Magnets

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Magnetism is covered by most physics curricula. Both at GCSE and A level. The topic can be very mathematically complicated . Fortunately, for a technician, the involvement is limited and only a few procedures are needed.

What is magnetism?

Actually the full theory is very complicated and beyond the scope of this article. Even if we were to restrict our question to “which materials are magnetic?”, this can be a difficult question to answer.
Some materials are attracted to magnets easier than others. Some retain their magnetism and some don't. Some materials are even repelled by magnets. Generally magnetism can be thought of as how a material reacts to a magnetic field. It is hoped that this article will help explain some of the various forms of magnetism, how to re-magnetise a weak magnet, how to demagnetise a magnet, precautions of storing, types of magnets and link to a few suggestions for practical investigations.

Simple Field Plotting

There are various methods of visualising the magnetic fields

Bviewers.jpg
  • Iron filings. These should be fine, and dispensed through a shaker. Simply place a sheet of paper over the magnets of which you wish to study and shake the filings onto the paper (it may be necessary to place a thin sheet of hardboard between the magnet and paper to keep a level surface . Hairspray or photo-mounting spray may be used to keep the patterns. Remove carefully so as not to spill non fixed filings onto the magnets. Any spillages onto the magnet may be removed with "Blu-tack" or similar material. Traditionally wax coated paper was used to hold the filings and these would hold the pattern as the wax set.
  • Magnetic field viewers. These are iron filings held within a transparent case containing an inert liquid. The 2D version contain water and 3D version contain oil. These are a lot less messy than Iron filings!. Water can be introduced and the 2D viewer re sealed if they become convex on storage.
  • Plotting compasses. These provide a simple directional method of tracing the field lines. A compass is used to follow the field line in a point to point method. The position of "North seeking pole" of the compass is marked on paper, then the compass moved so that the "South seeking pole" is placed over the point on the paper that the "North seeking pole" was. This is carried on until the magnet is reached (or paper runs out).

OHP compasses may be used as a class demo.

  • Magnetic Field Demonstrator. These are short iron bars held in a circular cell that align easily to the field. Quick and effective, these are ideal for projection.


Types of magnet

There are several types of magnets normally encountered in schools. A large Horseshoe, smaller bar magnets, rod magnets, ceramic or slab magnets, shaped magnets and rare earth magnets are all frequently encountered. Rod and bar magnets are made out of various high permeability materials under trade names like "Magnadur" and "Alcomax". Magnets often come with “keepers” which are pieces of “soft” iron that links one pole to another. Magnets are fragile. They require careful handling and storage.

typical magnets used in school


Compasses/ gimballed magnets

A compass is a special case of a bar magnet on a pivot. These may be used to illustrate the field of a magnet and how they interact. These are used in classic experiments such as Oersted's experiment where the field resulting from a current carrying conductor is shown. A gimballed magnet is a bar magnet able to pivot in the x, y and z planes.
For plotting compasses there are occasions when they face the "wrong way" see:Realigning a compass.


Soft and Hard magnetic materials

This does not refer to the tensile strength of the material, but to how the material retains magnetism after being placed in a magnetic field. Hysteresis curves for materials can show the difference but a much simpler demonstration can be used. Current limits on the coil restrict which laboratory coils may be used. It may be necessary to build one, but for most purposes the 240 coils of the “Westminster” transformer kits.
See also Magnetic Hardness


Uses of magnets

Magnets are used in several physics experiments. They are commonly used in schools, to show field lines pick up “magnetic” objects, provide an attractive or repulsive force, show the effect of breaking or eddy currents, or cause a current to be deflected. Field line plotting may be done using iron filings, or plotting compasses. Field viewers are available that have iron filings in a clear liquid (2 dimensional and 3 dimensional versions exist). Another commercial alternative is a set of encapsulated small bars that are free to orientate with the field.


Demagnetising a magnet

All to often this is done by accident , for example by dropping. A bar may be struck on an East-West orientation and eventually, it will reduce to zero. This is neither consistent nor convenient. However to get a magnet to do this in consistently a reducing magnetic field needs to be set up in a coil. What actually needs to happen is the hysteresis cycle gradually being reduced to zero. The initial magnetic field must be taken beyond the field strength of the magnet and then cycled down to zero. See Magnetization and Demagnetization

Re-magnetising a magnet

There are commercial magnetising devices available through most scientific suppliers. CLEAPSS also has an outline of how to build one. A solenoid of many coils of wire need to have a high d.c. current passed though them so as to create a high magnetic field. The wire size will limit the current that can be passed. Various methods can be employed to give the high current. A Capacitor discharge is possibly the neatest, especially if a high voltage is used. NOTE this is potentially dangerous. The coil will also need a protective diode to stop “back E.M.F”. Switching wth a relay or normal switch may prove troublesome through arcing so a SCR or thyristor could be employed.
The magnet may also be re-magnetised by stroking or being heated in a strong flame then placed in a strong field and allowed to cool below the "Curie Temperature".See Magnetization and Demagnetization

Removing Iron Filings

Often, after a class practical the magnets become covered in iron filings. Some can be removed easily, but there is frequently a substantial quantity that remains.A sacrificial lump of "blutak" or, better plasticine may be employed to collect the sticking filings. Just roll the warm plasticine over the magnet, folding into itself to present a clean surface to the next part. Eventually the magnet should be filing free.


Other sources of information:

CLEAPSS laboratory handbook Chapter 12.22. And for information on domain theory:
types of magnetism
B-H curves



The different forms of magnetism

This section may be skipped as it is gives technical definitions, it may be of use in understanding some of the differences in behaviour. However it suffers a bit from the stamp collecting syndrome of creating classifications within classifications. There are five types of magnetism:

  • Ferromagnetism
  • Ferrimagnetism
  • Antiferromagnetism
  • Paramagnetism
  • Diamagnetism

The differences are explained by domain theory.
The alignment of magnetic fields in the domain boundary within the body of the material influences the strength of the external magnetic field. The normal magnets used in a school are ferri- and ferro-magnets
Ferromagnetism has all the domains spontaneously aligned parallel to each other.
Ferrimagnetism is similar but with only a net quantity of domains aligned parallel in excess of the anti-parallel domains.
Antiferromagnetism is where the there is a net magnetic domain balance.
Paramagnetism is where a magnetic field produces a positive (same direction) alignment of domains whilst the magnetic field is applied. This is a weak effect.
Diamagnetism is the opposite of paramagnetism, i.e. the magnetic field repels the material. Again this is a weak effect.


Diamagnetism Demo

This can be demonstrated in a few of ways. The most spectacular is the "superconductivity" demonstration of the Meissner Effect. This is discussed in the Liquid Nitrogen Demos. Pyrolytic graphite is, probably the next most spectacular to use (over rare earth magnets), but most schools will not normally have access to this. A short (10cm rod of glass) is suspended from a clamp stand via a length of fishing line, so that it lies with the length parallel to the ground. When a rare earth magnet is brought close to it, it will move slowly away from the magnet.

Curie Temperature Demo

The Curie temperature of a ferromagnetic substance is the temperature above which it ceases being ferromagnetic and becomes paramagnetic. That is: it appears to lose it's "magnetism". Upon cooling it will revert to the ferromagnetic nature and, if left in a magnetic field, may become permanently magnetised.
This property can be shown by heating a ferromagnetic sample held, by a suitable pivot, near a permanent magnet. In order not to damage the permanent magnet, some heat shielding such as a piece of heat mat, is placed between the sample and the magnet. The pivot should be such that the sample would, in the absence of the permanent magnet fall away.
M.I.T. have a "youtube" clip which illustrates this by professor Walter Lewin
A modern alternative is to use a smart material. Mindset (formally MUTR) have a filament of Gadolinium (Rare earth metal) in polythene: http://www.mindsetsonline.co.uk/product_info.php?products_id=1010046 .
This "smart cord" may be used to demonstrate the "Curie point" at safer temperatures.
As Gadolinium's Curie point is at about room temperature, and the density of the cord is less than that of water. The use of two cups, one of hot water and one of ice water, allows a quick demonstration.
Two short lengths are cut from the filament of about 3 cm. These are then placed, one in the hot cup one in the cold cup.
As the filament floats a slab/ferrite/"magnadur"/westminster kit/ceramic magnet will easily cause an attraction in ice water, but not in the hot water when the magnet is place into the cup.
Do not be tempted to use a more powerful (rare earth) magnet (eg. a neodymium button magnet) as this will "swamp" the filament. The remaining paramagnetic attraction will mask the loss of ferromagnetism. Alternatively this can be used to emphasize the difference between para- and ferro- magnetism.
A youtube video of this is here Link (please excuse the amateur production quality and pronunciation of gadolinium).

--D.B.Ferguson 22:27, 15 February 2010 (UTC)

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