Download What is magnetism

What is magnetism?
•
•
•
•
•
•
•
•
•
•
•
Magnets got their name from a region in Greece called Magnesia
The 1st magnetic rocks called lodestones which has magnetite in them, were
found in this region nearly 3000 years ago
These rocks are magnetic all the time and so are called permanent magnets
You can turn any piece of iron into a permanent magnetic by stroking it several
times with a permanent magnet
Magnetic Pole-Like poles repel and opposite poles attract
Magnets have a pair of poles, a north pole and a south pole. These are called
Dipoles. The poles of magnets
exert a force on one another. Two
like poles such as south poles will
repel each other. Two unlike poles
attract each other.
Magnets always have a north and
south pole
When like poles repel each other
and non-like poles attract one
another this is due to an invisible
force called magnetic fields.
All magnets produce a magnetic
field
The force of the magnetic field
depends on the distance to the
magnet-the farther away, the
weaker the magnetic field is. This
is called the inverse square law.
(Force = 1/ distance 2)
The strength of magnetic fields is
shown with lines. The stronger the
magnetic field, the closer the lines
are. Field lines that are drawn
farther apart indicate a weaker
field.
What’s a Compass?
A compass is a device that tracks the Earth’s
magnetic field. Compasses are built with a magnet
suspended on top of a pivot so that the magnet can rotate freely
• Compasses align with the Earth’s magnetic field because the Earth is like one
giant magnet due to the circulation of electrons in the Earth’s liquid core.
Permanent Magnets versus Temporary Magnets
There are two basic kinds of magnets – permanent and
temporary. We’ll take on permanent first – which is appropriate,
because before humans could invent and exploit temporary
magnets, they had to discover the permanent kind first.
People first happened upon magnets sometime before 600 BC
with the discovery of the mineral magnetite, a naturally
occurring magnet. One type of magnetite, lodestone, has the
additional property of polarity – it aligns itself with the Earth’s
magnetic field. The lodestone made possible the invention of the
compass, which was used in navigation as early as 1086.
Unlike temporary magnets, the permanent ones (such as those on
your fridge) stick around for awhile (no pun intended). While
they may not last forever, you often have to go to some effort to
demagnetize them. You can’t turn them on and off with the power switch. Permanent
magnets all belong to a class of materials referred to as ferromagnetic.
The other major difference between permanent and temporary magnets is what the
magnetic fields look like on an atomic level. These are two different phenomena entirely.
All magnetism comes down to electrons. In the electromagnets we’ll discuss in a minute,
magnetic fields result from electron flow through a conductor. In the case of permanent
magnets, it’s the spinning of the electrons that creates magnetism, not their movement
through a conducting material.
You know that the Earth is a great big magnet. Well, electrons
are teeny tiny ones. They have a north and a south pole, too, and
spin around an axis. This spinning results in a very tiny but
extremely significant magnetic field. Every electron has one of
two possible orientations for its axis.
Most materials are either naturally magnetized, or can be induced
to be magnetic. In most materials, atoms are arranged in such a
way that the magnetic orientation of one electron cancels out the
orientation of another.
Iron and other ferromagnetic substances, though, are different. Their atomic makeup is
such that smaller groups of atoms band together into areas called domains, in which all
the electrons have the same magnetic orientation. Inside each of these domains, the
electrons are oriented in the same direction.
Magnets are atomic powered. The difference between a permanent magnet and a
temporary magnet is in their atomic structures. Permanent magnets have their atoms
aligned all the time. Temporary magnets have their atoms aligned only while under the
influence of a strong external magnetic field.
Make your own electroscope analysis
Name:________________________
After conducting the activity as described in class, answer the following questions:
1. On the picture below, draw in the piece of aluminum foil to show its reaction to
the statically charged foam. Then, show the distribution of charges on the can.
2. We learned that the force of attraction or repulsion between two charged objects
is inversely proportional to the square of the distance between them:
1
F 2
where r = distance (d)
r
a. This means that if the distance between two charged objects is doubled,
then the force between them will be ________ what it was.
b. If the distance is tripled, then the force will be ________ what it was.
c. If the distance is halved, then the force will be ________ what it was.
3. Assume there is an attractive force of strength 100 between two oppositely
charged objects that are 1 cm apart.
What would the attractive force be when the objects are 2 cm apart and when they
are 0.5 cm apart?
Magnetism from Electric Currents
***Magnetism can be produced by electric currents
***Electric currents produce a magnetic field
Use the right-hand rule to find the direction of a magnetic field produced by a
current
V= direction of current
B= direction of the magnetic field
A safe way to increase the magnetic field and therefore the current is to wrap the
conductive wire into a coil. Each loop of wire adds to the strength of the magnetic field.
The more loops, the stronger the magnetic field. This is called a solenoid. The result is a
strong magnetic field similar to those produced by a bar magnet. The strength of a
solenoid’s magnetic filed can be increased further by inserting an iron rod through the
center of the coils. This is called an electromagnet. The magnetic field of the solenoid
causes the iron rod to also become magnetic. The magnetic field of the rod then adds to
the coil’s field, creating a stronger magnet than the solenoid alone.
Hair dryers and stereo speakers are two examples of everyday objects that are powered
by electromagnets.
Electric Motors
Electric motors are devices that use magnetism to cause motion. Electric currents in a
wire cause the wire to develop a magnetic field, with north and south poles. When the
coil of wire comes in contact with the poles of a magnet, the coil of wire will spin
because like poles repel. If a coil of wire is attached to a shaft, it can do work. You can
increase the strength of a magnetic field by increasing the number of coils.
When a current flows through a coil of wire, the coil becomes an electromagnet. The
direction of the current determines the polarity of the magnetic field. One side of the
coil becomes the north pole, and the opposite side becomes the south pole. As with all
magnets, opposite magnetic poles attract and like magnetic poles repel. The magnet pile
placed under the coil attracts its opposite pole on the coil and repels its like pole, causing
the coil to spin.
Since the arms of the coil only have insulation on one half, electric current only passes
through when the uninsulated part of the wire touches the paperclips. When the coil
swings around and the insulated part of the wire touches the paperclips, the current is
interrupted, stopping the magnetic field for half a turn. When the current flows through
the coil again, the two magnetic poles either repel or attract each other once more. After
the coil starts spinning, momentum carries it through the part of its cycle when there is no
current.
Almost every mechanical movement that you see around you is caused by an AC
(alternating current) or DC (direct current) electric motor.