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Magnetism
Magnetic Properties
• Like magnetic poles repel
• Unlike magnetic poles attract
• Magnetic field – space around a magnet in
which a magnetic force is exerted.
Origin of Magnets
• Early Greeks discovered a
rare ore in a region of Turkey
called Magnesia.
• In the 12th century, the
Chinese found that a single
stone made of this ore if
suspended will always point
toward geographical north
Origin of magnets
• Lodestones – leading stone used by
navigators to always know the direction of
travel.
• Chinese invented the first compass which
they called lodestones.
Magnets
• N pole – North seeking geographic north
• S pole – South seeking geographic south
Compass
• A magnet that is free to move
• Responds to Earth’s magnetic field
What are some statements that we
just made that may seem
contradictory?
• The _________
south
pole of
the earth’s magnetic field
is located close to the
north
_________
geographic
pole.
• The N pole of a compass
needle is attracted to the
south
earth’s _________
magnetic pole.
Magnetic declination
• Difference between magnetic and
geographic poles.
Magnetic or Not
Paper clip
Penny
Aluminum
foil
Nail
Razor
blade
Straight pin
Paper
Wood
Quarter
Zinc
Ferromagnetic elements
• Are strongly attracted to magnets
• Paired electrons with opposite magnetic
spins cancel
• Unpaired electrons with same spins cause
magnetic fields to reinforce for an atom
Ferromagnetic elements
• Are strongly attracted to magnets
• Have many unpaired electrons spinning in
the same direction to produce identical
magnetic fields.
• Most common are Fe, Co, and Ni
Why aren’t all nails magnets?
• Attach paper clips to your magnets until
they will hold no more.
• Remove the magnet by holding the top
paperclip and taking the magnet away.
• What happened?
• Drop the paperclips. Try to pick up one
with another to see if they are still
magnetic.
Why aren’t all nails magnets?
• Domains – large groups of ferromagnetic
atoms whose magnetic fields are aligned.
• Randomly oriented in unmagnetized
substances.
• Align themselves with an external
magnetic field. (magnetic induction)
Iron for Magnets
• Hard ferromagnetic materials (alloys)
make better permanent magnets than soft
iron because their magnetic domains are
hard to align, but once they are, they will
stay aligned even without the magnetic
field.
How does dropping a magnet
weaken it?
• The magnetic domains of some atoms
become misaligned and therefore weaken
the magnetic field of the magnet.
How will a magnet be affected by
breaking it in two?
• Breaking a magnet will create two
magnets. Both with north and south poles
still.
How will heating a magnet affect it?
• Increases the thermal molecular motion
causing some of the atoms to become
misaligned and weakening the magnetic
field.
Show What You Know
1) Ferromagnetic materials
a) are strongly attracted to magnets
b) include iron, cobalt, and nickel
c) have many unpaired electrons, with identical
spins
d) all of these
2) Magnetic monopoles
a) would have north or south poles alone
b) do not exist
c) both a and b
d) neither a nor b
3) Microscopic regions of atoms whose electron
spins are the same a) are called domains
c) are always aligned
b) exist in all substances
d) all of these
4) Breaking a magnet in two
a) destroys its magnetic properties
b) makes two smaller magnets
5) Which of these actions will not damage the
polarity of a permanent magnet?
a) dropping it
b) heating it
c) breaking it in half
d) none of these