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Transcript
MAGNETISM AND ITS USES
CHAPTER 8
SLIDE 1
The Greeks found a
mineral, they called
(lode stone), now
called magnetite. It
had natural
magnetic properties.
What explanations do
you think they gave
for the behavior of
magnetite?
SECTION 1:
MAGNETISM
A. Magnetism – the properties an
interactions of magnets
1. Interaction between two magnets
called magnetic force increases as
magnets move closer together.
2. A magnetic field, which exerts the
magnetic force, surrounds a
magnet, and is strongest closest
to the magnet.
SLIDE 2
B.
Magnetic poles – the
regions of a magnet
where the magnetic
force exerted by the
magnet is strongest
1. All magnets have a
north
pole and a south pole.
2. Like poles repel.
Unlike poles attract.
SLIDE 3
The magnetic field
lines around
horseshoe and
disk magnets are
closest together
at the magnet’s
poles.
Magnets can attractSLIDE
or4
repel each other.
Unlike poles attract. When
unlike poles are brought
together, their magnetic
field lines seem to
connect with each other.
Like poles repel. When
like poles are brought
together, their magnetic
field lines seem to push
away from each other.
C.
Magnetic materialsSLIDE
– iron,
cobalt, and
5
nickel
1. The magnetic field created by each
atom exerted a force on nearby atoms.
2. Magnetic domains – groups of atoms
with aligned magnetic poles.
a. In a magnet, the like poles of all
the domains point in the same
direction.
b. Permanent magnets are made by
placing a magnetic material in a
strong magnetic field, forcing a large
number of magnetic domains to line
up.
SLIDE 6
3.
Earth has magnetic
poles.
a. A compass
needle is a
small bar magnet
that can freely
rotate.
b. A compass
needle always
points north.
A normal iron nail is made up of billions of domains that are
arranged randomly.
The domains will align themselves along the magnetic field lines
of a nearby magnet.
Each piece of a broken magnet still has a north and a south pole.
SECTION 2:
A.
ELECTRICITY & MAGNETISM
Moving charges and magnetic fields.
1. Moving charges, like those in an electric current,
produce magnetic fields.
a. The magnetic field around a current-carrying
wire forms a circular pattern about the wire.
b. The direction of the field depends on the
direction of the current.
c. The strength of the magnetic field depends on
the amount of current flowing in the wire
SLIDE 2
B. Electromagnet – a temporary magnet made
by placing a piece of iron inside a currentcarrying coil of wire.
1. Magnetic field is present only when
current is flowing in the wire.
2. Increase strength of the magnetic field
by adding more turns to the wire coil or
increasing the current passing through
the wire.
3. Magnetic properties of electromagnets can
be controlled by changing the electric
current flowing through the wire.
4. Converts electrical energy into mechanical
energy to do work.
SLIDE 2
B. When many loop of currentcarrying wire are formed into a coil,
the magnetic field is increased inside
the coil. The coil has a north pole and
a south pole.
A. Magnetic field lines circle around
a loop of current-carrying wire.
C. An iron core inserted into the coil
becomes a magnet.
SLIDE 3
The electromagnet in a
speaker turns
electrical energy into
mechanical energy to
produce sound.
A.
B.
SLIDE 4
When current flows
through the coil, an
electromagnet is formed
that is attracted to and
repelled by the poles of
the permanent magnet.
The magnetic forces on
the coil cause it to rotate,
aligning it with the field of
the permanent magnet.
C. Galvanometer – a device that uses an
electromagnetic to measure electric current.
D.
Electric motor – a device that changes
SLIDE 5 electrical energy into
mechanical energy.
1. Contains an electromagnet that is free to
rotate between the poles of a permanent,
fixed magnet. The coil in the electromagnet is
connected to a source of electric current.
2. When a current flows through the electromagnet, a
magnetic field is produced in the coil.
3. Changing the direction of the current causes the coil in an electric
motor to
keep rotating.
4. Rotation speed of electric motors can be controlled.
a. Vary the amount of current flowing through the coil.
b. When more current flows through the coil, the electromagnet’s
magnetic field becomes stronger, the magnetic force between turns faster.
A. A battery causes an electric current to
flow through the coil of the electromagnet.
B. Unlike poles of the two magnets attract each other, and the like
poles repel. This causes the coil to rotate until the opposite poles are
next to each other.
C. If the current in the coil is switched, the direction of the coil’s
magnetic field also switches. The north and south poles of the magnet
trade places.
D. The coil is repelled by and attracted once again to the poles of the
permanent magnet. The coil rotates until it is again aligned with the
permanent magnet’s field.
SECTION 3: PRODUCING ELECTRIC CURRENT
A.
From mechanical to electrical energy
1. Electromagnetic induction – the
production of an electric current by
moving a lop of wire through a
magnetic field or moving a magnet
through a wire loop
2. Generator – a device that produces
electric current by rotating a coil of
wire in a magnetic field
a. The wire coil is wrapped around an iron
core and placed between the poles of a
permanent magnet.
b. Coil is rotated by an outside source of mechanical
energy.
c. As the coil turns within the magnetic field of the
permanent magnet, an electric current flows through
the coil.
d. Direction of the current in the coil in a generator
changes twice with each revolution.
SLIDE 2

The electromagnet
in a generator is
rotated by some
outside source of
mechanical energy.
In this setup, a
student can rotate a
crank to turn the
electromagnet.
SLIDE 3

The direction that
current flows in a
wire coil depends
on how the wire coil
is aligned with the
permanent magnet.
SLIDE 4
3.
Generating electricity
a. Electricity used in the home comes from
a power plant with huge generators.
b. Coils of electromagnets in the generators
usually connected to a turbine – a large
wheel that rotates when pushed by
water, wind, or steam.