Download Magnetism - District 196

Magnetism
Elecricity is closely related to magnetism.
Magnetic forces and fields are produced by a moving
electric charge.
Why this happens? We do not really know. It is a
relativistic effect produced by the motion of the electric
charge.
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History
800 BC
The Greeks discovered Lodestone, a naturally occuring
magnet. This material was found in the region of Asia
Minor called Magnesia. (This is where the term "Milk
of Magnesia" comes from) (Ha, Ha)
12th. Century
The Chinese invent the compass and begin using it for
navigation. They would place a bar magnet on a piece
of wood and float it in a bowl of water.
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History
1269
Petrus Peregrinus, a military engineer in Europe, introduced
the idea of magnetic poles and explained how to determine
north and south.
16th. Century
Columbus, observed that the amount of dip of a compass
needle (magnetic declination) changed in relation to your
position on Earth.
William Gilbert wrote the book, On the Magnet. He was the
first to study magnetic lines of force. He made a large sphere
of lodestone and found a change in the declination an
inclination of a magnetic needle.
1820
Hans Christian Oersted discovers that an electric current will
deflect a compass. He did this while demonstrating
electricity to a physics class.
This discovery basically started the Industrial Revolution.
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William Gilbert
1544-1603
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Hans Christian Oersted
1777 - 1851
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Magnetic Poles
Magnetic Poles
Similar, yet DIFFERENT than electric charges.
Similarities
1. Can attract and repel without touching
2. The amount of attracting or repelling depends on
distance
3. Like poles repel, opposite poles attract.
Difference
Magnetic poles can NOT be separated. They always
occur in pairs.
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Magnetic Fields
Magnetic Fields
The space around a magnet in which a magnetic force is
exerted.
These are imaginary lines that scientists use to describe
the effect on a compass needle when it is near a magnet.
Rules for Magnetic Field lines.
1. They show the shape of the field
2. They exit the North pole and enter the South Pole.
3. If the lines are close together the field is stronger.
4. The lines may never cross.
5. Use arrows to show the direction the north pole of a
compass points.
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Magnetic Field Pictures
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Magnetic Field Pictures
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Magnetic Field Pictures
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Magnetic Field Pictures
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Atomic Theory of Magnetism
We now know today that magneic fields are produced
by the motion of electric charges.
The charges can spin or orbit.
Electrons have two magnetic fields, one due to the spin
and one due to its orbit about the nucleus. The field due
to the spin is stronger.
In most materials the spins of the electrons are in the
opposite directions so the fields cancel out.
Iron, Nickel, Cobalt.
In these naturally occuring materials the spins do not
cancel. Thus, they can be made into magnets if the
conditions are right.
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Magnetic Domains
Magnetic domains are microscopic clusters of aligned
atoms.
(The spins of the charges line up.)
Magnetic Induction
If a strong magnetic field is brought near a set of random
domains, the domains will align with the magnetic field.
The domains physically rotate.
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Magnetic Domain Pictures
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Magnetic Domains
Destroying Magnets
To destroy a magnet you must force the domains to
become random in their orientation.
This may be done by heating or hitting the magnet.
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