Download Magnetism Permanent magnetism Permanent magnets

L 28 Electricity
and Magnetism [5]
Magnetism
• two sources of magnetism
• permanent magnets
• electromagnets
Why are magnets magnets?
• the earth’s magnetic field
magnetism
„ magnetic forces
„ applications
„
• how does a compass work
• the north pole is really a south pole!
• Van Allen radiation belts
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Permanent magnetism
• certain minerals (magnetite,
Fe3O4) are naturally magnetic
• a piece of loadstone will attract
bits of iron
• a magnet produces a magnetic
field in the space around it, just
like the Sun has a gravitational
field that holds the planets in
their orbits
• the magnetic field can be
visualized with iron filings
Earth’s g-field
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• Are made from alloys of some of the rare
earth elements like neodymium and
samarium and cobalt.
• Always have a north and a south pole
• like poles repel and unlike poles attract
• if you break a magnet in half you get 2
magnets Æ cannot have just a north or
just a south pole
NS
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Bar Magnet
Permanent magnets
S
Solenoidal Coil
Electromagnet
• If you pass current through a loop of wire you get a
magnet Oersted discovered this
• Ampere figured out the formula relating currents to
magnetic fields (Ampere’s Law)
NN
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1
N S
S
N S
+
N S
MAGNETIZED
N S
N S
N
N
N S
N S
N S
N S
S
S
S
N
S
N S
N S
atomic
magnets
N S
N S
N S
N S
SOUTH
You can think of the nail as a collection of little
magnets that are randomly aligned. The magnetic
field of the coil aligns these little magnets giving a
larger field than that of the coil alone. We say that
the nail becomes “magnetized”, but the effect is
not permanent.
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N S
PARTIALLY
MAGNETIZED
NORTH
Duracell
N
N
N S
UNMAGNETIZED
N S
COIL
Iron nail
N S
Homemade magnets
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Magnetic materials
The earth is a big magnet
• some materials are naturally magnetic or
can be magnetized and retain their
magnetism Æ ferromagnetic materials
• other materials (iron) can be magnetized
temporarily by placing them near magnets
• some materials have essentially no
magnetic properties Æ copper, aluminum,
plastics...
• heat can destroy magnetism (Curie effect)
• The earth’s north
geographic pole is the
south pole of a big
magnet.
• A compass needle is
attracted to the earth’s
north geographic pole
• The earth’s magnetism is
the magnetic north pole is
due to currents flowing in
inclined about 14° from the
its molten core (not
geographic north pole, or
entirely understood!)
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by about 600 miles.
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Acnn
c tiv a te
Sun – Earth Connection:
space weather
CNN Report Dec. 12, 2003
Report: Earth's magnetic field fading
Slight chance of flipping magnetic poles
earth
The strength of the earth’s magnetic field had
Decreased10% over the last 150 years. At this
rate, the field will disappear altogether in
1,500 to 2,000 years.
solar eruption
North
SUN
time
solar eruption
Northern Lights
South
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Particles stay on the magnetic
field lines
Solar eruptions
electron that originated
on the sun Æ part of the
solar wind.
The earth’s magnetic
field traps charged
particles.
During periods of intense
solar activity, satellites can
be at risk.
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Van Allen Radiation Belts
Magnetic forces
• magnetic fields can
cause charges to turn
around.
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Magnetic forces on wires
Magnetic
deflection of
electrons in a
TV tube.
N
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S
Wire pushed
OUT
N
Wire pulled
IN
S
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Forces on parallel wires
Applications
Magnets are not just for
holding things on the
refrigerator!
Opposites repel
Likes attract
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The electric motor
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Magnets make speakers work
The force between the permanent magnet
and the voice coil moves the speaker cone
When a current is present in a coil, it
experiences a torque and rotates.
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MAGLEV Trains
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Magnetic tape recording
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