Magnetic Properties of substance

Magnetic properties of a substance. Curie temperature

What causes magnetism?

Any moving charge creates a magnetic field around it, and this is the reason that there is a magnetic field around a current carrying wire.

However, the source of the magnetic fields generated by materials like permanent magnets, which have no current flowing within them, is due to a property of the particles within the material (their ‘magnetic moment’), as well as the motion of electrons within the atoms.

Usually these cancel out, but in some materials they can align to produce an overall magnetic field.

Some materials respond to a magnetic field, and we are used to seeing this when some metals, like iron, are attracted to magnets. However, this is only one form of magnetism, ferromagnetism; materials can also be paramagnetic or diamagnetic.

Ferromagnetic materials are strongly attracted by magnetic fields and can retain their magnetic properties even after the field is removed. For this reason permanent magnets are made from ferromagnetic materials. Iron, nickel and cobalt are ferromagnetic.
 
Paramagnetic materials are weakly attracted by magnetic fields. This effect is usually so weak that it is not noticeable. After the magnetic field is removed paramagnetic material does not retain its magnetic properties.
 
Diamagnetic materials are repelled by magnetic fields, but the effect is usually very weak. Superconductors can be strongly diamagnetic and this means they can be levitated above powerful magnets.

How are magnets used in electricity generation?

Electricity is generated by moving an electrical conductor through a magnetic field. In a power station, steam or water is used to turn large turbines and drive generators.

These move wires next to magnets (or move magnets next to wires) and the changing magnetic fields cause the electrons in the wires to move, generating electrical currents. In power stations, which use fossil fuels, the coal, oil or gas is burned and the heat is used to boil water.

This produces steam to turn the turbine. Similarly, nuclear power stations use the heat generated by the nuclear fuel in the reactor to boil water to produce steam. Hydropower uses water to turn turbines.

What is magnetic resonance imaging?

Magnetic Resonance Imaging (MRI) uses powerful magnets to image the human body. It relies on protons in the water molecules within body tissue aligning with a strong magnetic field. When the field is switched off the protons return to their original state, and the energy released when this happens can be detected and used to produce images of tissue inside the body.
 
Unlike other scans, MRI does not use X-rays and so the patient is not exposed to radiation. However, care has to be taken to check that the patient does not have metal in their body, which could move and cause injury during scanning, or medical implants which could be affected by the magnetic fields.

How does magnetic levitation work?

Some trains use magnetic levitation. The train is suspended above the track, which reduces friction and allows it to reach very high speeds. There are two ways to achieve this. In ElectroDynamic Suspension (EDS) superconductors are placed on the train and these are repelled by electromagnets in the track.
Alternatively, in ElectroMagnetic Suspension (EMS), electromagnets can be used in an arrangement where part of the train reaches around and under the track. Electromagnets on this part are attracted to coils on the underside of the track. This pulls the train upwards, lifting it off the track.

What causes the Earth’s magnetic field?

The Earth has a magnetic field which protects us from the solar wind, a stream of charged particles which emanate from the Sun. It is caused by the flow of molten iron in the Earth’s outer core.
The Earth’s magnetic field lets us navigate using a compass, a small magnet which is free to move and so will orient itself in the direction of the field. The end of the magnet, which points north, is called the north pole.
However, as opposite poles attract, this means that the magnetic pole which is found in the north is actually the south pole of the Earth's magnetic field.

What is the Curie temperature?

If ferromagnetic materials, like iron, are heated above a certain temperature they lose their magnetism. This is known as the Curie temperature, and for iron is around 770°C. If the material is allowed to cool below the Curie temperature it will again be attracted by magnetic fields