1.
Magnets come in a variety of sizes, shapes, and strengths
Shapes
of magnets vary greatly. Common magnet shapes include rectangular bars,
horseshoes, cylinders, and doughnuts. Magnets may be as large as a car or as
small as a pinhead. A magnet's size is not a good indicator of strength. A
tiny neodymium magnet, for example, can be incredibly powerful and capable of
picking up many times its own weight.
2.
Magnets attract some materials.
Materials
that are attracted to a magnet are called magnetic materials. These
include iron, cobalt, nickel, and some rare earth elements. It should be noted
that all these materials are metals, but not all metals are magnetic.
Aluminum, copper, lead, gold, and silver are examples of metals that are not
attracted to a magnet. Materials that are not attracted to a magnet are non-magnetic.
3.
A magnet can both attract and repel another magnet.
The
test for whether an object is a magnet or not is whether some part of the
object can repel some part of a known magnet.
4.
All magnets have two kinds of poles.
The
two kinds of poles are called
north and south. These poles may
be located in a variety of positions: near
the ends, on opposite faces, or even on the edges of a magnet.
The poles may shift location over time as a magnet loses its strength.
It is also possible for a magnet to have more than one pole of the same
kind.
5.
The pole of a freely-suspended magnet that points toward the north is called a
north magnetic pole; the pole that points toward the south is called a south
magnetic pole.
If
a bar or other type of magnet is freely suspended, the magnet will naturally
come to rest with the same pole always pointed in the general direction of
north. This pole is called the north pole. The opposite pole is called the
south pole.
6.
A compass needle is a magnet.
6.
A compass needle is a magnet.
A
compass needle is a freely-suspended magnet. Most compasses have some type of
marking on the needle that indicates which end is the north pole. Usually, one
half of the needle is colored to indicate the north pole. But be careful;
sometimes the poles of a compass needle become accidentally reversed.
To
use a compass, let the needle come to rest. The colored end of the needle should
point toward north. Slowly turn the base of the compass until the “N”
printed on the compass face lies directly beneath the north pole of the
needle. One can then look at the compass face to tell the directions of north,
east, south, and west. The compass face is usually marked into 360 equal
parts, known as degrees. Compass measurements start at 0 degrees, which is
north. East is 90 degrees, south
is 180 degrees, and west is 270 degrees. North is both 0 and 360 degrees.
Degrees are usually symbolized with a small zero placed after, and just above,
the measurement; for example: 3600.
When using a compass, make sure that there are no magnets or magnetic
materials nearby that might affect the needle’s direction.
7.
The Earth acts like a magnet.
A
compass needle behaves as it does because Earth is a giant, weak magnet. Like
other magnets, Earth has a magnetic north pole and a magnetic south pole.
The
magnetic poles of Earth are close to, but not in the same place as, Earth’s
geographic poles. The magnetic pole in the northern hemisphere is in Bathurst
Island in northernmost Canada, about 1400 miles from Earth’s geographic
pole. This pole is actually a magnetic south pole, even though it is located
in the north. The magnetic pole in the southern hemisphere is found in Wilkes
Land, in Antarctica, about 1400 miles from the geographic south pole, and is
actually a magnetic north pole.
The
Earth’s magnetism is thought to be due to the presence of charged particles
in the moving liquid core of the Earth. There is evidence that at certain
times in history, the magnetic poles of the earth have shifted, reversed, and
even disappeared for periods of time.
8.
Like poles repel; unlike poles attract.
When
two magnets are brought near one another, they exert forces on each other. Like
magnetic poles (north/north or south/south) repel each other, while unlike magnetic
poles (north/south) are attracted to one another. This is called the Law of
Magnetic Poles.
9.
A magnet is strongest at its poles.
A
magnet’s greatest strength is found at its poles, and its strength
diminishes rapidly moving away from the poles. At a point about midway between
its two opposite poles, a bar magnet
exhibits no magnetism at all, because the effects of the opposite poles cancel
one another. The poles of a magnet are not necessarily of equal strength.
10.
Magnetic materials are composed of many “tiny magnets,” called magnetic
domains.
When
the magnetic domains are randomly arranged, the material does not act as a
magnet.
When
most of the magnetic domains in a material are lined up with their north and
south poles pointing in the same direction, the material as a whole acts like
a magnet.
Non-magnetic
materials - such as plastic, glass, or aluminum - are not made of magnetic
domains and cannot be made into magnets.
11.
It is possible to magnetize and demagnetize magnetic materials.
Stroking a magnetic material with a permanent magnet causes tiny magnetic
domains in the material to align. The material itself temporarily becomes a
magnet. A magnet can lose its magnetism if its magnetic domains are arranged so
they are no longer aligned. Heating,
shaking, or jarring a magnet can cause it to lose its magnetism.
A magnetizer is a device that uses electric current to quickly re-magnetize
magnets. It can be purchased from a
science education supplier for less than $100.
12.
Every magnet is surrounded by a magnetic field.
Magnetic
forces can be detected around every magnet.
These forces are caused by a magnetic field that surrounds the magnet.
This magnetic field exerts forces on other magnets, magnetic materials,
and moving charges located in the field. The amount of force, or strength of
the magnetic field, varies at different locations around a magnet but is
always strongest near the poles. The
magnetic field near the poles of a typical student bar magnet is typically
several hundred times stronger than the magnetic field of the earth near its
surface.
13.
Magnetic field lines are used to indicate the relative strength and direction of
the magnetic field.
The
distance between lines indicates the relative strength of the field.
The lines are closest together near the poles, where the magnetic field
is strongest. Arrows drawn on the lines show the direction of the magnetic
field. The direction of the
magnetic field is defined as the direction of the force on a north pole
located in the field. (A small
compass can be used to define the direction of the field at different points
around a magnet. Just notice the
direction in which the north pole of the compass needle points.)
Regardless of the shape of the magnet, magnetic field lines always form
closed loops, leaving the north pole and entering the south pole and then
passing through the magnet.
14.
All magnetic fields are produced by moving charges.
Whenever
there are moving charges, there are magnetic effects.
An example is a current-carrying wire.
When the ends of a bare copper wire are connected to the terminals of a
battery, electrons flowing through the wire cause it to behave like a magnet
and small staples or iron filings can be picked-up by the wire.
The magnetic field produced by the current in the wire can also deflect
a compass needle.
Coiling
the wire increases the strength of the magnetic field in much the same way
that stacking small magnets creates a stronger magnet. Even the magnetism of
bar, donut, horseshoe, and other physical magnets is caused by moving charges.
These moving charges are the negatively charged electrons in the atoms
of matter. Electrons, spinning
as they orbit the nuclei of atoms, create magnetic fields. The direction of spin of each electron determines the
direction of the magnetic field surrounding it.
Because atoms of most materials have electrons that are paired and spin
in opposite directions, these magnetic fields usually cancel one another.
But atoms of ferromagnetic materials, such as iron, cobalt, and nickel,
have several unpaired electrons. These
unpaired electrons all spin in the same direction.
The magnetic fields of these electrons reinforce one another and cause
the atom itself to behave as a tiny magnet.
Neighboring atoms may align themselves with their poles in the same
direction, creating magnetic domains. Each
magnetic domain can be thought of a region of the material that behaves
independently as a small magnet. Magnetic domains are separated by un-aligned atoms or atoms
of impurities in the metal. If
most of the domains of the material are aligned in the same direction, then
the material itself behaves as a magnet. Not all materials that have atoms
with unpaired electrons are ferro-magnetic. There are a number of elements
whose atoms have one or two unpaired electrons and thus behave as tiny
magnets. In some elements, these atoms do not align them-selves to form
magnetic domains. Such elements, like sodium and oxygen, are called
paramagnetic and are only weakly attracted to a very strong magnet. Only ferro-magnetic
materials are noticeably attracted to an ordinary magnet.
15.
An electromagnet is made with a coil of current-carrying wire wrapped around
an iron core.
Electricity
through the coiled wire creates a magnetic field which induces magnetism in
the iron core. The domains in the
core become aligned with the magnetic field of the coil, thus creating a
stronger magnet.
The
poles of an electromagnet can be located with a compass.
The polarity of an electromagnet depends on the direction of the
current through the coil. An
electromagnet can be made stronger by increasing the number of turns in the
coil, increasing the current through the coil, or increasing the size of the
iron core. Modern electromagnets,
used in industry for purposes such as loading scrap iron, can typically lift
50,000 pounds. Electromagnets are
used in a variety of devices including doorbells and circuit breakers.
16.
Magnetism is used to produce electricity.
An
electric current can be produced in a wire by simply moving a magnet back and
forth inside a coil of wire whose ends are connected to make a complete
circuit. No battery or other
power supply is needed! In fact,
electricity is produced when the magnet moves past the wire, the wire moves
past the magnet, or they both move past one another,
Electricity is induced by the relative motion between a wire and a
magnetic field. This is the
principle of generators, from the small hand-held generators you may use in a
classroom to the giant generators used in power plants to produce electricity
for whole cities.
17.
Magnetism and electricity can be used to produce motion.
When
current passes through a coil of wire that is free to spin, the pole of a
nearby magnet will attract one side of the coil and repel the opposite side.
If the magnet is positioned just right, it will cause the coil to turn.
If the current in the wire is quickly turned off and on, or if there is
alternating current flowing through the wire, then the magnet will alternately
attract and repel each side of the coil, causing the coil to spin.
This is the principle behind a motor.
