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Transcript
Magnetism Notes
I. Magnetism – refers to the properties and interactions of
magnets
A. Magnetic force is the interaction between two
magnets.
• A magnet is surrounded by a magnetic field that
exerts the magnetic force.
• The magnetic field is strongest closer to the
magnet.
B. Magnetic Field of a Bar Magnet
• Un-magnetized iron brought near a magnet
becomes magnetized.
• Iron filings line up around the bar magnet. This
shows the shape of the magnetic field.
C. Magnetic Poles – regions
where magnetic force is
the strongest
• All magnets have a
north and south pole.
• Magnetic field lines
always connect the
north pole and the
south pole of a
magnet.
D. Earth’s Magnetic Poles
• Earth is like a bar magnet, with
its south magnetic pole near
its geographic south pole.
• Scientists are NOT sure what
produces Earth’s magnetic
field.
E. Compass Needles
• A compass contains a small
bar magnet for a needle.
• If you place a bar magnet near
a compass, north and south
poles will attract.
The earth's magnetic field is just like the field of any
magnet - only LARGER and STRONGER. A compass is
simply another magnet. And the principles of
attraction and repulsion govern the earth magnet and
the compass magnet. The earth magnet is considered
stationary. Therefore, the compass magnet's north
pole is attracted to the earth's south pole and the
compass' south is attracted to the earth's north.
Which means that the compass' magnet, which is free
to turn, always points north. The confusing part of this
is that the NORTH POLE of the compass points to the
NORTH POLE of the earth. This apparently says "North
attracts North." Of course, this is NOT true. The
magnetic pole near the north geographic pole is
ACTUALLY A SOUTH MAGNETIC POLE. Common usage
has named this "the North Pole" - just remember that
MAGNETICALLY it's a SOUTH pole.
F. Magnetic Materials
• Only a few metals such as iron, cobalt, and nickel are
attracted to magnets or can be made into a
permanent magnet.
• These particular metals are magnetic because the
magnetic properties of the electrons don’t cancel out.
II. Magnets
A. Making a Metal a Magnet
• In order for a metal to become a magnet,
the atoms in the metal must align their
magnetic domains in the same direction.
• If you place a permanent magnet near a
nail, the magnet aligns the magnetic
domains and the nail becomes a
temporary magnet.
• A permanent magnet can be made by
placing the metal in a strong magnetic
field.
• Permanent magnets can lose their
magnetic behavior if heated or dropped.
B. What happens when a magnet is broken?
• In un-magnetized iron, the domains are randomly
oriented.
• In slightly magnetized iron, there is incomplete
alignment of domains.
• In strongly magnetized iron,
virtually all of the domains are
aligned.
• Even if the magnet were divided
into bits as small as a single
domain, it would still have
north and south poles.
C. North Poles Cannot Be Separated from South
• Each piece of broken magnet still has a north and
south pole.
• No matter how many times a bar magnet is cut in half,
there is always a north and south pole, even in the
smallest piece.
III. Electromagnets
A. Moving charges and Magnetic Fields
• When electric current flows
through a wire, a magnetic
field forms around the wire.
• The direction of the magnetic
field depends on the
direction of the current in
the wire.
B. Electromagnet – temporary magnet made by placing a piece of
iron inside a current
• The more coils there are, the more current the electromagnet
has.
• The gauges in a car use galvanometers, which are devices that
use an electromagnet to measure electric current.
C. Electric Motors – a device that changes electrical energy into
mechanical energy.
• A battery causes an electric current to flow through the coil of
the electromagnet.
• Unlike poles of the two magnets attract to each other and the
like poles repel. This causes the coil to rotate until the
opposite poles are next to each other.
• If the current in the coil is
switched, the direction of the
coils magnetic field also switches.
The north and south poles of the
magnet trade places.
• The coil is repelled by and
attracted once again to the poles
of the permanent magnet.
D. Generators
• A generator produces electric current by
rotating a coil of wire in a magnetic
field.
• Mechanical energy is turned in to
electrical energy
E. Current Direction
• A direct current (DC) flows
through only one direction
through a wire
• An alternating current (AC)
reverses the flow of the current
flow
F. Transformers
• A transformer is a device that increases or decreases voltage
of an alternating current.
• Made of two coils of wire called
primary and secondary coils.
• If the secondary coil has more
wires, then the transformer,
increases, or steps up the voltage.
• If the primary coil has more wires,
then the transformer steps down,
or reduces voltage.
*** This is how electricity gets to houses in power lines.
G. Power Transmission and Transformers
• In the U.S., power lines can carry up to 750,000 volts.
• This amount of power is dangerous and cannot be used with
household appliances.
• Step down transformers convert voltage to 120 V—a usable
amount of power for household appliances.