Magnetism
History of Magnets
• (~800 BC) Ancient Chinese and Greeks discovered
that certain stones would attract and magnetize
iron.
• Small slivers of the stone were found to align
themselves with the North Pole.
• Chinese were the first to use magnets for
navigation.
• The orienting properties were used to align streets
in cities in the North-South / East-West direction.
Poles of a Magnet
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Magnets have a North and South Pole.
Like poles repel.
Unlike poles attract.
What happens if you break a magnet in
half? Will you get two monopoles?
– No.
N
S
N
S + N
S
Oddly shaped
magnets still have a
north and a south
Magnets either attract or repel each other
South poles are attracted to north poles
Like poles repel
Unlike poles attract
Magnetic Field Lines vs. Electric Field
Lines
Magnetic Dipole
Electric Dipole
What happens when you bring a
compass near a bar magnet?
The north indicator of the compass will
point toward the south pole of a
magnet.
This is also why a compass points
north, it's lining up with the earth's
magnetic poles.
The Earth’s Magnetic Field
• The earth has a magnetic field that scientist believe is a
result of the dynamo effect due to electrical currents
created in the molten iron and nickel outer core.
• The Earth's Magnetic Field
• Bar Magnet - 3D
Sometimes the field
completely flips. The north
and the south poles swap
places. Such reversals,
recorded in the magnetism
of ancient rocks, are
unpredictable.
They come at irregular
intervals averaging about
300,000 years; the last one
was 780,000 years ago. Are
we overdue for another? No
one knows.
The magnetic North Pole is responsible for more than just the
direction a compass points. It's also the source of the aurora
borealis, the dramatic lights that appear when solar radiation
bounces off the Earth's magnetic field.
This happens at the South Pole
as well. In the southern
hemisphere, the lights are
called the aurora australas.
Source of Magnetic Fields
• Electrical Charge in motion.
– Currents occur at the atomic level in atoms due to the orbits of
electrons around the nucleus.
– The intrinsic spin (+1/2, -1/2) is critical in the case of magnetism.
Magnetic Domains
• A: Iron absent of a magnetic field.
• B: Iron in the presence of a magnetic field.
• C: A non-magnetic material.
A
B
C
Magnetic Domains = groups of
atoms with aligned poles
Magnets can be temporary (like
the needle used in the compass).
This nail has its atoms aligned, but
the effect is only temporary. You
can get this affect by rubbing the
nail on a magnet.
Neat fact: Hitting the nail can
demagnetize it, you are basically
scrambling the atoms.
Magnetism of Soft Ferromagnetic
Materials
How does a magnet attract screws, bolts nails, paperclips, etc.
when they are not magnetic to start with?
– Soft ferromagnetic material align their domains in the presence of an
external magnetic field creating a magnetic dipole.
• When the magnetic field is removed, the domains re-randomize resulting in
no magnetic attraction. They are temporary
• Soft ferromagnetic material is attracted to both the North pole and South
pole.
S
N
S
Types of Magnets
• Temporary: When charged particles move through space,
they induce a magnetic field (Electromagnets).
• Permanent: Electrons have an intrinsic magnetic field that
may add together in certain matter to create a magnetic
field (Speakers).
Temporary
Permanent
A permanent magnet is an object made from a material that is
magnetized and creates its own persistent magnetic field.
An everyday example is a refrigerator magnet used to hold
notes on a refrigerator door.
Metals that are magnetic: nickel, iron, cobalt
Things that are not magnetic: aluminum, plastic, glass
Ferromagnetic - a substance such as iron in which the
magnetic moments of the atoms spontaneously line up with
each other, making a large net magnetic moment.
Ferromagnets lose their ferromagnetism when heated above a
specific temperature , because the thermal energy melts the
magnetic alignment.
What else can cause a ferromagnet to lose its magnetism?
Focus on SPEAKERS
The loudspeakers in your radio, television or stereo system
consists of a permanent magnet surrounding an electromagnet
that is attached to the loudspeaker membrane or cone.
By varying the electric current through
the wires around the electromagnet,
the speaker cone moves back and
forth.
The resulting vibration of the speaker
cone will create sound waves,
including that from voice and music.
Maglev Trains
Magnetic + Levitation =
maglev
Maglev vehicles “float” over
an electromagnetically
powered fixed steel
guideway and are propelled
by the current with no
motors, wheels, moving
parts or additional energy
sources. The system is
environmentally friendly,
energy efficient and runs in
all weather conditions.
http://dsc.discovery.com/video
s/extreme-engineeringseason-1-shorts-maglevtrain.html
Make your own compass
http://adventure.howstuffworks.com/outdooractivities/hiking/compass2.htm
Applications
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Computer disc drives (hard and floppy)
VCR and cassette tape
Credit cards
Speakers
Motors (Both AC and DC)
Speed sensors
Solenoids for relays, valves, etc.
Magnetos (piston engine aircraft)
Key Ideas
• All magnets have North and South Poles
• Magnetic field lines originate in the North and end at the
south pole.
• Magnetic field lines do not cross.
• Magnetism exists at the atomic level.
• Magnetism is the result of moving charges.
• Some magnets are temporary while others are permanent.
• Types of Magnetism.
– Ferromagnetism.
– Paramagnetism.
– Diamagnetism.
Quiz Time - check out this page first
http://www.magnet.fsu.edu/education/tutorials/index.html
Check out the *Science Demonstrations link for some neat
tricks with magnets.
Or check out this video about "LINEMEN" who repair
power lines: http://www.youtube.com/watch?v=7gl-_Liz_0o
1. All magnets are surrounded by an invisible force called the
________________ _______________
2. The north pole of one magnet will be attracted to the ______
pole of another magnet.
3. The poles of the earth can move. True or False?
4. In the far north, radiation from the sun is reflected off the
earth's magnetic field and creates patterns of colors. This effect
is known as the ___________ ________________
5. A nail is normally not magnetized. How can you magnetize
it?
6. Who discovered that magnets are affected by electricity?
7. You can make an ______________________ by wrapping a
wire coil carrying a current around an iron core.
8. What car part is attached to your starter and is needed to
start the car?
9. This is a device used to measure electric current.
10. A generator converts ___________ energy into electrical
energy.