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Magnetism

Magnetism is a fascinating subject which is often explained using arcane terms and complex equations that make it difficult to understand. Here is my attempt to explain it using everyday language. It's not rigorous but I hope it helps to explain to some degree when and how magnetism works.

Why Does a Magnet Attract Iron?

At first it seems like the answer is obvious: it's just what magnets do, isn't it? But it isn't quite so simple. Magnets attract each other because of their magnetic fields. But how does a magnet attract iron? A piece of (unmagnetised) iron doesn't have a magnetic field, and two lumps of iron don't attract each other, so how does a magnet do it? The answer is that the magnet turns the iron into a magnet, and then the magnet and the iron are attracted to each other just like any two magnets.

This seemingly innocuous questions opens up a whole topic of conversation. Iron has the property that it can become magnetised. This happens when it comes within the field of a magnet, or the magnetic field of an electric current. When the the magnet and the iron are separated, or the electric current is turned off, the iron may return to a completely unmagnetised state, or it may retain some magnetism.

All elements can be classified by their magnetic properties into three groups:

• Diamagnetic: usually thought of as having no magnetic properties, but in fact they have a small magnetic repulsion. This is usually too small to be noticed.
• Paramagnetic: elements that can become temporarily magnetic when subjected to a magnetic field. When the field is removed their magnetism disappears.
• Ferromagnetic: elements that can become permanently magnetic.

Iron is ferromagnetic, in fact the term comes from the Latin name for iron, ferrum, because it was in iron, or iron bearing, materials that magnetism was first noticed. But other elements are also capable of being permanently magnetised like iron, such as cobalt or nickel, and the strongest magnets are made from exotic and "rare earth" elements such as neodymium and samarium. All of these elements are ferromagnetic.

If the magnetic field is weak, then a ferromagnetic material can behave like a paramagnetic material, i.e. become magnetic when the field is present but lose that magnetism when the field is removed.

Magnetic Domains

Ferromagnetic materials contain magnetic domains, areas approximately a millimetre across that contain billions of atoms with their magnetic moments aligned. Each of these domains produces a magnetic field. If the magnetic domains in the material are randomly arranged their fields cancel each other out and there is no overall magnetic field. If the magnetic domains are aligned so that their individual fields are all pointing in the same direction, then an overall magnetic field is produced.

When iron is in a molten state the magnetic domains are free to move around. If the iron was cooled and set in a strong magnetic field the magnetic domains would line up and the resulting material would be magnetic. Usually iron is not cast in a strong magnetic field, and the earth's magnetic field is not strong enough to have an effect, so the material is not magnetic. In the absence of an external magnetic field, a magnetic material spontaneously forms magnetic domains that are oriented at random in order to minimise its internal energy. Aligning the magnetic domains so that an overall magnetic field is produced requires work, called magnetostatic energy, and in the absence of an external force materials naturally arrange themselves to minimise this work.

Ferromagnetic material in an unmagnetised condition material is attracted to a magnet because its magnetic domains align themselves with the field of the magnet and the material becomes temporarily magnetised, some of which magnetism may remain after the two pieces are separated. When the unmagnetised item becomes a temporary magnet, the two magnets are attracted to each other. Two unmagnetised pieces of ferromagnetic material will not be attracted to each other. When a ferromagnetic material is magnetised, its magnetic domains are permanently reorientated so that their individual magnetic fields are aligned producing a net magnetic field. The strength of the field depends on the proportion of the magnetic domains that become permanently aligned, the more there are, the stronger the resulting field.

Soft Iron? Hard and Soft Magnetic Materials

The phrase "soft iron" sounds like some sort of ferromagnetic putty, or self contradictory; how can iron be soft? It would be if it meant "soft" in the usual sense. It actually refers to the magnetic property of the iron and not its physical hardness, which remains, well, as hard as iron. Soft magnetic material has low coercivity and high permeability; it is easily magnetised, and also easily demagnetized.

These properties are useful in electromagnets. An electromagnet is made by wrapping coils of wire around a soft iron core. When a current is passed through the wire a magnetic field is produced. The iron core becomes magnetised, which increases the strength of the field considerably. The magnet should hold the load firmly while the current is applied, and release it as soon as the current is stopped. By using a soft iron core, when the current is turned off the magnetic field in the core also almost completely disappears.

Not all of the magnetism induced in the core of an electromagnet disappears when the current is stopped, the core remains a weak permanent magnet. The residual or remanent magnetization can be removed by applying a small current in the reverse direction. If the full current was applied in the reverse direction and then stopped, the core would become a weak permanent magnet in the reverse direction. This phenomenon of not returning to zero after the current is removed is called hysteresis.

Note that electromagnets are energised by direct current. If an alternating current was used, the magnetic field would reverse when the current reversed, and the North and South poles of the magnet would swap around.

One of the uses of soft iron is to shield items from magnetic fields. Sensitive items are encapsulated in soft iron and the high permeability material channels the magnetic field through itself and away from whatever is inside it.

Soft iron ideally would have a zero carbon content, but steels with a very low carbon content are also magnetically soft. However, as the carbon content in steel rises the material also becomes magnetically harder. Harder to magnetise, and harder to demagnetise. Permanent magnets are made of steel, or more usually today of even magnetically harder materials.

Is a Magnetic Shield a Faraday Cage?

Note that a magnetic shield is not a "Faraday cage", which is used to protect items against electromagnetic radiation such as radio waves. A Faraday cage does not block static or slowly varying magnetic fields, a normal magnetic compass will still work inside a Faraday cage.

The easiest way think about this is that a Faraday cage can be made from copper wires formed into a cage, the gaps between the wires depending on the wavelength of the electromagnetic radiation that protection is required against. However, a magnetic shield must be made of a ferromagnetic material in order to divert lines of magnetic flux around the interior.

How to Make a Magnet

A permanent magnet can be made by stroking a piece of ferromagnetic material with a magnet, or by exposing it to a magnetic field generated by an electric current. A magnet made by stroking with a magnet is never as strong as the first magnet, and the strongest magnets are made using electric currents to generate a powerful magnetic field.

All ferromagnetic materials contain large numbers of very small magnetic domains. When the material is unmagnetised, the domains are oriented at random. By applying a magnetic field to the material, some of the domains are made to align themselves with the applied field and the material becomes a magnet. Some of the domains will slip back when the magnetic field is removed, which is why the magnet is not as strong as the one that was used to magnetise it. If the material is magnetically soft, more domains will slip back than if it is magnetically harder, but this is why it is harder to magnetise a magnetically hard material.

A bar of iron can be magnetised by aligning its axis with the earth's magnetic field and then striking it with a hammer, which jolts loose some of the domains and they align with the earth's field. This happens to iron and steel ships when they are being built due to the hammering on the hull, and was probably more significant when ships were riveted together. The ship retains a residual magnetism from the alignment it was in when it was built, which the ships compass needs to be corrected for.

How to Demagnetise a Magnet

Before electricity was artificially generated and distributed to households and businesses it was very difficult to demagnetise something that had accidentally become magnetised. The only practical way was to heat it to the materials Curie point, the temperature at which the thermal activity in the material jumbles up all its magnetic domains so they point in random directions and there is not net magnetic field. Once alternating current electricity became widely available it became very easy to demagnetise things. An electromagnet supplied with alternating current reverses the direction of its magnetic field when the current reverses, 50 or 60 times a second. A magnetised item placed in this field has its magnetism repeatedly reversed by the alternating field. Drawing the item slowly out of the alternating field results in its magnetic domains settling at random in new orientations so there is no overall field.

Magnetic and Electric Fields

There are two types of field in electricity: magnetic and electric.

A magnetic field exists around a permanent magnet, or around a moving electric charge such as a current flowing in a wire. Magnetic fields give rise to the relatively familiar phenomena of magnetism and induction, which is used in electricity generation, i.e. making the electricity that powers lights, machines and computers.

Electric fields are less familiar, they exist around an electrical potential; a voltage. The most familiar evidence for the existence of an electric field is "static electricity", where an electric charge builds up on something, raising it to a higher voltage than its surroundings. This can cause non-sticky things like a sheet of plastic film to "stick" to you, or an electric shock when the charge is discharged, such as by touching a door handle after walking across a room with a carpet that contains nylon. Such a shock is the same as the weather event of lightning, where an electric charge builds up on a cloud, creating an electrical field around it, until the electrical potential, the voltage difference between the cloud and the earth, becomes so large that the charge in the cloud is suddenly discharged to earth in a bolt or streak of lightning. Electric fields are not used as widely as magnetic fields, they are used in photocopiers and laser printers, and also in electrostatic coating where an electric field is used to attract paint or powder to the work piece.