General Properties of Magnets
1. Magnets have polarity – north and south poles.
2. Like poles repel, opposite poles attract
3. Some metals can be permanently magnetized, and some ores in
the earth are already magnetized – magnetite and lodestones are
naturally occurring magnets.
4. Some metals can be turned into temporary magnets (polarized) by
being brought near another magnet. This is magnetizing by
induction.
5. Magnets cannot be broken or separated into separate north and
south poles. If you break a magnet, you simply get two smaller
magnets. North and south cannot exist independently.
6. The earth itself is a giant magnet and generates a magnetic field.
This can only be observed by its effect on other magnets. If a bar
magnet is suspended perfectly horizontally with a string, it will
rotate to align itself pointing directly towards magnetic north.
7. Electric fields can induce magnetism and magnetic fields can
induce electrical currents. This is the principle behind electric
motors and generators.
Microscopic Picture of Magnetic
Materials
Materials which can be easily magnetized are known as
ferromagnetic. Examples of ferromagnetic materials include iron,
nickel and cobalt. If any of these metals are placed inside a
magnetic field, they can become magnetized (induced). They can
also be used to create electromagnets by placing them within an
electric field.
Each electron in an atom acts like a tiny electromagnet. The
magnetic fields in a group of neighbouring electrons can add
together. Such a group is called a “domain”. When a piece of
ferromagnetic material is not in a magnetic field, the domains all
point in random directions (ie, electrons move freely, etc). If,
however, the material is placed inside a field, the domains more or
less tend to line up in one direction, creating positive and negative
poles.
Some alloys of ferromagnetic materials can be used to create
permanent magnets through induction. For example, AlNiCo, an
alloy of aluminum, nickel and cobalt, can be used to create
permanent magnets. When induced within a magnetic field, the
domains remain aligned even after removal from the field.
Ferromagnetic substance,
domains are not aligned.
Not magnetic
Same substance in a magnetic
field. The domains or dipoles
now align. It is magnetic.
Paramagnetic materials can also be magnetized when placed
within a strong magnetic field. However, these materials are
not nearly as magnetically permeable, and thus will not create
nearly as strong a magnet as a ferromagnetic material will.
Also, when removed from the magnetic field, they will
immediately revert to being non-magnetic. In other words,
they cannot be permanently magnetized.
Examples of common paramagnetic materials include
aluminum, barium, calcium, uranium and magnesium.
Diamagnetism is a very weak form of magnetism that is
exhibited only in the presence of a very strong external
magnetic field. It is the result of changes in the orbital
motion of electrons because of the presence of the
magnetic field. It results in a repulsive force between the
diamagnetic material and the magnetic field. Thus,
diamagnetic materials repel magnetic fields, but this
repulsion is so weak that it is difficult to observe in every
day life.
ALL materials are diamagnetic in nature. However, in
ferromagnetic and paramagnetic materials, their magnetic
properties overshadow their diamagnetic ones.
As a result, the term diamagnetic is generally used to refer to
non-magnetic materials, like water, wood, plastic, etc.
Creating a Permanent Magnet
Magnetic Permeability – the extent to which an
external magnetic field can permeate a substance
and realign the magnetic dipoles. Ferromagnetic
materials are highly magnetically permeable;
paramagnetic materials are much less permeable.
Curie Point – the temperature at which, if a magnetic
material is heated, it loses all magnetic properties.
Alloy – a blend of metals. In the case of permanent
magnets, a blend of ferro- and paramagnetic
metals is used. A common one is AlNiCo.
Steps to create a permanent magnet:
1. Create an alloy of ferro and paramagnetic
materials.
2. Place the alloy in a strong external magnetic
field
3. Heat the alloy to the curie point. This will give
the dipoles lots of kinetic energy to easily align
themselves.
4. While still inside the field, cool the magnet to
room temperature. The paramagnetic nature of
the alloy will “freeze” the dipoles in their
magnetic alignment.
5. A permanent magnet has been created.