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Magnetism
THE PROPERTY OF A MATERIAL/OBJECT
TO ATTRACT OTHER OBJECTS MADE
OUT OF IRON, MAGNETITE, OR STEEL
Magnets and Technology
 Computers use them to read and write data
 Your speakers and headphones use them
 Credit cards and ATM cards store data on
magnetized strips
 Junkyards use them to lift scrap metal
 They are used in many security systems
Two Categories
 Permanent:
 Retains its magnetic properties all the time regardless of
whether it’s near another magnetic object.
 Objects that can be magnetized:
 When brought close to another magnet
 i.e. a refrigerator magnet
 There are no liquid magnets
 This is because magnetic effects come from the small scale
organization of atoms within a material.
 Disrupting this orderly arrangement (i.e. heating) destroys
magnetic effects.
Poles
 All magnets contain two magnetic poles.
 They have an opposite polarity, called north and south pole.
 If you cut a permanent magnet in half, each will have a north
and south pole.
 You can separate + and – charges but not N and S
poles of a magnet.
Attraction and Repulsion
 When two magnets are close, they exert an
attractive/repulsive force depending on how the
poles are aligned.
 Opposites attract!!!
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
When north and south are put together, the magnets will
attract each other.
North-north or south-south will repel.
 The force between two magnets at shorter distances
increase more rapidly causing the forces to become
very strong and hard to separate.
Attraction and Repulsion (cont.)
 Because magnets are dipoles (two poles), the
strength decreases rapidly as they are separated.
 Opposite magnetic forces on the two ends of abar
magnet can create a torque (rotational motion) that
causes the magnet to twist/rotate.
Force Field
 An organization of energy in space that creates a
force on any receptive matter that passes within its
‘influence’.
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
Gravity is an example.
The sun’s gravitational field causes forces on each planet
 The force between magnets acts through a magnetic
field.
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

The poles create a magnetic field that extends outward in the
surrounding space.
Other magnets ‘feel’ forces through their interaction with this
field.
This is a non-contact force.
The Magnetic Field
 It describes the strength and direction of magnetic
forces around a magnet.



A compass is a freely spinning magnet sensitive to magnetic
fields.
Each location we place a compass tell us the direction of the
magnetic force.
The red end typically is the magnetic north pole.
 Uses lines and arrows to represent
Magnetic Field
Drawing
the force exerted on the north
magnetic pole.



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They point into south poles and out of
north poles.
The closer the field lines the stronger the
magnetic force.
Field lines never cross
Field lines always make closed loops
 Our planet has a magnetic field
approximately aligned with its
north-south axis.


Migratory birds sense this field and use it
(remember, The Core)
Humans use a compass.
Compass
 Why does a compass point north?
 Earth’s north geographic pole is a south magnetic pole.
 People labeled the north magnetic pole of a magnet as the pole
that pointed north.
 Earth’s magnetic field completely reverses itself
every million years or so!
Diamagnetism, Paramagnetism
and Ferromagnetism:
 Diamagnetism:


The effect of materials to slightly repel magnets of either polarity.
Silver, lead, and copper
 Paramagnetism:


Slightly attract magnets of either polarity
Aluminum, Magnesium and Tungsten
 These are substances that can become magnetized and
experience strong magnetic forces in the presence of
external magnets.


Paperclips
Iron, Cobalt, and nickle
 The property that makes iron
Magnetic
Domains
ferromagnetic is that it only takes
a small amount of energy for an
iron atom to flip its magnetic axis.
 Groups of Iron atoms form these
domains in which atoms within a
single domain have a similar
magnetic alignment.
Magnetic Domains
 When an external magnet is brought near iron,
magnetic domains attracted to the magnet gain
atoms and grow because the iron atoms easily
change their magnetic orientation.
 Domains repelled by external magnets easily lose
atoms and shrink and vice versa
 Ferromagnetic materials are crucial to many
technologies such as motors and generators.
 Each magnet has a north-seeking and south-seeking
pole
 Exert a magnetic force on the space around them =
magnetic field
 Lines
closer together – stronger field
 Where
is the field strength the greatest on a magnet?
Nature of a Magnetic Field
 Produced by the motion of an electric charge
(current)


Current carrying wire can produce a magnetic field
(macroscopic)
Electrons around an atom produce a tiny current and thus a
magnetic field (microscopic)
 SI unit for magnetic field is the Tesla
 Different examples of magnetic fields
Magnetic field (cont.)
 Two e- spinning in same direction = stronger
magnetic field
 Two e- spinning in different direction = zero
magnetic field
 Common magnets are made of iron, nickel, and
cobalt.

The spin of their e- do not cancel => magnets
Magnetic Domains
 Individual atoms align to form magnetic domains, a
cluster of aligned atoms.
 How can a magnet become weaker?
 How does a magnet attract a piece of metal that is
not magnetized?
Electric Currents and Magnetic Fields
 A current-carrying wire will produce magnetic fields
in a pattern of concentric circles
 When wire is made into loops the magnetic field
bunches up.
 More loops = more magnetic field = electromagnet

(made stronger by adding nail/ iron core)
Practical applications of electromagnets
 Maglev trains – page 569, http://www.o-keating.com/hsr/maglev.htm
 Stereo speaker, http://electronics.howstuffworks.com/speaker3.htm
 Alarm systems
 Junkyard crane
 MRI, http://electronics.howstuffworks.com/mri2.htm
Magnetic Forces on moving charged
particles
 A charged particle must move relative to the
magnetic field in order to be affected by the
magnetic field
 A charged particle that is moving perpendicular to
a magnetic field will be forced upward
Force of a Magnetic Field on a moving e-
The force that a magnetic field places on a charged
particle is …


greatest when e- move perpendicular to field lines
Zero when e- move parallel to field lines
Magnetic Forces on
Current-carrying wires
 If a moving charged particle is deflected by a
magnetic field, so will a wire that contains moving
e-
 A wire will be forced …
 up if current is flowing one direction
 Down if current is flowing in opposite direction
 We see this exhibited when using a compass
under a wire.
Meters
 Galvanometer-detects current
 When calibrated it can measure current (ammeter)
or voltage (voltmeter)
Motors
 By modifying a galvanometer you can make an
electric motor
 http://electronics.howstuffworks.com/motor1.htm
Investigation
When we moved a compass around the magnet
during the experiment yesterday, what
occurred?