FAQ

Magnets FAQ

In an electromagnet the magnetic field is created through electric current in a wire-wound coil and strengthened by a soft-iron core. As soon as you turn off the power, the soft-iron core loses its magnetisation.
A permanent magnet is made of ferromagnetic material, which is magnetised by a strong external magnetic field. The magnetically hard material that is used keeps part of its magnetisation after the external magnetic field is turned off.

The attraction between magnets is a little stronger than the repulsion. That is due to the alignment of the molecular magnets in the magnet. The attraction as well as the repulsion of magnets decrease significantly with increasing distance.

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

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.

Remanence Br is a measurement for the magnetic induction or magnetic flux density that, after successful magnetisation, remains in the magnet. Simply said: the higher this value is, the "stronger" the magnet. T (Tesla) is used as the unit of measurement for magnetic induction and, respectively, magnetic flux density. The unit of measurement previously used was G (Gauss). 1 Tesla = 10 000 Gauss.

Coercive force Hc describes the force that is necessary to completely demagnetise a magnet. Simply said: the higher this number is, the better a magnet retains its magnetism when exposed to an opposing magnetic field. There are differences between the Coercive Force Hcb of flux density and the Intrinsic Coercive Force Hcj of polarisation. If a magnet is exposed to a demagnetising field strength of Hcb, the magnetic flux density in the magnet disappears. The magnet itself is still magnetic, but the flux density that the magnet generated is contrarily exactly the same size as the flux density of the demagnetised field, so that the two cancel each other out. The magnet only loses its magnetic polarisation, and thus its total magnetism, by a demagnetising field strength of Hcj.
The standard unit of measurement used for a magnetic field strength is A/m (Ampere per metre). You will also often see the old standard measurement, Oe (Oersted).

The maximum energy product is a measurement for the maximum amount of magnetic energy stored in a magnet. It is the maximally attainable product of flux density B and field strength H for a material. The unit of measurement is kJ/m³ (Kilojoule per cubic meter) or MGOe (Mega-Gauss-Oersted).
Simply put, the maximum energy product is an indicator of magnet strength. You can either use a small magnet with a higher energy product or a large magnet with a lower energy product for the same application.

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

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

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.

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.

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".

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

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.

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

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

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).

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.

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.

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.

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

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.

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

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.

The magnetic flux density of a magnet is also called "B field" or "magnetic induction". It is measured in tesla (SI unit) or gauss (10 000 gauss = 1 tesla).
A permanent magnet produces a B field in its core and in its external surroundings. A B field strength with a direction can be attributed to each point within and outside of the magnet. If you position a small compass needle in the B field of a magnet, it orients itself toward the field direction. The justifying force is proportional to the strength of the B field.
There are no simple formulas that calculate this field for the various magnetic shapes. Computer programs were developed for that purpose There are simple formulas for less complex symmetrical geometries, which indicate the B field on a symmetry axis in north-south pole direction. Subsequently, we are glad to share these with you.

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

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

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.

A material having a permeability slightly greater than 1.

The inverse of reluctance, analogous to conductance in electrical circuits.

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.

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

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.

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).

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.

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.

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.

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.

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

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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