MAGNETS
A condensed History of Magnets
The ancient Greeks
and Chinese discovered that certain rare stones, called lodestones, were
naturally
magnetized.
These stones could attract small pieces of iron in a magical way, and were
found to always
point in the same
direction when allowed to swing freely suspended by a piece of string.
The name
comes from Magnesia,
a district in Thessaly, Greece.
What is a magnet?
A magnet is an object
made of certain materials which create a magnetic field. Every magnet
has at least
one north pole and
one south pole. By convention, we say that the magnetic field lines
leave the North
end of a magnet
and enter the South end of a magnet. This is an example of a magnetic
dipole ("di"
means two, thus
two poles). If you take a bar magnet and break it into two pieces,
each piece will again
have a North pole
and a South pole. If you take one of those pieces and break it into
two, each of the
smaller pieces will
have a North pole and a South pole. No matter how small the pieces
of the magnet
become, each piece
will have a North pole and a South pole. It has not been shown to
be possible to end
up with a single
North pole or a single South pole which is a monopole ("mono" means one
or single, thus
one pole).
What are magnets used for?
Magnets are used for all kinds of things. Some people even think that without magnets we wouldn't have fresh food, because magnets are the things that help a fridge door stay together. Magnets are also used for motors, cars,and all kinds of stuff. There are also other neat things that you can do with a magnet.
PROPERTIES OF MAGNETS
1. Attract many, but not all metals. (Iron, nickel = magnetic)
2. They exert a force
at a distance. (Magnets don't have to touch an object
to move it.)
3. Attract or repel another object. Magnets have direction!! A COMPASS is an instrument used to detect magnetic fields; the needle is magnetic and therefore is sensitive to other magnets.
4. Can make another object magnetic.
5. PERMANENT vs.
TEMPORARY magnet
What would happen if we took the magnet with two nails hanging
from it and carefully removed the magnet? If you carefully
removed the magnet, the nails would retain their magnetism for a short
time. These are TEMPORARY magnets. Permanent magnets will stay
magnetic forever unless something disrupts their chemistry.
Ten
Facts about Magnets
(from the book Driving
Force)
1. North poles point north, south poles point south.
2. Like poles repel, unlike poles attract.
3. Magnetic forces attract only magnetic materials.
4. Magnetic forces act at a distance.
5. While magnetized, temporary magnets act like permanent magnets.
6. A coil of wire with an electric current flowing through it becomes a magnet.
7. Putting iron inside a current-carrying coil increases the strength of the electromagnet.
8. A changing magnetic field induces an electric current in a conductor.
9. A charged particle
experiences no magnetic force when moving parallel to a magnetic field,
but when it
is moving perpendicular
to the field it experiences a force perpendicular to both the field and
the direction
of motion.
10. A current-carrying
wire in a perpendicular magnetic field experiences a force in a direction
perpendicular to
both the wire and the field.
What
types of magnets are there?
There are three
main types of magnets:
Permanent magnets
Temporary magnets
Electromagnets
Permanent Magnets
Permanent magnets
are those we are most familiar with, such as the magnets hanging onto our
refrigerator doors.
They are permanent in the sense that once they are magnetized, they retain
a level of
magnetism.
As we will see, different types of permanent magnets have different characteristics
or
properties concerning
how easily they can be demagnetized, how strong they can be, how their
strength
varies with temperature,
and so on.
Temporary Magnets
Temporary magnets
are those which act like a permanent magnet when they are within a strong
magnetic
field, but lose
their magnetism when the magnetic field disappears. Examples would
be paperclips and
nails and other
soft iron items.
Electromagnets
An electromagnet
is a tightly wound helical coil of wire, usually with an iron core, which
acts like a
permanent magnet
when current is flowing in the wire. The strength and polarity of
the magnetic field
created by the electromagnet
are adjustable by changing the magnitude of the current flowing through
the
wire and by changing
the direction of the current flow.
There
are four classes of permanent magnets:
Neodymium Iron Boron (NdFeB or NIB)
Samarium Cobalt (SmCo)
Alnico
Ceramic or Ferrite
Shapes of Magnets
Permanent magnets
can be made in most any shape imaginable. They can be made into round
bars,
rectangular bars,
horseshoes, rings or donuts, disks, rectangles, multi-fingered rings, and
other custom
shapes. Some
are cast into a mold and require grinding to achieve final dimensions.
Others start as a
powder which is
pressed into a mold or pressure bonded or sintered.
This
is a horseshoe magnet...a popular magnet shape.
MOTORS
Parts of an Electric Motor
Let's start by looking at the overall
plan of a simple two-pole DC electric motor. A simple motor has six parts,
as shown in the diagram below:
Armature or rotor
Commutator
Brushes
Axle
Field magnet
DC power supply of some sort
An electric motor is all about magnets and magnetism: A motor uses magnets to create motion. If you have ever played with magnets you know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). Inside an electric motor, these attracting and repelling forces create rotational motion.
In the diagram you can see two magnets
in the motor: The armature (or rotor) is an electromagnet, while the field
magnet is a permanent magnet (the field magnet could be an electromagnet
as well, but in most small motors it isn't in order to save power).
Electromagnets and Motors
To understand how an electric motor works, the key is to understand how the electromagnet works. You can learn more about electromagnets by reading How Electromagnets Work.
An electromagnet is the basis of an electric motor. You can understand how things work in the motor by imagining the following scenario. Say that you created a simple electromagnet by wrapping 100 loops of wire around a nail and connecting it to a battery. The nail would become a magnet and have a north and south pole while the battery is connected.
Now say that you take your nail electromagnet, run an axle through the middle of it and suspend it in the middle of a horseshoe magnet as shown in the figure below. If you were to attach a battery to the electromagnet so that the north end of the nail appeared as shown, the basic law of magnetism tells you what would happen: The north end of the electromagnet would be repelled from the north end of the horseshoe magnet and attracted to the south end of the horseshoe magnet. The south end of the electromagnet would be repelled in a similar way. The nail would move about half a turn and then stop in the position shown.
You can see that this half-turn of motion
is simple and obvious because of the way magnets naturally attract and
repel one another. The key to an
electric motor is to then go one step further so that, at the moment that
this half-turn
of motion completes, the field of
the electromagnet flips. The flip causes the electromagnet to complete
another
half-turn of motion. You flip the
magnetic field simply by changing the direction of the electrons flowing
in the wire (you
do that by flipping the battery
over). If the field of the electromagnet flipped at just the right moment
at the end of each
half-turn of motion, the electric
motor would spin freely.
The armature takes the place of the nail in an electric motor. The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core.
The diagram at the below shows how
the commutator and brushes work together to let current flow to the
electromagnet, and also to flip the direction
that the electrons are flowing at just the right moment. The contacts of
the commutator are attached to the axle of the electromagnet, so they spin
with the magnet. The brushes are just two pieces of springy metal or carbon
that make contact with the contacts of the commutator.
Putting It All Together
When you put all of these parts
together, what you
have is a complete electric motor:
Armature
In this figure, the armature winding
has been left out so that it is easier to see the commutator in action.
The key thing
to notice is that as the armature
passes through the horizontal position, the poles of the electromagnet
flip. Because
of the flip, the north pole of the
electromagnet is always above the axle so it can repel the field magnet's
north pole
and attract the field magnet's south
pole.
If you ever have the chance to take
apart a small electric motor, you will find that it contains the same pieces
described above: two small permanent
magnets, a commutator, two brushes, and an electromagnet made by
winding wire around a piece of metal.
Almost always, however, the rotor will have three poles rather than the
two
poles as shown in this article.
There are two good reasons for a motor to have three poles:
It causes
the motor to have better dynamics. In a two-pole motor, if the electromagnet
is at the balance point,
perfectly
horizontal between the two poles of the field magnet when the motor starts,
you can imagine the
armature
getting "stuck" there. That never happens in a three-pole motor.
Each time
the commutator hits the point where it flips the field in a two-pole motor,
the commutator shorts out
the battery
(directly connects the positive and negative terminals) for a moment. This
shorting wastes energy
and drains
the battery needlessly. A three-pole motor solves this problem as well.
It is possible to have any number
of poles, depending on the size of the motor and the specific application
it is being
used in.
Thanks to "How Stuff Works" - www.howstuffworks.com-
