# Magnetism

The force that can attract (pull closer) or repel (push away) magnetic objects

## What is magnetism?

Magnetism is a force that can attract (pull closer) or repel (push away) magnetic objects.

All magnets have two poles (ends) – the north pole and the south pole.

If the poles are the same, they repel one another. If the poles are different, they attract one another. Opposites attract, and likes repel.

Magnetic poles almost always appear in pairs. If you cut a magnet in half, each half would still have a north and south pole.

The lines represent magnetic fields and are called magnetic field lines. They show the direction and strength of the magnetic force.

If the lines are closer together, they are stronger. Magnetic fields are strongest at the poles.

Magnetism is caused by the motion of electric charges. Any moving electric charge generates a magnetic field perpendicular (at right angles) to it.

In an atom, electrons have a negative electric charge. Electrons spin on their own axis to create a small magnetic field. The strength of this magnetic field is called a magnetic moment.

There are two types of electrons in an atom: bound and free electrons.

In most substances, there are pairs of bound electrons. Bound electrons are bound (stuck) to individual atoms. The two electrons spin in opposite directions, which cancels out their magnetism (as shown in the above LEGO video).

However, in some atoms (in iron, nickel and cobalt) you have one or more free electrons. These free electrons are unpaired (unbound). So, the magnetic moments of these electrons aren’t cancelled out.

If all the magnetic moments of the electrons do not completely cancel out, the atom will act like a tiny magnet.

To turn something into a magnet (magnetise it), you need to put it in a magnetic field.

There are two types of magnets: permanent and temporary magnets.

A permanent magnet has a permanent magnetic field, even when no external magnetic field is present. A temporary magnet loses its magnetic field when an external magnetic field is removed.

You can further divide a material’s response to a magnetic field into one of five basic categories of magnetism:

1.  Diamagnets (temporary): the material weakly repels the external magnetic field, except in superconductors.
2. Paramagnets (temporary): the material is slightly attracted to the external magnetic field.
3. Ferromagnets (permanent): the material is strongly attracted to the external magnetic field.
4. Ferrimagnets (permanent): similar to ferromagnetism, the material is strongly attracted to the external magnetic field. But the magnetic moments of opposing electrons are unequal and the electrons order in a different way to ferromagnets.
5. Antiferromagnetism (temporary – above a certain temperature): opposite to ferromagnetism, the magnetic moments of electrons point in opposite directions. The material has no net magnetic moment. Below a certain temperature (called the Néel temperature) the material does not respond to the external magnetic field. Above the Néel temperature, the material is weakly attracted to the external magnetic field (it is now a paramagnet).

## So what?

Magnetism is used in a wide range of applications (places).

Many computers use magnets to store data. Magnets are used in electric generators to turn mechanical energy into electricity (and vice versa). They can sort recyclable materials and mining materials.

In medicine, we use Magnetic Resonance Imaging (MRI) technology to scan the human body and diagnose disease. Magnets can also eradicate and detect cancerous cells.

In transport, Maglev trains use magnetic fields to propel and levitate these superfast trains. While using magnetic compasses is an established concept, scientists are also trying to understand if animals use the Earth’s magnetic field to navigate.

## What else?

Did you know the Earth is one giant magnet? Its solid iron core is surrounded by a molten layer of iron. This churning iron creates a magnetic field. But it’s very weak (about 1,000 times weaker than a bar magnet). We call the magnetic fields around planets magnetospheres.

The most powerful magnet is the universe is a special kind of dead star called a magnetar. Its magnetic field is 1,000 trillion(1,000,000,000,000,000,000,000,000) times stronger than the Earth’s magnetic field.

Magnets always have two poles – but we do not understand why a magnetic monopole (single pole) does not exist. However, some theories do predict their existence. The MoEDAL experiment at the Large Hadron Collider at CERN is currently looking for magnetic monopoles.

You may have heard of dark matter and dark energy, but scientists recently put forward the idea of dark magnetism. Dark magnetism could help scientists explain why the universe is expanding.

## Want more?

Magnetism is a really complicated topic. This is a great crash course on the topic:

#### The National MagLab

The largest and highest-powered magnet laboratory in the world.

#### Introducing Cosmo Learning

With 36 informative video lectures on electricity and magnetism.

#### A Very Short Introduction

An introduction to magnetism from the “Very Short Introductions” series.

### Special relativity

Quantum Mechanics
November 8, 2019

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