مغناطيس كهربائي

يجذب المغناطيس الكهربائي مشابك الورق عند تطبيق التيار مما يؤدي إلى إنشاء مجال مغناطيسي، ويفقدها عند إزالة التيار والمجال المغناطيسي

ينتج التيار (I) عبر سلك مجالاً مغناطيسياً (B). يتم توجيه الحقل وفقاً لـ قاعدة اليد اليمنى.

المغنطيس الكهربائي Electromagnet مغنطيس مؤقت يتكون فقط عندما يمر تيار كهربائي خلال سلك أو موصل آخر. تتكون معظم المغانط الكهربائية من سلك ملفوف حول قلب حديدي. يُصنع هذا القلب من حديد خفيف المغنطة، أي يفقد المغنطة بسرعة حالما يتوقف سريان التيار المار في السلك. وتقوم المغانط الكهربائية بتشغيل أجراس الأبواب ومرحلات التيار وتنتج المجالات المغنطيسية اللازمة لتشغيل المحركات والمولدات الكهربائية. وتُستخدم المغانط القوية لرفع الأجزاء الثقيلة من المخلفات الحديدية. وهناك مغانط كهربائية مصممة خصيصًا لإنتاج مجالات مغنطيسية بالغة القوة، تُستخدم لتوجيه الجسيمات الذرية في مسارات محددة في معجلات الجسيمات.

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تاريخ

مغناطيس سترجون الكهربائي، 1824

أحد مغناطيسات هنري الكهربائية التي يمكن أن ترفع مئات الجنيهات، في ثلاثينيات القرن التاسع عشر

صورة مقربة لمغناطيس هنري الكهربائي

اكتشف العالم الفيزيائي الدنماركي هانز اورستد في عام 1820، أن التيار الكهربائي يستطيع إنتاج مجال مغنطيسي. وأوضح الكهربائي الإنجليزي وِليام سترجون أن القلب المغنطيسي يقوي المجال المغنطيسي الناشئ عن ملف لولبي، فاخترع المغناطيس الكهربائي في 1824.^[1]^[2]كان أول مغناطيس كهربائي له عبارة عن قطعة حديد على شكل حدوة حصان ملفوفة بحوالي 18 لفة من الأسلاك النحاسية العارية (لم يكن سلك المعزول موجوداً بعد). تم عزل الحديد لعزله عن اللفات. عندما يمر تيار عبر الملف، يصبح الحديد ممغنطاً ويجذب قطعاً أخرى من الحديد؛ عندما يتوقف التيار، يفقد المغنطة. أظهر سترجون قوته من خلال إظهار أنه على الرغم من أنه يزن سبع أونصات فقط (حوالي 200 جرام)، إلا أنه يمكن أن يرفع تسعة أرطال (حوالي 4 كيلوغرامات) عندما يتم تطبيق التيار الكهربائي لخلية واحدة. ومع ذلك، كانت مغناطيسات سترجون ضعيفة لأن السلك غير المعزول الذي استخدمه لا يمكن إلا أن يتم لفه في طبقة واحدة متباعدة حول اللب، مما يحد من عدد الدورات.

وبدءاً من عام 1830، تمكن العالم الفيزيائي الأمريكي جوسف هنري، منهجياً، من تحسين وترويج المغناطيس الكهربائي.^[3]^[4] باستخدام الأسلاك المعزولة بخيوط الحرير، والمستوحاة من استخدام شڤايگر لفات متعددة من الأسلاك لعمل الگلڤانومتر،^[5]لقد كان قادراً على لف طبقات متعددة من الأسلاك على النوى، مما يخلق مغناطيساً قوياً بآلاف لفات الأسلاك، بما في ذلك واحد يمكنه دعم 936 kg. كان أول استخدام رئيسي للمغناطيسات الكهربائية في أجهزة التلغراف.

تم اقتراح نظرية المجال المغناطيسي لكيفية عمل النوى المغناطيسية لأول مرة في عام 1906 من قبل الفيزيائي الفرنسي پيير وايس، كما تم وضع النظرية الحديثة المفصلة لميكانيكا الكم للمغناطيسية الحديدية في عشرينيات القرن الماضي بواسطة ڤرنر هايزنبرگ، ليڤ لانداو، فليكس بلوك وآخرون.

إستخدامات المغناطيس الكهربائي

مغناطيس كهربائي صناعي لرفع الخردة، 1914

المغناطيس الكهربائي المحمول هو مغناطيس مصمم لمجرد تثبيت المواد في مكانها؛ مثال على ذلك هو مغناطيس الرفع. يطبق مغناطيس الجر قوة ويحرك شيئاً ما^[6]

المغناطيس الكهربائي يستخدم في العديد من الأجهزة الكهربائية ومنها:

المحركات و المولدات
المحولات
المرحلات
الأجراس الكهربائية والصفارات
مكبرات الصوت وسماعات الرأس
المشغلات مثل الصمامات
تخزين مغناطيسي ومعدات تخزين البيانات: مسجل شريط، VCR، الأقراص الصلبة
آلات MRI
المعدات العلمية مثل مطيافية الكتلة
معجلات الجسيمات
الأقفال المغناطيسية
مستقبلات الآلات الموسيقية
معدات الفصل المغناطيسي، والتي تستخدم لفصل المواد المغناطيسية عن المواد غير المغناطيسية، على سبيل المثال فصل المعادن الحديدية عن المواد الأخرى في الخردة.
مغناطيسات الرفع الصناعية
التعليق المغناطيسي، المستخدم في القطار المغناطيسي أو في القطارات العادية.
التسخين التحريضي للطبخ والتصنيع و العلاج بفرط الحرارة

مغناطيس كهربائي خاص للمختبر. تنتج مجال 2 T بتيار 20 أمبير.

المغناطيس في مطيافية الكتلة

مغناطيس كهربائي ذو تيار متردد على الجزء الثابت في المحركات الكهربائية

مغناطيس في الجرس الكهربائي

مغناطيس سكتپول البؤري في السنكروترون

Simple solenoid

Magnetic field produced by a solenoid. 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 $F$ is^[6]

F=CAnI/l

where $C$ is a proportionality constant, $A$ is the cross-sectional area of the plunger, $n$ 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).^[7] For example, a 12-inch long coil ( $l =12 in$ ) with a long plunger of 1-square inch cross section ( $A =1 in 2$ ) and 11,200 ampere-turns ( $n I =11,200 Aturn$ ) had a maximum pull of 8.75 pounds (corresponding to $C =0.0094 psi$ ).^[8]

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^[9]

P=AnI[(nI/l_{\mathrm {a} }^{2}C_{1}^{2})+(C/l)]=(An^{2}I^{2}/l_{\mathrm {a} }^{2}C_{1}^{2})+(CAnI/l)

Here $l a$ is the distance between 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, 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").^[10]^[11] The magnetic return path, just as the stop, has little impact until the air gap is small.

Physics

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

The magnetic field lines of a current-carrying loop of wire pass through the center of the loop, concentrating the field there

The 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 below). To concentrate the magnetic field, in an electromagnet the wire is wound into a coil with many turns of wire lying side by side.^[12] The magnetic field of all the turns of wire passes through the center of the coil, creating a strong magnetic field there.^[12] A coil forming the shape of a straight tube (a helix) is called a solenoid.^[13]^[12]

The direction of the magnetic field through a coil of wire can be found from a form of the right-hand rule.^[14]^[15] 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.

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.^[13]^[12]^[16]^[17] 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 the material.^[13]^[12] This is called a ferromagnetic-core or iron-core electromagnet. However, not all electromagnets use cores, and the very strongest electromagnets, such as superconducting and the very high current electromagnets, cannot use them due to saturation.

Ampere's law

For definitions of the variables below, see box at end of article.

The magnetic field of electromagnets in the general case is given by Ampere's Law:

\int \mathbf {J} \cdot d\mathbf {A} =\oint \mathbf {H} \cdot d\mathbf {l}

which says that the integral of the magnetizing field $\mathbf {H}$ around any closed loop is equal to the sum of the current flowing through the loop. Another equation used, that gives the magnetic field due to each small segment of current, is the Biot–Savart law. Computing the magnetic field and force exerted by ferromagnetic materials 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 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.

Magnetic core

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 iron core point in random directions, so their tiny magnetic fields cancel each other out, and 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 the field, and the magnetic field passes through the core more easily than 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.

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 the field disappears. However, some of the alignment persists, because the domains have difficulty turning their direction of magnetization, leaving the core 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, the core's magnetization is constantly reversed, and the remanence contributes to the motor's losses.

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Magnetic circuit – the constant B field approximation

Magnetic field (green) of a typical electromagnet, with the iron core C forming a closed loop with two air gaps G in it.
B – magnetic field in the core
B_F – "fringing fields". In the gaps G the magnetic field lines "bulge" out, so the field strength is less than in the core: B_F < B
B_L – leakage flux; magnetic field lines which don't follow complete magnetic circuit
L – average length of the magnetic circuit used in eq. 1 below. It is the sum of the length L_core in the iron core pieces and the length L_gap in the air gaps G.
Both the leakage flux and the fringing fields get larger as the gaps are increased, reducing the force exerted by the magnet.

In many practical applications of electromagnets, such as motors, generators, transformers, lifting magnets, and loudspeakers, the iron core is in the form of a loop or magnetic circuit, possibly broken by a few narrow air gaps.^[12] This is because the magnetic field lines are in the form of closed loops. 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.^[12]

Since most of the magnetic field is confined within the outlines of the core loop, this allows a simplification of the mathematical analysis.^[12] 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 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.^[12] This just 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 the outlines of the core before curving back to enter the next piece of core material, reducing the field strength in the gap.^[12] The bulges (B_F) are called fringing fields.^[12] 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 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. Therefore, the equations in this section are valid for electromagnets for which:

the magnetic circuit is a single loop of core material, possibly broken by a few air gaps
the core has roughly the same cross sectional area throughout its length.
any air gaps between sections of core material are not large compared with the cross sectional dimensions of the core.
there is negligible leakage flux

The main nonlinear feature of ferromagnetic materials is that the B field saturates at a certain value,^[12] which is around 1.6 to 2 teslas (T) for most high permeability core steels.^[18]^[19]^[20] The B field increases quickly with increasing current up to that value, but above that value the field levels off and becomes almost constant, regardless of how much current is sent through the windings.^[12] So the maximum strength of the magnetic field possible from an iron core electromagnet is limited to around 1.6 to 2 T.^[18]^[20]

Magnetic field created by a current

The magnetic field created by an electromagnet is proportional to both the number of turns in the winding, N, and the current in the wire, I, hence this product, NI, in ampere-turns, is given the name magnetomotive force. For an electromagnet with a single magnetic circuit, of which length L_core of the magnetic field path is in the core material and length L_gap is in air gaps, Ampere's Law reduces to:^[12]^[21]^[22]

NI=H_{\mathrm {core} }L_{\mathrm {core} }+H_{\mathrm {gap} }L_{\mathrm {gap} }\,

NI=B\left({\frac {L_{\mathrm {core} }}{\mu }}+{\frac {L_{\mathrm {gap} }}{\mu _{0}}}\right)\qquad \qquad \qquad \qquad (1)\,

where

\mu =B/H\,

is the magnetic permeability of the core material at the particular B field used.

\mu _{0}=4\pi (10^{-7})\ \mathrm {N} \cdot \mathrm {A} ^{-2}

is the permeability of free space (or air); note that

\mathrm {A}

in this definition is amperes.

This is a nonlinear equation, because the permeability of the core, μ, varies with the magnetic field B. For an exact solution, the value of μ at the B value used must be obtained from the core material hysteresis curve.^[12] 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 will be approximately the saturation value B_sat for the material, and won't 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, $\mu _{r}=\mu /\mu _{0}\approx 2000-6000\,$ .^[22] So in equation (1) above, the second term dominates. Therefore, in magnetic circuits with an air gap, the strength of the magnetic field B depends strongly on the length of the air gap, and the length of the flux path in the core doesn't 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.

Force exerted by magnetic field

The force exerted by an electromagnet on a section of core material is:

F={\frac {B^{2}A}{2\mu _{0}}}\qquad \qquad \qquad \qquad \qquad \qquad (2)\,

where $A$ is the cross-sectional area of the core. 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^[18]^[20] mentioned above sets a limit on the maximum force per unit core area, or magnetic pressure, an iron-core electromagnet can exert; roughly:

{\frac {F}{A}}={\frac {B_{sat}^{2}}{2\mu _{0}}}\approx 1000\ \mathrm {kPa} =10^{6}\mathrm {N/m^{2}} =145\ \mathrm {lbf} \cdot \mathrm {in} ^{-2}\,

In more intuitive units it's useful to remember that at 1 T the magnetic pressure is approximately 4 atmospheres, or kg/cm².

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.

Closed magnetic circuit

Cross section of lifting electromagnet like that in above photo, showing cylindrical construction. The windings (C) are flat copper strips to withstand the Lorentz force of the magnetic field. The core is formed by the thick iron housing (D) that wraps around the windings.

For a closed magnetic circuit (no air gap), such as would be found in an electromagnet lifting a piece of iron bridged across its poles, equation (1) becomes:

B={\frac {NI\mu }{L}}\qquad \qquad \qquad \qquad \qquad \qquad (3)\,

Substituting into (2), the force is:

F={\frac {\mu ^{2}N^{2}I^{2}A}{2\mu _{0}L^{2}}}\qquad \qquad \qquad \qquad \qquad (4)\,

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 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.

Force between electromagnets

The above methods are applicable to electromagnets with a magnetic circuit and do not apply when a large part of the magnetic field path is outside the core. An example would be a magnet with a straight cylindrical core like the one shown at the top of this article. For electromagnets (or permanent magnets) with well defined 'poles' where the field lines emerge from the core, the force between two electromagnets can be found using the a magnetic-charge model which assumes the magnetic field is produced by fictitious 'magnetic charges' on the surface of the poles, with pole strength m and units of Ampere-turn meter. Magnetic pole strength of electromagnets can be found from:

$m={\frac {NIA}{L}}$

The force between two poles is:

$F={\frac {\mu _{0}m_{1}m_{2}}{4\pi r^{2}}}$

Each electromagnet has two poles, so 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. This model assumes point-like poles instead of the finite surfaces, and thus it only yields a good approximation when the distance between the magnets is much larger than their diameter.

Side effects

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.

Ohmic heating

Large aluminum busbars carrying current into the electromagnets at the LNCMI (Laboratoire National des Champs Magnétiques Intenses) high field laboratory.

The only power consumed in a DC electromagnet under steady state conditions 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 = I²R, increases with 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 the ohmic losses. For this reason, electromagnets often have a significant thickness of windings.

However, the limit to increasing N or lowering the resistance is that the windings take up more room between the magnet's core pieces. If the area available for the windings is filled up, more turns require going to a smaller diameter of wire, which has higher resistance, which cancels the advantage of using more turns. So in large magnets there is a minimum amount of heat loss that can't be reduced. This increases with the square of the magnetic flux B².

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Inductive voltage spikes

An electromagnet has significant inductance, and resists changes in the current through its windings. Any sudden changes in the winding current cause large voltage spikes across the windings. This is because when the current through the magnet is increased, such as when it is turned on, energy from the circuit must be stored in the magnetic field. When it is turned off the energy in the field is returned to the circuit.

If an ordinary switch is used to control the winding current, this can cause sparks at the terminals of the switch. This does not occur when the magnet is switched on, because the limited supply voltage causes the current through the magnet and the field energy to increase slowly, but when it is switched off, the energy in the magnetic field is suddenly returned to the circuit, causing a large voltage spike and an arc across the switch contacts, which can damage them. With small electromagnets a capacitor is sometimes used across the contacts, which reduces arcing by temporarily storing the current. More often a diode is used to prevent voltage spikes by providing a path for the current to recirculate through the winding until the energy is dissipated as heat. The diode is connected across the winding, oriented so it is reverse-biased during steady state operation and does not conduct. When the supply voltage is removed, the voltage spike forward-biases the diode and the reactive current continues to flow through the winding, through the diode and back into the winding. A diode used in this way is called a freewheeling diode or flyback diode.

Large electromagnets are usually powered by variable current electronic power supplies, controlled by a microprocessor, which prevent voltage spikes by accomplishing current changes slowly, in gentle ramps. It may take several minutes to energize or deenergize a large magnet.

Lorentz forces

قالب:Unsourced section In powerful electromagnets, the magnetic field exerts a force on each turn of the windings, due to the Lorentz force $q\mathbf {v} \times \mathbf {B} \,$ acting on the moving charges within the wire. The Lorentz force is perpendicular to both the axis of the wire and the magnetic field. It can be visualized as a pressure between the magnetic field lines, pushing them apart. It has two effects on an electromagnet's windings:

The field lines within the axis of the coil exert a radial force on each turn of the windings, tending to push them outward in all directions. This causes a tensile stress in the wire.
The leakage field lines between each turn of the coil exert an attractive force between adjacent turns, tending to pull them together.^{[بحاجة لمصدر]}

The Lorentz forces increase with B². In large electromagnets the windings must be firmly clamped in place, to prevent motion on power-up and power-down from causing metal fatigue in the windings. In the Bitter design, below, used in very high-field research magnets, the windings are constructed as flat disks to resist the radial forces, and clamped in an axial direction to resist the axial ones.

Core losses

In alternating current (AC) electromagnets, used in transformers, inductors, and AC motors and generators, the magnetic field is constantly changing. This causes energy losses in their magnetic cores that is dissipated as heat in the core. The losses stem from two processes:

Eddy currents: From Faraday's law of induction, the changing magnetic field induces circulating electric currents inside nearby conductors, called eddy currents. The energy in these currents is dissipated as heat in the electrical resistance of the conductor, so they are a cause of energy loss. Since the magnet's iron core is conductive, and most of the magnetic field is concentrated there, eddy currents in the core are the major problem. Eddy currents are closed loops of current that flow in planes perpendicular to the magnetic field. The energy dissipated is proportional to the area enclosed by the loop. To prevent them, the cores of AC electromagnets are made of stacks of thin steel sheets, or laminations, oriented parallel to the magnetic field, with an insulating coating on the surface. The insulation layers prevent eddy current from flowing between the sheets. Any remaining eddy currents must flow within the cross-section of each individual lamination, which reduces losses greatly. Another alternative is to use a ferrite core, which is a nonconductor.
Hysteresis losses: Reversing the direction of magnetization of the magnetic domains in the core material each cycle causes energy loss, because of the coercivity of the material. These losses are called hysteresis. The energy lost per cycle is proportional to the area of the hysteresis loop in the BH graph. To minimize this loss, magnetic cores used in transformers and other AC electromagnets are made of "soft" low coercivity materials, such as silicon steel or soft ferrite. The energy loss per cycle of the AC current is constant for each of these processes, so the power loss increases linearly with frequency.

High-field electromagnets

Superconducting electromagnets

The most powerful electromagnet in the world, the 45 T hybrid Bitter-superconducting magnet at the US National High Magnetic Field Laboratory, Tallahassee, Florida, USA

When a magnetic field higher than the ferromagnetic limit of 1.6 T is needed, superconducting electromagnets can be used. Instead of using ferromagnetic materials, these use superconducting windings cooled with liquid helium, which conduct current without electrical resistance. These allow enormous currents to flow, which generate intense magnetic fields. Superconducting magnets are limited by the field strength at which the winding material ceases to be superconducting. Current designs are limited to 10–20 T, with the current (2017) record of 32 T.^[23]^[24] 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 nonsuperconducting electromagnets of a design invented by Francis Bitter in 1933, called Bitter electromagnets.^[25] 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 B². 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 37.5 T اعتبارا من 31 مارس 2014 (2014-03-31)^[تحديث], produced by a Bitter electromagnet at the Radboud University High Field Magnet Laboratory in Nijmegen, the Netherlands.^[26] The previous record was 35 T.^[24] The strongest continuous magnetic field overall, 45 T,^[25] was achieved in June 2000 with a hybrid device consisting of a Bitter magnet inside a superconducting magnet.

The factor limiting the strength of electromagnets is the inability to dissipate the enormous waste heat, so more powerful fields, up to 100 T,^[24] have been obtained from resistive magnets by sending brief pulses of high current through them; the inactive period after each pulse allows the heat produced during the pulse to be removed, before the next pulse.

Explosively pumped flux compression

A hollow tube type of explosively pumped flux compression generator. The hollow copper tube acts like a single turn secondary winding of a transformer; when the pulse of current from the capacitor in the windings creates a pulse of magnetic field, this creates a strong circumferential current in the tube, trapping the magnetic field lines within. The explosives then collapse the tube, reducing its diameter, and the field lines are forced closer together increasing the field.

The most powerful manmade magnetic fields^[27] 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 1000 T^[25] for a few microseconds. While this method may seem very destructive, it is possible to redirect the brunt of the blast radially outwards so that neither the experiment nor the magnetic structure are harmed. These devices are known as destructive pulsed electromagnets.^[28] They are used in physics and materials science research to study the properties of materials at high magnetic fields.

تعريف المصطلحات

$A\,$	square meter	cross section area of core
$B\,$	tesla	مجال مغناطيسي
$F\,$	newton	Force exerted by magnetic field
$H\,$	ampere per meter	Magnetizing field
$I\,$	ampere	Current in the winding wire
$L\,$	meter	Total length of the magnetic field path $L_{\mathrm {core} }+L_{\mathrm {gap} }\,$
$L_{\mathrm {core} }\,$	meter	Length of the magnetic field path in the core material
$L_{\mathrm {gap} }\,$	meter	Length of the magnetic field path air gap
$m_{1},m_{2}\,$	ampere meter	Pole strength of the electromagnets
$\mu \,$	newton per square ampere	Permeability of the electromagnet core material
$\mu _{0}\,$	newton per square ampere	Permeability of free space (or air) = 4π(10^-7)
$\mu _{r}\,$	-	Relative permeability of the electromagnet core material
$N\,$	-	Number of turns of wire on the electromagnet
$r\,$	meter	Distance between the poles of two electromagnets

انظر أيضاً

Dipole magnet - Electromagnet used in particle accelerators
كهرومغناطيسية
Electropermanent magnet - a magnetically hard electromagnet arrangement
Explosively pumped flux compression generator
Field coil
Magnetic bearing
Pulsed field magnet
Quadrupole magnet – a combination of magnets and electromagnets used mainly to affect the motion of charged particles

المصادر

الموسوعة المعرفية الشاملة

^ Sturgeon, W. (1825). "Improved Electro Magnetic Apparatus". Trans. Royal Society of Arts, Manufactures, & Commerce. 43: 37–52. cited in Miller, T.J.E (2001). Electronic Control of Switched Reluctance Machines. Newnes. p. 7. ISBN 978-0-7506-5073-1. Archived from the original on 2016-12-03.
^ Windelspecht, Michael. Groundbreaking Scientific Experiments, Inventions, and Discoveries of the 19th Century Archived 2017-01-11 at the Wayback Machine, xxii, Greenwood Publishing Group, 2003, ISBN 0-313-31969-3.
^ Cavicchi, Elizabeth. Series and Parallel Experimenting with Electromagnets. Pavia Project Physics, Univ. of Pavia, Italy. Archived from the original. You must specify the date the archive was made using the |archivedate= parameter. http://ppp.unipv.it/Collana/Pages/Libri/Saggi/Volta%20and%20the%20History%20of%20Electricity/V%26H%20Sect4/V%26H%20387-407.pdf. Retrieved on August 22, 2015.
^ Sherman, Roger (2007). "Joseph Henry's contributions to the electromagnet and the electric motor". The Joseph Henry Papers. The Smithsonian Institution. Archived from the original on 2012-06-08. Retrieved 2008-08-27.
^ "Schweigger Multiplier – 1820". Maglab. National High Magnetic Field Laboratory. Archived from the original on 17 October 2017. Retrieved 17 October 2017.
^ ^أ ^ب Dawes, Chester L. (1967). "Electrical Engineering". In Baumeister, Theodore (ed.). Standard Handbook for Mechanical Engineers (7th ed.). McGraw-Hill. p. 15-105.
^ Dawes 1967, p. 15-105–15-106
^ Dawes 1967, p. 15-106, Table 25
^ Dawes 1967, p. 15-106
^ Dawes 1967, p. 15-106
^ Underhill, Charles R. (1906). The Electromagnet. D. Van Nostrand. p. 113. Archived from the original on 2016-05-01.
^ ^أ ^ب ^ت ^ث ^ج ^ح ^خ ^د ^ذ ^ر ^ز ^س ^ش ^ص ^ض Merzouki, Rochdi; Samantaray, Arun Kumar; Pathak, Pushparaj Mani (2012). Intelligent Mechatronic Systems: Modeling, Control and Diagnosis. Springer Science & Business Media. pp. 403–405. ISBN 978-1447146285. Archived from the original on 2016-12-03.
^ ^أ ^ب ^ت Nave, Carl R. (2012). "Electromagnet". Hyperphysics. Dept. of Physics and Astronomy, Georgia State Univ. Archived from the original on September 22, 2014. Retrieved September 17, 2014.
^ Millikin, Robert; Bishop, Edwin (1917). Elements of Electricity. Chicago: American Technical Society. pp. 125.
^ Fleming, John Ambrose (1892). Short Lectures to Electrical Artisans, 4th Ed. London: E.& F. N. Spon. pp. 38–40. Archived from the original on 2017-01-11.
^ Gates, Earl (2013). Introduction to Basic Electricity and Electronics Technology. Cengage Learning. p. 184. ISBN 978-1133948513. Archived from the original on 2017-01-10.
^ Shipman, James; Jerry, Wilson; Todd, Aaron (2009). Introduction to Physical Science (12 ed.). Cengage Learning. pp. 205–206. ISBN 978-1111810283. Archived from the original on 2017-01-11.
^ ^أ ^ب ^ت "Saturation flux levels of various magnetic materials range up to 24.5 kilogauss" (2.5 T) p.1 "Silicon steel saturates at about 17 kilogauss" (1.7 T) p.3 Pauley, Donald E. (March 1996). "Power Supply Magnetics Part 1: Selecting transformer/inductor core material". Power Conversion and Intelligent Motion. Archived from the original on December 24, 2014. Retrieved September 19, 2014.
^ The most widely used magnetic core material, 3% silicon steel, has saturation induction of 20 kilogauss (2 T). "Material Properties, 3% grain-oriented silicon steel". Catalog. Magnetic Materials Co. 2013. p. 16. Archived from the original on September 20, 2014. Retrieved September 19, 2014.
^ ^أ ^ب ^ت "Magnetic steel fully saturates at about 2 T" Short, Thomas Allen (2003). Electric Power Distribution Handbook. CRC Press. p. 214. ISBN 978-0203486504.
^ Feynman, Richard P. (1963). Lectures on Physics, Vol. 2. New York: Addison-Wesley. pp. 36–9 to 36–11, eq. 36–26. ISBN 978-8185015842.
^ ^أ ^ب Fitzgerald, A.; Kingsley, Charles; Kusko, Alexander (1971). Electric Machinery, 3rd Ed. USA: McGraw-Hill. pp. 3–5.
^ "32 Tesla All-Superconducting Magnet". National High Magnetic Field Laboratory, USA. 2018.
^ ^أ ^ب ^ت "Mag Lab World Records". Media Center. National High Magnetic Field Laboratory, USA. 2008. Archived from the original on 2008-10-07. Retrieved 2008-08-31.
^ ^أ ^ب ^ت Coyne, Kristin (2008). "Magnets: from Mini to Mighty". Magnet Lab U. National High Magnetic Field Laboratory. Archived from the original on 2008-09-17. Retrieved 2008-08-31.
^ "HFML sets world record with a new 37.5 tesla magnet". High Field Magnet Laboratory. 31 March 2014. Archived from the original on 4 September 2015. Retrieved 21 May 2014.
^ "What is the strongest magnet in the world?". Apex magnets. November 2014. Archived from the original on February 5, 2017. Retrieved February 5, 2017.
^ Coyne, Kristin (2008). "7. Pulsed Magnets: Brief Shining Moments". Magnets from Mini to Mighty. National High Magnetic Field Laboratory. Archived from the original on 2014-12-20. Retrieved 2014-05-21.

وصلات خارجية

Magnets from Mini to Mighty: Primer on electromagnets and other magnets National High Magnetic Field Laboratory
Magnetic Fields and Forces Cuyahoga Community College
Fundamental Relationships School of Geology and Geophysics, University of Oklahoma

الكلمات الدالة:

المغناطيس الكهربائي

[1] Sturgeon, W. (1825). "Improved Electro Magnetic Apparatus". Trans. Royal Society of Arts, Manufactures, & Commerce. 43: 37–52. cited in Miller, T.J.E (2001). Electronic Control of Switched Reluctance Machines. Newnes. p. 7. ISBN 978-0-7506-5073-1. Archived from the original on 2016-12-03.

[2] Windelspecht, Michael. Groundbreaking Scientific Experiments, Inventions, and Discoveries of the 19th Century Archived 2017-01-11 at the Wayback Machine, xxii, Greenwood Publishing Group, 2003, ISBN 0-313-31969-3.

[Cavicchi-3] Cavicchi, Elizabeth. Series and Parallel Experimenting with Electromagnets. Pavia Project Physics, Univ. of Pavia, Italy. Archived from the original. You must specify the date the archive was made using the |archivedate= parameter. http://ppp.unipv.it/Collana/Pages/Libri/Saggi/Volta%20and%20the%20History%20of%20Electricity/V%26H%20Sect4/V%26H%20387-407.pdf. Retrieved on August 22, 2015.

[Sherman-4] Sherman, Roger (2007). "Joseph Henry's contributions to the electromagnet and the electric motor". The Joseph Henry Papers. The Smithsonian Institution. Archived from the original on 2012-06-08. Retrieved 2008-08-27.

[5] "Schweigger Multiplier – 1820". Maglab. National High Magnetic Field Laboratory. Archived from the original on 17 October 2017. Retrieved 17 October 2017.

[Marks_105-6] أ ^ب Dawes, Chester L. (1967). "Electrical Engineering". In Baumeister, Theodore (ed.). Standard Handbook for Mechanical Engineers (7th ed.). McGraw-Hill. p. 15-105.

[Marks_105-6-7] Dawes 1967, p. 15-105–15-106

[8] Dawes 1967, p. 15-106, Table 25

[9] Dawes 1967, p. 15-106

[10] Dawes 1967, p. 15-106

[11] Underhill, Charles R. (1906). The Electromagnet. D. Van Nostrand. p. 113. Archived from the original on 2016-05-01.

[Merzouki-12] أ ^ب ^ت ^ث ^ج ^ح ^خ ^د ^ذ ^ر ^ز ^س ^ش ^ص ^ض Merzouki, Rochdi; Samantaray, Arun Kumar; Pathak, Pushparaj Mani (2012). Intelligent Mechatronic Systems: Modeling, Control and Diagnosis. Springer Science & Business Media. pp. 403–405. ISBN 978-1447146285. Archived from the original on 2016-12-03.

[Hyperphysics-13] أ ^ب ^ت Nave, Carl R. (2012). "Electromagnet". Hyperphysics. Dept. of Physics and Astronomy, Georgia State Univ. Archived from the original on September 22, 2014. Retrieved September 17, 2014.

[14] Millikin, Robert; Bishop, Edwin (1917). Elements of Electricity. Chicago: American Technical Society. pp. 125.

[15] Fleming, John Ambrose (1892). Short Lectures to Electrical Artisans, 4th Ed. London: E.& F. N. Spon. pp. 38–40. Archived from the original on 2017-01-11.

[Gates-16] Gates, Earl (2013). Introduction to Basic Electricity and Electronics Technology. Cengage Learning. p. 184. ISBN 978-1133948513. Archived from the original on 2017-01-10.

[Shipman-17] Shipman, James; Jerry, Wilson; Todd, Aaron (2009). Introduction to Physical Science (12 ed.). Cengage Learning. pp. 205–206. ISBN 978-1111810283. Archived from the original on 2017-01-11.

[Pauley-18] أ ^ب ^ت "Saturation flux levels of various magnetic materials range up to 24.5 kilogauss" (2.5 T) p.1 "Silicon steel saturates at about 17 kilogauss" (1.7 T) p.3 Pauley, Donald E. (March 1996). "Power Supply Magnetics Part 1: Selecting transformer/inductor core material". Power Conversion and Intelligent Motion. Archived from the original on December 24, 2014. Retrieved September 19, 2014.

[MagneticMaterials-19] The most widely used magnetic core material, 3% silicon steel, has saturation induction of 20 kilogauss (2 T). "Material Properties, 3% grain-oriented silicon steel". Catalog. Magnetic Materials Co. 2013. p. 16. Archived from the original on September 20, 2014. Retrieved September 19, 2014.

[Short-20] أ ^ب ^ت "Magnetic steel fully saturates at about 2 T" Short, Thomas Allen (2003). Electric Power Distribution Handbook. CRC Press. p. 214. ISBN 978-0203486504.

[21] Feynman, Richard P. (1963). Lectures on Physics, Vol. 2. New York: Addison-Wesley. pp. 36–9 to 36–11, eq. 36–26. ISBN 978-8185015842.

[Fitzgerald-22] أ ^ب Fitzgerald, A.; Kingsley, Charles; Kusko, Alexander (1971). Electric Machinery, 3rd Ed. USA: McGraw-Hill. pp. 3–5.

[nationalmaglab-23] "32 Tesla All-Superconducting Magnet". National High Magnetic Field Laboratory, USA. 2018.

[MagnetLab-24] أ ^ب ^ت "Mag Lab World Records". Media Center. National High Magnetic Field Laboratory, USA. 2008. Archived from the original on 2008-10-07. Retrieved 2008-08-31.

[MagnetLabU-25] أ ^ب ^ت Coyne, Kristin (2008). "Magnets: from Mini to Mighty". Magnet Lab U. National High Magnetic Field Laboratory. Archived from the original on 2008-09-17. Retrieved 2008-08-31.

[Dutch_record-26] "HFML sets world record with a new 37.5 tesla magnet". High Field Magnet Laboratory. 31 March 2014. Archived from the original on 4 September 2015. Retrieved 21 May 2014.

[Apex-27] "What is the strongest magnet in the world?". Apex magnets. November 2014. Archived from the original on February 5, 2017. Retrieved February 5, 2017.

[MagnetLab-pulsed-28] Coyne, Kristin (2008). "7. Pulsed Magnets: Brief Shining Moments". Magnets from Mini to Mighty. National High Magnetic Field Laboratory. Archived from the original on 2014-12-20. Retrieved 2014-05-21.

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