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Magnets

Magnetism has been known for thousands of years. A natural occurring mineral, known as lodestone, was found in a part of Greece caled Magnesia. Lodestones could be used for navigation, since they would align themselves with the Earth's magnetic field.

Lodestone
Lodestones were the first magnets and compasses: this stone is covered in iron filings to demonstrate the magnetic field it is generating

But without an understanding of the atom and charge (the electron was not discovered till 1898), people had no idea how magnetism worked.

Oersted's Great Discovery

Oersted demonstration
Oersted's experiment demonstrated the previously unknown relationship between an electrical field and a magnetic field, opening the way to all the discoveries and inventions of electromagnetism

In 1820, a Danish scientist, Hans Christian Oersted, noticed by chance that a compass needle was caused to change direction when near to a wire through which an electrical current was passing. Incidently, Oersted was the inventor of the term 'thought experiment', which is curious for a man who made one of the most important practical discoveries in the history of electricity!

With this new insight about the relationship between electricity and magnetic fields, people like Faraday were able to experiment and develop everything we now know about electricity. Soon there were machines using electromagnetic induction, such as motors and generators, and an understanding of electromagnetic fields led to the invention of radio.

The age of electricity and telecommunications had begun. After Oersted's discovery the world would never be the same!

What is Magnetism?

Within a metal such as iron, nickel, cobalt, and aluminium, or their alloys (mixtures), electrons are spinning and have a certain ability to move. When their spins are caused to align in the same direction, by an electrical or magnetic field, the metal itself becomes a magnet.

Any movement of charge sends out an electrical field. Any other charge will be affected by this field. Therefore, electrons spinning in one metal will cause electrons in another metal to be pushed or pulled by their electrical field. If the electrons in one metal are all spinning in different directions, as they are in most materials, then all the little electrical fields push and pull in different directions, so cancel each other out, and there is no net electrical field.

However, if the electrons can be caused to spin in the same way, or orientation (up/down, or side-to-side), their electrical fields all add together and the electrons of another material feel a net electrical field. This 'permanent' field is known as a magnetic field, even though it is caused by electrons and their electrical fields.

A distinction is made to electrostatic force, in that the magnetic force, although caused by electrical fields of electrons, is not the result of an accumulation of electrical charge in one region of a material, but is an intrinsic and uniform distribution of electron spin orientation in the crystalline structure of the metal.

Magnetic Materials

There are electrons around atoms in all materials, so why do magnets only pull or push certain materials, such as metals, and not even all metals?

Well, when a magnet is near to a piece of metal like iron or steel, the metal becomes temporarily magnetised. This is because their crystalline structure is of a type that allows the electrons to align their spins in the same way as the magnet's. This realignment is not possible in non-magnetic materials, such as paper and stone.

If a strong magnetic field is applied to a metal from a class of metals called ferromagnetic, while it is being manufactured, and is still molten, the magnetic field due to electron spin alignment becomes permanent as the metal cools and solidifies in that condition. This is how fridge and other permanent magnets are made. Some metals and alloys retain their magnetism longer than others.

Summary: Magnetism is the fixed presence of an electrical field in one material which can magnetise temporarily some other types of material, causing a force of attraction or repulsion due to the respective orientations of electron spins. Magnetic fields and electrical fields are two forms of the same thing, and are described by the laws of electromagnetism.

Magnetic Fields

Magnetic field
Magnetic field around a single magnet: the direction of the arrows is from north to south

Magnets have magnetic fields. This is an area of space where magnetised particles will experience a force.

Fields are drawn with force lines, known as magnetic field lines, indicating the direction a north polarised magnet (such as the needle on a compass) would move.

A magnetic field passes from the north pole to the south pole of a magnet. Since a magnet has two poles, we say it is 'dipolar'.

Earth's magnetic field
Earth's magnetic field: geographic north is magnetic south! Currently, the magnetic dipole is 10° to the rotation axis.

With two magnets, the field lines are the sum of all the forces from the four poles.

Magnetic field around two dipolar magnets
Magnetic field around two dipolar magnets
Magnets
Photograph of two magnets with unlike poles facing, and iron filings

Two magnets with unlike poles facing each other have a field with almost straight lines directly between them.

Magnets
Photograph of two magnets with like poles facing, and iron filings
SchoolScienceCH
Magnetic field around two dipolar magnets with like poles facing each other

Two magnets with like poles facing each other have field lines as shown here.

A current from a battery runs through a magnet, providing an electric field which pushes against the magnetic field of the magnet from one side only, causing it to spin.

Content © Andrew Bone. All rights reserved. Created : April 10, 2014 Last updated :February 27, 2016

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Bernhard Riemann, 1826 - 1866, German mathematician
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