Most materials are non-magnetic, that is
they cannot be magnetised to create their own independent magnetic field.
You might ask the question 'what is magnetism', the
answer is the same for gravity, nobody is really quite sure and the
answer will lie somewhere in some deep quantum theory, well above school
physics level and my little grey cells!
However, the causes and effects of magnetic fields
are well understood and can be described by 'working models' e.g.
little atomic magnets to do with electrons in atoms like iron, magnetic
fields from compass plotting and accurate mathematical models from very
simple equations to the extremely complex!
Magnetism is a non-contact force because it
causes objects to attract or repel without the two objects having to
make contact.
Only
relatively few materials display magnetic properties - that is, they are
naturally magnetic, or more usually, capable of being magnetised.
Magnetic
materials
Typical
magnetic materials are iron and its alloys like steel, nickel and its
alloys and cobalt and its alloys - the alloys, in particular, are used to produce very strong permanent magnets.
Other common structural metals like aluminium,
copper and lead are not magnetic.
Jewellery metals
like gold, platinum and silver are also not magnetic.
Actually most materials are NOT magnetic!
A
bar or rod of magnetic material can be shown to have two poles, one at
each end - a north seeking pole (north) and a south seeking pole
(south).
e.g. a bar magnet is suspended by
a fine thread, one end will point to the Earth's magnetic north and
the other end to the Earth's magnetic south.
This is evidence that the
metallic core of the Earth (lots of iron) is permanently
magnetic.
As with electric charges, when you bring poles
together, one of two things can happen.
Either the poles attract or they repel each other.
In other words any two magnetic objects exert a
force on each other - attraction or repulsion.
Experiments demonstrating magnetic pole rules
The rule is quite simply like poles repel
(<= N N => or <= S S =>) and
unlike poles attract (N => <= S).
You can easily demonstrate these rules with
two bar
magnets (of known polarity) suspend on string and bringing the
various ends together (as in diagram above).
Previously
magnetised materials will attract other magnetic materials whether
they were or were not, already magnetised.
e.g. a magnet will always and readily attract an
item of iron or steel.
In other words when a magnet
is placed near a magnetic material the two objects always
experience an attractive force - it doesn't matter which pole of
the magnet is placed nearest the object - see induction later.
A magnetic force is a non-contact force, even if
the objects eventually touch.
Electrical and
gravitational forces are also non-contact forces.
Already magnetised materials are surrounded by
their own magnetic field.
=>
<=
=>
or <=