-   MAGNETIZING YOUR FUTURE

FAQ overview

Tekniske spørgsmål til magneter
  • Hvad afgør den mængde af magnetisk materiale, som jeg har brug for?

    Det gør energiproduktet BHmax. Jo større energiproduktet er, jo mindre magnet har du brug for.

  • Hvad bestemmer en magnets trækkraft?

    <p>Det gør remanencen Br. Gererelt gælder det, at jo større remanens jo større trækkraft.</p>

  • Hvad bestemmer, hvorvidt en magnet vil forblive magnetisk, når du anvender den?

    Det gør “den tvingende kraft” Hc. Jo større Hc er, jo større er magnetens modstand mod demagneti-sering. Magnet type Br Hc BHmax (Gauss) (Oersteds) (gauss-Oersteds) Fleksibel 1,725 1,325 0.6 Ferrit (keramisk) 2,200 1,900 1.1 SmCo 8,600 7,200 18 NdFeB 35 12,300 11,300 35 Skemaet viser typiske størrelsesordner af konstanter for forskellige magnettyper.

  • Hvad er BHmax - det maksimale energiprodukt?

    BHmax (enhed er Gauss-Oersteds) indikerer hvilken volumen af magnetisk materiale, der skal til, for at skabe en bestemt magnetisk flux.

  • Hvad er Br – remanensen, eller restinduktionen?

    Br (måles i Gauss eller Tesla) indikerer, hvor stærk magneten kan blive og hvor stor trækkraften er.

  • Hvad er Curie-temperaturen?

    Magnetiseringen aftager med stigende temperatur. Når temperaturen overstiger en vis grænse, den såkaldte Curie-temperatur som karakteriserer det pågældende materiale, falder magnetiseringen til nul. For jern er Curie-temperaturen 1.043 grader C.

  • Hvad er diamagnetisme?

    Når et materiale placeres i et ydre magnetfelt, vil der i materialet induceres strømme som genererer et modfelt. Fænomenet betegnes diamagnetisme.


    Hvis man placerer et stykke superledende materiale i et ydre magnetfelt som ikke er for stærkt, vil der i superlederen induceres elektriske strømme som giver ophav til et magnetfelt der er lige så stærkt og modsat rettet det ydre magnetfelt. I den forstand er en superleder en perfekt diamagnet. Superlederes evne til at fortrænge et ydre magnetfelt fuldstændigt kaldes Meissner-effekten. Den gør det bl.a. muligt for et lille stykke superleder at svæve i et ydre magnetfelt.

  • Hvad er elektromagneter?

    Elektromagneter fremstilles ved at placere en jernkerne (for det meste en jernlegering) i en spole, hvor der i vindingerne transporteres en elektrisk strøm. Elektriciteten i spolen skaber et magnetisk felt, som ledes gennem jernkernen. Dets styrke afhænger af strømstyrken og antallet af vinder i spo-len. Polariteten bestemmes af retningen af den elektriske strøm. Når strømmen påtrykkes opfører jernkernen sig som en magnet, men når strømmen afbrydes, mister jernkernen sine magnetiske egenskaber.

  • Hvad er en ikke-permanent magnet?

    Jern og visse jernlegeringer magnetiseres let, selv i et svagt magnetfelt. Så snart magnetfeltet fjernes mister jernet sin magnetisme. Jern og visse jernlegeringer er derfor gode ikke-permanente magneter, som kan bruges i f.eks. telefoner og elektromotorer.

  • Hvad er en magnetisk dipol?

    En meget almindelig kilde til magnetisme i naturen er en magnetisk dipol med en sydpol og en nordpol. Nordpolen på en given magnet tiltrækkes af sydpolen på en anden magnet og sydpolen af denne tiltrækkes af nordpolen på en tredje osv. 

  • Hvad er en permanent magnet?

    En permanent magnet er en magnet fremstillet af et materiale, som forbliver magnetisk. Permanente magneter er lavet af ”hårde” ferromagnetiske materialer.

  • Hvad er et magnetfelt?

    Et magnetfelt indeholder energi, og i fysikken bevæger materialer eller tilstande med højere energi sig mod materiale eller tilstande med lavere energi. En magnetisk dipol i et magnetfelt oplever et vridningsmoment (dvs. magnetfelt bevirker faktisk, at magneten tvinges til at rotere om sin egen akse, hvis det er muligt). Dette udtrykkes vha. feltstyrken og det magnetiske dipolemoment.

  • Hvad er Ferrimagnetisme?

    I visse materialer er uparrede elektronspin orienteret antiparallelt på en sådan måde at de delvis ud-ligner hinandens magnetfelt. Fænomenet betegnes ferrimagnetisme. Jernmineralet magnetit er ferrimagnetisk.

  • Hvad er Ferritmagneter?

    Ferritmagneter er en gruppe af kemiske stoffer med formlen AB2O4, hvor A and B repræsenterer forskellige positivt ladede metalioner for det meste jernione). Ferritmagneter adskiller sig fra andre magneter.

  • Hvad er ferromagnetisme?

    Visse materialer kan magnetiseres permanent. Det gælder bl.a. jern, nikkel og kobolt. Fænomenet kaldes ferromagnetisme. Det skyldes at uparrede elektronspin orienterer sig parallelt inden for små områder, de såkaldte domæner. Magnetiseringen i de forskellige domæner peger i forskellige retninger, men når det ferromagnetiske materiale placeres i et ydre magnetfelt, magnetiseres det i samme retning som det ydre felt. Det skyldes dels at domæner drejer sig, dels at de domæner som i forvejen vender rigtigt, vokser på bekostning af de øvrige.


    Når alle domæner peger i samme retning, kan magnetiseringen ikke øges yderligere. Mætnings-magnetiseringen yder typisk et bidrag til den magnetiske fluxtæthed der er tusindvis af gange større end det ydre felt som afstedkom magnetiseringen. Herpå beror anvendelsen af jernkerner i elektro-magneter.

  • Hvad er iHc - ”Den tvingende kraft” (Coercive Force)?

    iHc (enhed er Oersteds) indikerer, hvor vanskeligt det er at demagnetisere magneten.

  • Hvad er magnetisk mætning?

    Magnetisk mætning er, når den tilstand, hvor det felt, der påføres en magnet fra ekstern kilde, ikke kan magnetisere magneten yderligere. Dette ses ofte i ferro-magnetiske materialer.

  • Hvad er Néel-temperaturen?

    Den magnetiske orden aftager med stigende temperatur. Når temperaturen overstiger en vis grænse, den såkaldte Néel-temperatur som karakteriserer det pågældende materiale, er den magnetiske or-den helt forsvundet.

  • Hvad er paramagnetisme?

    Visse stoffer magnetiseres midlertidigt når de placeres i et ydre magnetfelt. De tiltrækkes af en permanent magnet. Fænomenet betegnes paramagnetisme. Det optræder i stoffer som indeholder atomer eller nanopartikler med permanente magnetiske momenter.

  • Hvad er sjældne jordmagneter?

    Sjældne jordmagneter er stærke permanente magneter, som er fremstillet af legeringer af sjældne grundstoffer fra jordens overflade. Sjældne jordmagneter er stærkere end ferrit- og AlNiCo-magneter. Sjældne jordmagneter er ekstremt sprøde, hvorfor de tit indlejres i nikkel for at beskytte dem mod at gå i stykker.

  • Hvordan kan man bestemme en magnets nordpol, hvis ikke man kender den i forvejen?

    Man kan ikke umiddelbart se det. Hvis man derimod placerer et kompas tæt på magneten, vil kom-passets nål, som normalt peger mod jordens nordpol pege mod magnetens sydpol.

  • Hvordan måler man styrken af et magnetfelt?

    Som mål for styrken af et magnetfelt haves den magnetiske fluxtæthed. Ligningen F = qBv define-rer den magnetiske fluxtæthed. Den angiver kraften (F) på en partikel med ladning q der bevæger sig med hastigheden v i et felt hvor den magnetiske fluxtæthed er B. Af ligningen fremgår bl.a. at kraftpåvirkningen er nul hvis partiklen er i hvile. Kraften virker i øvrigt vinkelret på både hastighed og magnetfelt.
    Den magnetiske fluxtæthed måles i tesla (T). 1 T er en stor enhed. De kraftigste elektromagneter kan generere magnetiske fluxtætheder på mellem 10 og 100 tesla. Jordens magnetfelt er langt svagere, nemlig af størrelsesorden 70 mikrotesla ved polerne.

  • Hvorfor bruge Neodymium magneter?

    Fordi de har den største remanens, coercivitet (tvingende kraft, Hc) og BHmax af alle magneter, som er tilgængelige på markedet. Neodymiummagneten er en sjælden jordmagnet.

  • Kan en magnet, som har mistet sin magnetisme blive re-magnetiseret?

    Ja, det kan de godt, hvis de ikke er blevet ødelagt af ekstreme temperaturer.

  • Kan magneter miste deres kraft over tid?

    Nej, moderne magneter mister kun meget lidt kraft over tid. For SmCo-magneter drejer det sig typisk om 1% over en tiårs periode.

English encyclopedia
  • Air Gap

    A low permeability gap in the flux path of a magnetic circuit. Often air, but inclusive of other materials such as paint, aluminum, etc.

  • Anisotropic Magnet

    A magnet having a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in one preferred direction.

  • Closed Circuit

    This exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit.

  • Coercive Force, Hc

    The demagnetizing force, measured in Oersteds, necessary to reduce observed induction, B, to zero after the magnet has previously been brought to saturation.

  • Curie Temperature

    TcThe temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the material is no longer able to hold magnetization.

  • Demagnetization Curve

    The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use. Also known as the B-H Curve.

  • Eddy Currents

    Circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux. Eddy currents can be harnessed to perform useful work (such as damping of movement), or may be unwanted consequences of certain designs, which should be accounted for or minimized.

  • Electromagnet

    A magnet, consisting of a solenoid with an iron core, which has a magnetic field existing only during the time of current flow through the coil.

  • Energy Product

    Indicates the energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetization curve. Calculated as Bd x Hd, and measured in Mega Gauss Oersteds, MGOe.

  • Ferromagnetic Material

    A material whose permeability is very much larger than 1 (from 60 to several thousand times 1), and which exhibits hysteresis phenomena.

  • Flux

    The condition existing in a medium subjected to a magnetizing force. This quantity is characterized by the fact that an electromotive force is induced in a conductor surrounding the flux at any time the flux changes in magnitude. The cgs unit of flux is the Maxwell.

  • Fluxmeter

    An instrument that measures the change of flux linkage with a search coil.

  • Fringing Fields

    Leakage flux particularly associated with edge effects in a magnetic circuit.

  • Gauss

    Lines of magnetic flux per square centimeter, cgs unit of flux density, equivalent to lines per square inch in the English system, and Webers per square meter or Tesla in the SI system.

  • Gaussmeter

    An instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one of the following: the Hall effect, nuclear magnetic resonance (NMR), or the rotating coil principle.

  • Hysteresis Loop

    A closed curve obtained for a material by plotting corresponding values of magnetic induction, B, (on the abscissa) against magnetizing force, H, (on the ordinate).

  • Induction, B

    The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the cgs system of units.

  • Intrinsic Coercive Force, Hci

    Measured in Oersteds in the cgs system, this is a measure of the material's inherent ability to resist demagnetization. It is the demagnetization force corresponding to zero intrinsic induction in the magnetic material after saturation. Practical consequences of high Hci values are seen in greater temperature stability for a given class of material, and greater stability in dynamic operating conditions.

  • Intrinsic Induction, Bi

    The contribution of the magnetic material to the total magnetic induction, B. It is the vector difference between the magnetic induction in the material and the magnetic induction that would exist in a vacuum under the same field strength, H. This relationship is expressed as: BI = B-H.

  • Irreversible Loss

    Defined as the partial demagnetization of a magnet caused by external fields or other factors. These losses are only recoverable by re-magnetization. Magnets can be stabilized to prevent the variation of performance caused by irreversible losses.

  • Isotropic Magnet

    A magnet material whose magnetic properties are the same in any direction, and which can therefore be magnetized in any direction without loss of magnetic characteristics.

  • Keeper

    A piece of soft iron that is placed on or between the poles of a magnet, decreasing the reluctance of the air gap and thereby reducing the flux leakage from the magnet.

  • Knee of the Demagnetization Curve

    The point at which the B-H curve ceases to be linear. All magnet materials, even if their second quadrant curves are straight line at room temperature, develop a knee at some temperature. Alnico 5 exhibits a knee at room temperature. If the operating point of a magnet falls below the knee, small changes in H produce large changes in B, and the magnet will not be able to recover its original flux output without re-magnetization.

  • Leakage Flux

    That portion of the magnetic flux that is lost through leakage in the magnetic circuit due to saturation or air-gaps, and is therefore unable to be used.

  • Length of air-gap, Lg

    The length of the path of the central flux line in the air-gap.

  • Load Line

    A line drawn from the origin of the Demagnetization Curve with a slope of -B/H, the intersection of which with the B-H curve represents the operating point of the magnet. Also see Permeance Coefficient.

  • Magnetic Circuit

    An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.

  • Magnetizing Force, H

    The magnetomotive force per unit length at any point in a magnetic circuit. Measured in Oersteds in the cgs system.

  • Magnetomotive Force, F

    Analogous to voltage in electrical circuits, this is the magnetic potential difference between any two points.

  • Maximum Energy Product, BHmax

    The point on the Demagnetization Curve where the product of B and H is a maximum and the required volume of magnet material required to project a given energy into its surroundings is a minimum. Measured in Mega Gauss Oersteds, MGOe.

  • North Pole

    That pole of a magnet which, when freely suspended, would point to the north magnetic pole of the earth. The definition of polarity can be a confusing issue, and it is often best to clarify by using "north seeking pole" instead of "north pole" in specifications.

  • Oersted, Oe

    A cgs unit of measure used to describe magnetizing force. The English system equivalent is Ampere Turns per Inch, and the SI system's is Ampere Turns per Meter.

  • Orientation Direction

    The direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties. Also known as the "axis", "easy axis", or "angle of inclination".

  • Paramagnetic Material

    A material having a permeability slightly greater than 1.

  • Permeance Coefficient,Pc

    Ratio of the magnetic induction, BD, to its self demagnetizing force, HD PC = BD / HD This is also known as the "load line", "slope of the operating line", or operating point of the magnet, and is useful in estimating the flux output of the magnet in various conditions. As a first order approximation, BD / HD = Lm/Lg, where Lm is the length of the magnet, and Lg is the length of an air gap that the magnet is subjected to. PC is therefore a function of the geometry of the magnetic circuit.

  • Pole Pieces

    <p>Ferromagnetic materials placed on magnetic poles used to shape and alter the effect of lines of flux.</p>

  • Relative Permeability

    The ratio of permeability of a medium to that of a vacuum. In the cgs system, the permeability is equal to 1 in a vacuum by definition. The permeability of air is also for all practical purposes equal to 1 in the cgs system.

  • Reluctance, R

    Analogous to resistance in an electrical circuit, reluctance is related to the magnetomotive force, F, and the magnetic flux by the equation R = F/(Magnetic Flux), paralleling Ohm's Law where F is the magnetomotive force (in cgs units).

  • Remanence, BD

    The magnetic induction that remains in a magnetic circuit after the removal of an applied magnetizing force. If there is an air gap in the circuit, the remanence will be less than the residual induction, Br.

  • Residual Induction, Br

    This is the point at which the hysteresis loop crosses the B axis at zero magnetizing force, and represents the maximum flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials.

  • Return Path

    Conduction elements in a magnetic circuit which provide a low reluctance path for the magnetic flux

  • Reversible Temperature Coefficient

    A measure of the reversible changes in flux caused by temperature variations.

  • Saturation

    The condition under which all elementary magnetic moments have become oriented in one direction. A ferromagnetic material is saturated when an increase in the applied magnetizing force produces no increase in induction. Saturation flux densities for steels are in the range of 16,000 to 20,000 Gauss.

  • Search Coil

    A coil conductor, usually of known area and number of turns that is used with a fluxmeter to measure the change of flux linkage with the coil.

  • Stabilization

    Exposure of a magnet to demagnetizing influences expected to be encountered in use in order to prevent irreversible losses during actual operation. Demagnetizing influences can be caused by high or low temperatures, or by external magnetic fields.

  • Temperature Coefficient

    A factor, which describes the change in a magnetic property with change in temperature. Expressed as percent change per unit of temperature.

  • Weber

    The practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric circuit during an interval of 1 second, will induce in this circuit an electromotive force of 1 volt.

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