Magnetism

The types of fields

• Gravitational fields → are created by objects with mass

• Electric fields → are created by objects with charge

• Magnetic fields → are created by moving charged particles, and exert forces on moving charges

Magnetic fields

• Magnets create magnetic fields and experience forces due to magnetic fields because of moving charged particles (electrons that they contain)

• There are two distinct poles to a magnet, and the magnetic force is concentrated at the ‘poles’:

  • Opposite poles attract (e.g + & -)

  • Like poles repel (e.g + & +, - & -)

• A magnetic field, ‘B’, is measured in Teslas, ‘T’, but everyday fields are measured in μT, as Tesla is a large unit.

• The direction of the field is defined as the direction that is an isolated magnetic pole would move.

• Therefore the vectors of a magnetic field always point outwards from a north pole and inwards towards a south pole .

Earth’s magnetic field

• The earth acts as if there was a bar magnet inside it

• Since the north pole of the magnet is attracted towards Earth’s geographical north pole, Earth’s geographical north pole is actually a magnetic south pole

Ferromagnetism

• Materials that can be magnetised are called ferromagnetic, where ‘ferro’ means for iron. ‘Ferrous metals’ are metals containing iron that is mixed with some carbon and may include another metal mixed in.

• Materials that only have paired electrons are not ferromagnetic, as paired electrons create opposite magnetic fields and therefore cancel each other out. Ferro magnetism arises from unpaired electrons in the shells, as there is a tiny magnetic imbalance in the atom.

• The total magnetic field of an electron is the sum of their magnetic field created by their orbital motion and spin.

Magnet behaviours

• In ferromagnetic materials, the fields due to the unpaired electrons tend to line up. Usually this happens only in small local regions. These small local regions of alignment are called magnetic domains.

  • All the different domains are randomly arranged so the lump of iron does not act like a magnet itself. However it “feels” the attraction of a magnetic field.

• If the local magnetic fields can be made to line up, the result is a material that has a large magnetic field – a magnet.

• When two magnets are brought close together, they exert a force on each other. Like poles repel, while opposite poles attract

Permanent magnets

• If a strong field comes near a lump of soft steel, all the randomly-oriented