2. Magnetic Poles
• Like poles repel,
opposite poles attract
– it’s a bit like the electric
force in this way
• Every magnet has a
north and a south pole
• Break a bar magnet in
half and you get two bar
magnets.
3. Magnetic Fields
• Inside magnet field goes
south to north.
• Outside field goes from north
to south.
• Denser lines = stronger field.
• A smaller magnet will tend to
align its poles with the larger
magnet.
– See: Compass
4. Demo: Magnetic Fields
• Sprinkle some iron filings over top of a
permanent magnet.
• Multiple magnets.
5. Magnetism is the result of moving
charges.
• The electrons in an atom
are in constant motion.
• Electrons (and nuclei)
may be ‘spinning’ which
is also motion of charge
and may produce a
magnetic field.
6. Magnetic Domains
• In most atoms, for every ‘spin’ in
one direction there is a ‘spin’ in the
opposite direction, canceling out
the magnetism.
• Some atoms (iron, nickel, cobalt)
have spins that add up in the
same direction.
• The magnetic fields of many
atoms may become aligned,
resulting in ‘domains’ with aligned,
stronger magnetic fields.
• When these domains tend to line
up you get a permanent magnet.
7. Example Problem: Breaking a magnet
• When you break a permanent magnet in half,
you get two permanent magnets. Explain, in
physical terms, how this could be so.
8. Magnetic Fields around Electric
Currents
• If a moving charge
creates a magnetic field,
what would you expect
from a current in a wire?
– Current in a wire
generates a magnetic
field. It curls around the
wire.
– Curl the wire and you can
get an electromagnet.
9. Demo
• Compass and current in a wire.
– I’m not sure how well this will work...
10. Example Question
• What is the cause of a magnetic field about a
permanent magnet, and about a current
carrying wire?
11. Electromagnets
• A coil of wire (often with an
iron core).
– Current flows through the wire.
– Due to the wrapping, the small
magnetic field from each
individual wire adds up.
– Lots of coils + high current =
strong electromagnet.
• May also be
superconducting
– must be very cold
– has near zero resistance to
current
13. Magnetic Forces
• If a moving charge creates a magnetic
field it stands to reason a magnetic
field affects a moving charge.
• A moving charge is deflected when it
moves through a magnetic field.
– if it is parallel it experiences no force.
– This effect is often used for something
called a ‘mass spectrometer’
• This effect is very important for
deflecting high-energy particles away
from the earth’s surface.
– This is why there is some concern
about magnetic field reversals.
14. Electric Motors
• So if we have a coil of
wire in a magnetic field
we can force the loop to
make a partial turn.
• If we cleverly arrange for
the direction of current
to reverse at the right
point, we get a loop that
will turn continuously.
16. Electromagnetic Induction
• A moving charge creates a
magnetic field.
• A charged object may
experience a force due to a
magnetic field by moving
through the field.
• What if we attempt to move
the field around a stationary
charge?
– Can we induce the charge to
move?
• Indeed – this principle lies
behind many things such as
metal detectors and
generators.
17. Faraday’s Law
• The induced voltage in a
coil is proportional to the
number of loops,
multiplied by the rate at
which the magnetic field
changes within those
loops.
– Current induced will be
proportional to the
resistance of the coil.
18. Sample Problem: Guitar pickups
• Electric guitars use steel strings. Under each string is
a coil of wire containing a permanent magnet. The coil
is connected to an amplifier and a speaker.
• When you pluck a string, the string vibrates above the
magnet and coil but does not make contact with it.
• Explain how the vibrating string can cause an
oscillating current to appear in the coil even though
the string isn’t connected to the coil.
19. Generators and AC
• A generator works in much
the same way as an electric
motor – but backwards.
– By rotating a coil in a magnetic
field you induce a current in the
coil (Faraday’s Law).
• The voltage induced has a
changing magnitude over
time, depending on how
quickly the number of
magnetic field lines through
the coil is changing.
20. Demo: Electric Power Generation
• Turning the handle spins a permanent magnet
inside of a coil.
21. Power Transmission: why AC?
• Transformers (not the robots).
– Changing voltage is as easy as having different numbers of
coils.
• All back to faraday’s law – two linked sets of coils with different
numbers of windings.
– Why does this matter?
• Remember P = I*V
• By increasing the voltage we decrease the current.
• Power dissipated by resistive heating: P = I2
R
– By increasing V we can dramatically decrease power loss in the lines.
• Example problem: Why is the use of AC preferred for
powerlines?
