Download Magnetism - TeacherWeb

Electromagnetism
What is a Magnet?
• The earliest magnets were found naturally in
the mineral magnetite which is abundant the
rock-type lodestone. These magnets were
used by the ancient peoples as compasses to
guide sailing vessels.
• Magnetism is the force of attraction
or repulsion of a magnetic material
due to the arrangement of its atoms,
particularly its electrons.
What is a Magnet?
• Magnets have two ends
or poles: North & South
• Unlike poles of magnets
attract each other and
like poles of magnets
repel.
• No Monopoles: If you cut
a magnet in half, you get
two poles on each
Magnetic Domains
• Atoms have magnetic properties due to electron
spin (more about this in a minute!)
• Small areas where the groups (billions) of atoms
are aligned are called Domains
• Domains align in the same direction when placed
in a magnetic field.
Magnetic Domains
• Ferromagnetic materials: Iron, Nickel, Cobalt
• Often magnets are made of alloys; e.g. AlNiCo,
Steel
• To create a temporary magnet: heat it or beat it in
presence of a magnetic field
• Even ferromagnets can lose magnetism
Magnetic Fields
• Iron filings will align around a
magnet
• Long-Range force: Field Force
• These “lines of force” are called
Magnetic field lines
• Magnetic field lines go from North
to South…
• And make a complete loop back
around!
• Lines closest together (e.g. at
poles) field is strongest
Earth: A Giant Magnet
• Magnets & compasses always
orient themselves in a NorthSouth direction in Earth’s
magnetic field
• But… Opposite poles attract!
• So the North pole of a compass
magnet is attracted to Earth’s
magnetic South, and the
compasses’ South pole is
attracted to Earth’s geographic
North pole.
Earth: A Giant Magnet
• The magnetic field of the Earth is very weak
compared with the strength classroom ceramic
magnets.
• Both the strength of the Earth’s magnetic field and
the location of the north and south magnetic poles
can switch places.
• Today, the Earth’s magnetic field is losing
approximately 7 percent of its strength every 100
years.
Earth: A Giant Magnet
• The magnetic field
of the Earth is
affected by solar
wind (charged
particles)
• Most are deflected
by magnetosphere
• Some get in near
the magnetic poles
and interact with
atmosphere
Aurora
• Borealis: Near
North Geographic
Pole
• Austrialis: Near
South Geographic
Pole
Electromagnetism
• 1820: Hans Christian Oersted laid a compass under
a wire, expecting the needle to point toward the wire
or in the same direction as the current in the wire
• Instead, the needle rotated until it pointed
perpendicular to the wire
Electromagnetism
• Reversing the current
caused the needle to
reverse directions.
• A magnetic field (full
loops!) is set up around
any current-carrying
wire
• Magnetic Field is
perpendicular to
current
• Right Hand Rule:
Wrap hand around
wire, thumb with
current
Electromagnetism
• All magnetic fields originate from moving
electric charges.
• Electricity and Magnetism are interchangeable: Moving charges create a
magnetic field, changing magnetic fields
cause charges to move
Electromagnets
• When current is passed through a coil, the
magnetic field loops through (and around)
• Adding an iron core strengthens the field
• Electromagnets are very strong and can lift a lot of
heavy metal!
Electromagnets
• Electromagnets are also used to make speakers
• Varying electric currents in the wire changes the
magnet field of an electromagnetic coil
• The changing field exerts forces on the permanent
magnet
• The moving
permanent magnet
creates vibrations in
the diaphragm (cone)
• The vibrating air
pressure is detected
as sound waves.
Galvanometer
• An electromagnet that interacts with a permanent
magnet: The stronger the electric current passing
through the electromagnet, the more is interacts
with the permanent magnet.
• Galvanometers
are used as gauges
in cars and many
other applications.
Magnetic Force on Current
• If a current-carrying wire is
placed in a magnetic field,
a perpendicular force is
exerted on it
• Right Hand Rule again:
Point fingers in direction of
Magnetic Field, curl in
direction of current, thumb
points in direction of Force
Electric Motors
• An electric motor is a device which changes
electrical energy into mechanical energy.
• As current flows through the loop of wire, the
magnetic field exerts an upward force on one side
and downward force on the other side causing it to
rotate
Electromagnetic Induction
• So if moving charges through a magnetic field can
cause a loop of wire to move…
• Then moving a magnet through a loop of wire can
also cause charges to move!
• 1831: Michael Faraday found that if there is relative
motion between a magnet and a coil of wire, a
current is “induced” in the wire
• Electromagnetic Induction
Electromagnetic Induction
• So if moving charges through a magnetic field can
cause a loop of wire to move (motor) …
• Then moving a magnet through a loop of wire can
also cause charges to move!
• For current to be induced, there must be a change
in magnetic “flux” (or a change in the # of field
lines going through the coil)
Generators
• Heinrich Emil Lenz determined
that the current induced produces
a field that tries to “restore” the
field or counteract the change
(Lenz’s Law)
• As the flux increases, current
induced produces a magnetic field
opposing the change
• This follows the law of
conservation of energy!
Generators
• So if the flux increases, current goes one direction;
as flux decreases, current goes other direction
• So if a loop of wire is rotated in a magnetic field, the
flux will alternate
• This sets up and alternating current: AC!
Motors vs. Generators
• Motors use electrical energy, change it into
mechanical energy (kinetic)
• Generators use mechanical energy (kinetic) and
change it into electrical energy
Transformers
• Two unconnected coils wrapped
around a soft iron core
• AC is sent through the first coil
• This induces a changing
magnetic field in the core
• The changing magnetic field in
the core induces an alternating
current in the second wire
• Depending on the number of
coils in each wire, a transformer
will “step up” or “step down” the
voltage
Transformers
• To save on power loss, voltage
is “stepped up” (to reduce
current) at the plant for
transmission over long distances
• (Secondary coil has more turns)
• Where power will be used, it
must then be “stepped down” to
a usable voltage and higher
current
• (Secondary coil now has fewer
turns)