnet is magnetized , a demagnetizing field will be created inside it. As the name suggests, the demagnetizing field will work to demagnetize the magnet, decreasing its magnetic properties. The strength of the demagnetizing field H d {\displaystyle H_{d}} is proportional to the magnet's magnetization M {\displaystyle M} and shape, according to Here, N d {\displaystyle N_{d}}

1691-403: A piece of iron bridged across its poles, equation ( 2 ) becomes: Substituting into ( 1 ), the force is: It can be seen that to maximize the force, a core with a short flux path L and a wide cross-sectional area A is preferred (this also applies to magnets with an air gap). To achieve this, in applications like lifting magnets (see photo above) and loudspeakers a flat cylindrical design

1780-400: A point where the north end of a compass points downward The South magnetic pole of planet Earth, a point where the south end of a compass points downward Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title Magnetic pole . If an internal link led you here, you may wish to change the link to point directly to

1869-410: A simple magnetic dipole; for example, quadrupole and sextupole magnets are used to focus particle beams . Electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current . Electromagnets usually consist of wire wound into a coil . A current through the wire creates a magnetic field which is concentrated in the hole in the center of

SECTION 20

#1732855473024

1958-400: A strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. The overall strength of a magnet

2047-444: Is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic ). These include the elements iron , nickel and cobalt and their alloys, some alloys of rare-earth metals , and some naturally occurring minerals such as lodestone . Although ferromagnetic (and ferrimagnetic) materials are

2136-401: Is a lifting magnet. A tractive electromagnet applies a force and moves something. Electromagnets are very widely used in electric and electromechanical devices, including: 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

2225-426: Is called a solenoid . 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

2314-416: Is called the demagnetizing factor, and has a different value depending on the magnet's shape. For example, if the magnet is a sphere , then N d = 1 3 {\displaystyle N_{d}={\frac {1}{3}}} . The value of the demagnetizing factor also depends on the direction of the magnetization in relation to the magnet's shape. Since a sphere is symmetrical from all angles,

2403-443: Is convenient to think of a magnet as having distinct north and south magnetic poles, the concept of poles should not be taken literally: it is merely a way of referring to the two different ends of a magnet. The magnet does not have distinct north or south particles on opposing sides. If a bar magnet is broken into two pieces, in an attempt to separate the north and south poles, the result will be two bar magnets, each of which has both

2492-562: Is defined as the pole that, when the magnet is freely suspended, points towards the Earth's North Magnetic Pole in the Arctic (the magnetic and geographic poles do not coincide, see magnetic declination ). Since opposite poles (north and south) attract, the North Magnetic Pole is actually the south pole of the Earth's magnetic field. As a practical matter, to tell which pole of a magnet

2581-433: Is defined to be the north pole . For definitions of the variables below, see box at end of article. Much stronger magnetic fields can be produced if a " magnetic core " 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 μ of

2670-433: Is due to the resistance of the windings, and is dissipated as heat. Some large electromagnets require water cooling systems in the windings to carry off the waste heat . Since the magnetic field is proportional to the product NI , the number of turns in the windings N and the current I can be chosen to minimize heat losses, as long as their product is constant. Since the power dissipation, P = I R , increases with

2759-405: Is highest for alnico magnets at over 540 °C (1,000 °F), around 300 °C (570 °F) for ferrite and SmCo, about 140 °C (280 °F) for NIB and lower for flexible ceramics, but the exact numbers depend on the grade of material. An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops, known as a solenoid . When electric current flows through

Magnetic pole - Misplaced Pages Continue

2848-509: Is known, then the pole model gives the magnetic field H . Outside the magnet, the field B is proportional to H , while inside the magnetization must be added to H . An extension of this method that allows for internal magnetic charges is used in theories of ferromagnetism. Another model is the Ampère model, where all magnetization is due to the effect of microscopic, or atomic, circular bound currents , also called Ampèrian currents, throughout

2937-462: Is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization . An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of "soft" ferromagnetic material such as mild steel , which greatly enhances

3026-464: Is north and which is south, it is not necessary to use the Earth's magnetic field at all. For example, one method would be to compare it to an electromagnet , whose poles can be identified by the right-hand rule . The magnetic field lines of a magnet are considered by convention to emerge from the magnet's north pole and reenter at the south pole. The term magnet is typically reserved for objects that produce their own persistent magnetic field even in

3115-419: 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. The above methods are applicable to electromagnets with a magnetic circuit and do not apply when a large part of the magnetic field path

3204-417: Is outside the core. A non-circuit example would be a magnet with a straight cylindrical core like the one shown at the top of this article. Only focusing on the force between two electromagnets (or permanent magnets) with well-defined "poles" where the field lines emerge from the core, a special analogy called a magnetic-charge model which assumes the magnetic field is produced by fictitious 'magnetic charges' on

3293-476: Is specified by two properties: In SI units, the strength of the magnetic B field is given in teslas . A magnet's magnetic moment (also called magnetic dipole moment and usually denoted μ ) is a vector that characterizes the magnet's overall magnetic properties. For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole, and the magnitude relates to how strong and how far apart these poles are. In SI units,

3382-400: Is subject to a torque. A wire in the shape of a circle with area A and carrying current I has a magnetic moment of magnitude equal to IA . The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M , with units A / m . It is a vector field , rather than just a vector (like the magnetic moment), because different areas in

3471-501: Is the number of turns in the solenoid, I is the current through the solenoid wire, and ℓ is the length of the solenoid. For units using inches, pounds force, and amperes with long, slender, solenoids, the value of C is around 0.009 to 0.010 psi (maximum pull pounds per square inch of plunger cross-sectional area). For example, a 12-inch long coil ( ℓ = 12 in ) with a long plunger of 1-square inch cross section ( A = 1 in ) and 11,200 ampere-turns ( N I = 11,200 Aturn ) had

3560-410: Is used to generate the magnets. The pole-to-pole distance is on the order of 5 mm, but varies with manufacturer. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used. For magnetic compounds (e.g. Nd 2 Fe 14 B ) that are vulnerable to a grain boundary corrosion problem it gives additional protection. Rare earth ( lanthanoid ) elements have

3649-406: Is why the very strongest electromagnets, such as superconducting and very high current electromagnets, cannot use cores. The main nonlinear feature of ferromagnetic materials is that the B field saturates at a certain value, which is around 1.6 to 2 teslas (T) for most high permeability core steels. The B field increases quickly with increasing current up to that value, but above that value

Magnetic pole - Misplaced Pages Continue

3738-515: The horseshoe magnet was invented by Daniel Bernoulli in 1743. A horseshoe magnet avoids demagnetization by returning the magnetic field lines to the opposite pole. In 1820, Hans Christian Ørsted discovered that a compass needle is deflected by a nearby electric current. In the same year André-Marie Ampère showed that iron can be magnetized by inserting it in an electrically fed solenoid. This led William Sturgeon to develop an iron-cored electromagnet in 1824. Joseph Henry further developed

3827-429: The 1990s, it was discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different from conventional magnets that store information at a magnetic domain level and theoretically could provide a far denser storage medium than conventional magnets. In this direction, research on monolayers of SMMs is currently under way. Very briefly,

3916-575: The Earth's magnetic field would leave the iron permanently magnetized. This led to the development of the navigational compass , as described in Dream Pool Essays in 1088. By the 12th to 13th centuries AD, magnetic compasses were used in navigation in China, Europe, the Arabian Peninsula and elsewhere. A straight iron magnet tends to demagnetize itself by its own magnetic field. To overcome this,

4005-689: The absence of an applied magnetic field. Only certain classes of materials can do this. Most materials, however, produce a magnetic field in response to an applied magnetic field – a phenomenon known as magnetism. There are several types of magnetism, and all materials exhibit at least one of them. The overall magnetic behavior of a material can vary widely, depending on the structure of the material, particularly on its electron configuration . Several forms of magnetic behavior have been observed in different materials, including: There are various other types of magnetism, such as spin glass , superparamagnetism , superdiamagnetism , and metamagnetism . The shape of

4094-680: The amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field. Electromagnets are widely used as components of other electrical devices, such as motors , generators , electromechanical solenoids , relays , loudspeakers , hard disks , MRI machines , scientific instruments, and magnetic separation equipment. Electromagnets are also employed in industry for picking up and moving heavy iron objects such as scrap iron and steel. Danish scientist Hans Christian Ørsted discovered in 1820 that electric currents create magnetic fields. In

4183-660: The availability of magnetic materials to include various man-made products, all based, however, on naturally magnetic elements. Ceramic, or ferrite , magnets are made of a sintered composite of powdered iron oxide and barium / strontium carbonate ceramic . Given the low cost of the materials and manufacturing methods, inexpensive magnets (or non-magnetized ferromagnetic cores, for use in electronic components such as portable AM radio antennas ) of various shapes can be easily mass-produced. The resulting magnets are non-corroding but brittle and must be treated like other ceramics. Alnico magnets are made by casting or sintering

4272-416: The coil. The magnetic field disappears when the current is turned off. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron ; the magnetic core concentrates the magnetic flux and makes a more powerful magnet. The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling

4361-405: The core material ( C ). Within the core the magnetic field ( B ) will be approximately uniform across any cross-section, so if in addition, the core has roughly constant area throughout its length, the field in the core will be constant. This leaves the air gaps ( G ), if any, between core sections. In the gaps, the magnetic field lines are no longer confined by the core. So they 'bulge' out beyond

4450-423: The core's magnetization is constantly reversed, and the remanence contributes to the motor's losses. 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 {\displaystyle \mathbf {H} } around any closed loop is equal to the sum of the current flowing through the loop. Another equation used, that gives

4539-403: The current, depending on the nonlinear relation between B and H for the particular core material used. For precise calculations, computer programs that can produce a model of the magnetic field using the finite element method are employed. In many practical applications of electromagnets, such as motors, generators, transformers, lifting magnets, and loudspeakers, the iron core is in the form of

SECTION 50

#1732855473024

4628-461: The demagnetizing factor only has one value. But a magnet that is shaped like a long cylinder will yield two different demagnetizing factors, depending on if it's magnetized parallel to or perpendicular to its length. Because human tissues have a very low level of susceptibility to static magnetic fields, there is little mainstream scientific evidence showing a health effect associated with exposure to static fields. Dynamic magnetic fields may be

4717-416: The electromagnet into a commercial product in 1830–1831, giving people access to strong magnetic fields for the first time. In 1831 he built an ore separator with an electromagnet capable of lifting 750 pounds (340 kg). The magnetic flux density (also called magnetic B field or just magnetic field, usually denoted by B ) is a vector field . The magnetic B field vector at a given point in space

4806-448: The end of the stop and the end of the plunger. The additional constant C 1 for units of inches, pounds, and amperes with slender solenoids is about 2660. The second term within the bracket represents the same force as the stop-less solenoid above; the first term represents the attraction between the stop and the plunger. Some improvements can be made on the basic design. The ends of the stop and plunger are often conical. For example,

4895-405: The entire core circuit, and thus will not contribute to the force exerted by the magnet. This also includes field lines that encircle the wire windings but do not enter the core. This is called leakage flux . The equations in this section are valid for electromagnets for which: The magnetic field created by an electromagnet is proportional to both N and I , hence this product, NI , is given

4984-438: The field disappears. However, some of the alignment persists, because the domains have difficulty turning their direction of magnetization, leaving 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 are used in motors,

5073-417: The field levels off and becomes almost constant, regardless of how much current is sent through the windings. The maximum strength of the magnetic field possible from an iron core electromagnet is limited to around 1.6 to 2 T. When the current in the coil is turned off, in the magnetically soft materials that are nearly always used as cores, most of the domains lose alignment and return to a random state and

5162-407: The field, and the magnetic field passes through the core in lower reluctance than when it would pass through air. The larger the current passed through the wire coil, the more the domains align, and the stronger the magnetic field is. Finally, all the domains are lined up, and further increases in current only cause slight increases in the magnetic field: this phenomenon is called saturation . This

5251-451: The first magnetic compasses . The earliest known surviving descriptions of magnets and their properties are from Anatolia, India, and China around 2,500 years ago. The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia Naturalis Historia in the 1st century AD. In 11th century China, it was discovered that quenching red hot iron in

5340-403: The following ways: Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways: Many materials have unpaired electron spins, and the majority of these materials are paramagnetic . When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as magnetic). Because of

5429-474: The intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Magnetic_pole&oldid=1255244276 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Magnet A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example

SECTION 60

#1732855473024

5518-400: The iron has no large-scale magnetic field. When a current is passed through the wire wrapped around the iron, its magnetic field penetrates the iron, and causes the domains to turn, aligning parallel to the magnetic field, so their tiny magnetic fields add to the wire's field, creating a large magnetic field that extends into the space around the magnet. The effect of the core is to concentrate

5607-479: The magnetic field due to each small segment of current, is the Biot–Savart law . Likewise on the solenoid, the force exerted by an electromagnet on a conductor located at a section of core material is: The force 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 mentioned above sets

5696-601: The magnetic field produced by the coil. Ancient people learned about magnetism from lodestones (or magnetite ) which are naturally magnetized pieces of iron ore. The word magnet was adopted in Middle English from Latin magnetum "lodestone", ultimately from Greek μαγνῆτις [λίθος] ( magnētis [lithos] ) meaning "[stone] from Magnesia", a place in Anatolia where lodestones were found (today Manisa in modern-day Turkey ). Lodestones, suspended so they could turn, were

5785-403: The magnetic moment is specified in terms of A·m (amperes times meters squared). A magnet both produces its own magnetic field and responds to magnetic fields. The strength of the magnetic field it produces is at any given point proportional to the magnitude of its magnetic moment. In addition, when the magnet is put into an external magnetic field, produced by a different source, it is subject to

5874-621: The magnetomotive force is well above saturation, so the core material is in saturation, the magnetic field will be approximately the saturation value B sat for the material, and will not vary much with changes in NI . For a closed magnetic circuit (no air gap) most core materials saturate at a magnetomotive force of roughly 800 ampere-turns per meter of flux path. For most core materials, μ r ≈ 2000 – 6000 {\displaystyle \mu _{r}\approx 2000{\text{–}}6000\,} . So in equation (2) above,

5963-437: The magnets can pinch or puncture internal tissues. Magnetic imaging devices (e.g. MRIs ) generate enormous magnetic fields, and therefore rooms intended to hold them exclude ferrous metals. Bringing objects made of ferrous metals (such as oxygen canisters) into such a room creates a severe safety risk, as those objects may be powerfully thrown about by the intense magnetic fields. Ferromagnetic materials can be magnetized in

6052-479: The material. Not all electromagnets use cores, so this is called a ferromagnetic-core or iron-core electromagnet. This is because the material of a magnetic core (often made of iron or steel) is composed of small regions called magnetic domains that act like tiny magnets (see ferromagnetism ). Before the current in the electromagnet is turned on, the domains in the soft iron core point in random directions, so their tiny magnetic fields cancel each other out, and

6141-416: The material. For a uniformly magnetized cylindrical bar magnet, the net effect of the microscopic bound currents is to make the magnet behave as if there is a macroscopic sheet of electric current flowing around the surface, with local flow direction normal to the cylinder axis. Microscopic currents in atoms inside the material are generally canceled by currents in neighboring atoms, so only the surface makes

6230-416: The name magnetomotive force . For an electromagnet with a single magnetic circuit , Ampere's Law reduces to: This is a nonlinear equation , because the permeability of the core μ varies with B . For an exact solution, the value of μ 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

6319-533: The only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism . Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron , which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in

6408-451: The outlines of the core before curving back to enter the next piece of core material, reducing the field strength in the gap. The bulges ( B F ) are called fringing fields . However, as long as the length of the gap is smaller than the cross-section dimensions of the core, the field in the gap will be approximately the same as in the core. In addition, some of the magnetic field lines ( B L ) will take 'short cuts' and not pass through

6497-401: The outlines of the core loop, this allows a simplification of the mathematical analysis. See the drawing at right. A common simplifying assumption satisfied by many electromagnets, which will be used in this section, is that the magnetic field strength B is constant around the magnetic circuit (within the core and air gaps) and zero outside it. Most of the magnetic field will be concentrated in

6586-409: 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 F is where C is a proportionality constant, A is the cross-sectional area of the plunger, N

6675-413: 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. An electric current flowing in a wire creates

6764-467: The same year, the French scientist André-Marie Ampère showed that iron can be magnetized by inserting it in an electrically fed solenoid. British scientist William Sturgeon invented the electromagnet in 1824. His first electromagnet was a horseshoe-shaped piece of iron that was wrapped with about 18 turns of bare copper wire. ( Insulated wire did not then exist.) The iron was varnished to insulate it from

6853-409: The second term dominates. Therefore, in magnetic circuits with an air gap, B depends strongly on the length of the air gap, and the length of the flux path in the core does not matter much. Given an air gap of 1mm, a magnetomotive force of about 796 Ampere-turns is required to produce a magnetic field of 1T. For a closed magnetic circuit (no air gap), such as would be found in an electromagnet lifting

6942-421: The square of the current but only increases approximately linearly with the number of windings, the power lost in the windings can be minimized by reducing I and increasing the number of turns N proportionally, or using thicker wire to reduce the resistance. For example, halving I and doubling N halves the power loss, as does doubling the area of the wire. In either case, increasing the amount of wire reduces

7031-549: The strongest. These cost more per kilogram than most other magnetic materials but, owing to their intense field, are smaller and cheaper in many applications. Temperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie point , it loses all of its magnetism, even after cooling below that temperature. The magnets can often be remagnetized, however. Additionally, some magnets are brittle and can fracture at high temperatures. The maximum usable temperature

7120-707: The surface of the poles. This model assumes point-like poles instead of the really existing surfaces, and thus it only yields a good approximation when the distance between the magnets is much larger than their diameter, so it is useful just for a force between them. Magnetic pole strength of electromagnets can be found from: m = N I A L {\displaystyle m={\frac {NIA}{L}}} The force between two poles is: F = μ 0 m 1 m 2 4 π r 2 {\displaystyle F={\frac {\mu _{0}m_{1}m_{2}}{4\pi r^{2}}}} Each electromagnet has two poles, so

7209-420: The total force on a given magnet due to another magnet is equal to the vector sum of the forces of the other magnet's poles acting on each pole of the given magnet. There are several side effects which occur in electromagnets which must be provided for in their design. These generally become more significant in larger electromagnets. The only power consumed in a DC electromagnet under steady-state conditions

7298-429: The two main attributes of an SMM are: Most SMMs contain manganese but can also be found with vanadium, iron, nickel and cobalt clusters. More recently, it has been found that some chain systems can also display a magnetization that persists for long times at higher temperatures. These systems have been called single-chain magnets. Some nano-structured materials exhibit energy waves , called magnons , that coalesce into

7387-579: 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 2,063 lb (936 kg). The first major use for electromagnets

7476-459: The way their regular crystalline atomic structure causes their spins to interact, some metals are ferromagnetic when found in their natural states, as ores . These include iron ore ( magnetite or lodestone ), cobalt and nickel , as well as the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring ferromagnets were used in the first experiments with magnetism. Technology has since expanded

7565-473: The weakest types. The ferrite magnets are mainly low-cost magnets since they are made from cheap raw materials: iron oxide and Ba- or Sr-carbonate. However, a new low cost magnet, Mn–Al alloy, has been developed and is now dominating the low-cost magnets field. It has a higher saturation magnetization than the ferrite magnets. It also has more favorable temperature coefficients, although it can be thermally unstable. Neodymium–iron–boron (NIB) magnets are among

7654-427: 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

7743-421: The wire, a magnetic field is generated. It is concentrated near (and especially inside) the coil, and its field lines are very similar to those of a magnet. The orientation of this effective magnet is determined by the right hand rule . The magnetic moment and the magnetic field of the electromagnet are proportional to the number of loops of wire, to the cross-section of each loop, and to the current passing through

7832-540: The wire. If the coil of wire is wrapped around a material with no special magnetic properties (e.g., cardboard), it will tend to generate a very weak field. However, if it is wrapped around a soft ferromagnetic material, such as an iron nail, then the net field produced can result in a several hundred- to thousandfold increase of field strength. Uses for electromagnets include particle accelerators , electric motors , junkyard cranes, and magnetic resonance imaging machines. Some applications involve configurations more than

7921-410: 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. A portative electromagnet is one designed to just hold material in place; an example

#23976