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History of Magnets and Magnetism
The Chinese discovered the magnetic compass as early as 200 BC. At first
fortune-tellers used it. Later people realised that it was a way to find the
direction of North and South.
The ancient Greeks knew that the lodestone or magnetite attracted iron towards
it. It is known that the Vikings used a lodestone to navigate. Later at the end
of the twelfth century Europeans were using this simple compass to aid
navigation.
During the 16th century Sir William Gilbert discovered that the properties of
the lodestone could be transferred to ordinary pieces of iron by rubbing them
with a lodestone.
What is a Magnet?
The first magnets were made of iron. These days they are:
alloy magnets that contain metals such as
iron nickel copper cobalt aluminium
ceramic magnets that are made from powders called ferrites which contain iron
oxide and barium oxide.
Permanent magnets do not readily lose their magnetism.
Descriptions of magnetic behaviors
There are many forms of magnetic behavior, and all materials exhibit at least
one of them. Magnets vary both in the permanency of their magnetization, and in
the strength and orientation of the magnetic field they create. This section
describes, qualitatively, the primary types of magnetic behavior that materials
can show. The physics underlying each of these behaviors is described in the
next section below, and can also be found in more detail in their respective
articles.
Most popularly found in paper clips, paramagnetism is exhibited in substances
which do not produce fields by themselves, but which, when exposed to a magnetic
field, reinforce that field by becoming magnetized themselves, and thus get
attracted to that field. A good example for this behavior can be found in a
bucket of nails - if you pick up a single nail, you can expect that other nails
will not follow. However, you can apply an intense magnetic field to the bucket,
pick up one nail, and find that many will come with it.
Unscientifically referred to as 'non-magnetic,' diamagnets actually do exhibit
some magnetic behavior - just to very small magnitudes. In fact, diamagnetic
materials, when exposed to a magnetic field, will magnetize (slightly) in the
opposite direction, getting (slightly) repelled from the applied field.
Superconductors are strongly diamagnetic.
Ferromagnetic and ferrimagnetic materials are the 'popular' perception of a
magnet. These materials can retain their own magnetization; a common example is
a traditional refrigerator magnetA magnet is a material or object that produces
a magnetic field. A low-tech means to detect a magnetic field is to scatter iron
filings and observe their pattern, as in the accompanying figure. A "hard" or
"permanent" magnet is one that stays magnetized, such as a magnet used to hold
notes on a refrigerator door. Permanent magnets occur naturally in some rocks,
particularly lodestone, but are now more commonly manufactured. A "soft" or
"impermanent" magnet is one that loses its memory of previous magnetizations.
"Soft" magnetic materials are often used in electromagnets to enhance (often
hundreds or thousands of times) the magnetic field of a wire that carries an
electric current and is wrapped around the magnet; the field of the "soft"
magnet increases with the current.
Two measures of a material's magnetic properties are its magnetic moment and its
magnetization. A material without a permanent magnetic moment can, in the
presence of magnetic fields, be attracted (paramagnetic), or repelled
(diamagnetic). Liquid oxygen is paramagnetic; graphite is diamagnetic.
Paramagnets tend to intensify the magnetic field in their vicinity, whereas
diamagnets tend to weaken it. "Soft" magnets, which are strongly attracted to
magnetic fields, can be thought of as strongly paramagnetic; superconductors,
which are strongly repelled by magnetic fields, can be thought of as strongly
diamagnetic.
Electro-Magnets
A wire which has a current flowing through it has a magnetic field around it.
This can be shown using plotting compasses or non filings.
If you reverse the current you reverse the polarity of the field.
A coil of wire with electricity flowing through it acts as a bar magnet.
However, you can control magnetism, you can turn it on and off by using a switch
to turn the current and off. You can also reverse the chaining the connections
battery. Placing a piece of iron into the coil induces a magnetic effect in the
iron when the current is flowing in the coil and so turning it into an
electro-magnet to the polarity by on
The strength of the electro-magnet can be improved by the following:
increasing number of coils
increasing the current
using an iron core (iron magnetises and demagnetises quickly, whereas steel
takes time to magnetise and demagnetise)
Use of electro-magnets
Electric generators
Electric Motors
Loudspeakers
Telephones
Tapes - flexible magnets
Beware - if you try and put too much current (ie more than 4-6v) through the
coil it may get hot! It is suggested that you do not use Labpak mains voltage
reducing devices to investigate electro-magnetism - it can cause a short circuit
and may overheat.
Having made an electro-magnet it can now be used to produce movement.
Iron or steel can be attracted towards the end of the electro-magnet when it is
switched on.
Bar magnets and other electro-magnets may be repelled away the nearest pole is
the same (like poles repel ...... etc).
Metal may also spring back when the current is turned off.
One final bit of theory - and the reverse of the initial electro-mag theory - if
a magnet or magnetic field is moved near a wire then an electric current flows
in the wire. This is important to understanding why many things work.
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