Transcript Magnetism Objectives

Magnetism Objectives
1. Explain how a magnet exerts a force on an object.
2. Describe the properties of temporary and permanent
magnets.
3. Explain why some materials are magnetic and others
are not.
4. Model magnetic behavior using magnetic domains.
5. Describe the magnetic field produced by an electric
current.
6. Explain how an electromagnet produces a magnetic field.
7. Describe how electromagnets are used.
8. Explain how an electric motor operates.
9. Define electromagnetic induction.
10. Describe how a generator produces an electric current.
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Magnetism Objectives
11. Distinguish between alternating and direct current.
12. Explain how a transformer can change the voltage of
an alternating current.
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Magnetism & Its Uses
magnetism: properties and interactions of magnets
Magnets can exert forces on each other:
-can attract (different polarity-N & S)
-can repel (same polarity-S & S or N & N)
The strength of the force depends on the distance
between the magnets
-increases as the magnets get closer together
-magnetic force creates a magnetic field around the
magnet
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magnetic field: force exerted on
other objects made of magnetic
material or other magnets
-represented by lines of
force (magnetic field lines)
-has direction, as shown by
the arrows, which always are
drawn from the north end to the
south end.
Notice that at the ends of the bar,
the lines are close together
-these regions are called
magnetic poles
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magnetic poles: regions of a magnet where the
magnetic force exerted by the magnet is the
strongest
-all magnets have a north and
south pole
~bar magnets has their poles
at opposite ends
~horseshoe magnets have
their poles on the same side
~a round magnet has its poles on
opposite sides (top and bottom)
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Magnets can interact with one another when they
come into close contact with one another.
-see p 226, Figure 4
-unlike poles attract and like poles
repel
Magnetic fields can be detected with
a compass:
-the needle (small bar magnet) will
rotate until it lines up with the magnetic
field lines
-the needle always points from magnetic north to
magnetic south
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Earth also has magnetic poles:
-acts like a huge bar magnet
and is surrounded by a
magnetic field that extends
into space
-north pole of a magnet points
toward the geographic north
pole, which is actually the
magnetic south pole
~the magnetic south pole is about 1500 km from
the geographic north pole and is slowly
changing as time passes
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-no one is sure what produces Earth’s magnetic field,
but some believe that the circulation of molten iron
and nickel in the outer core is the cause
Not all materials are attracted to magnets, so what
makes a material magnetic?
-in most materials, the electron’s charge in the atoms
cancel out (even though each electron has magnetic
properties)
-iron, cobalt and nickel have magnetic properties
because the magnetic properties of the electrons do
not cancel out.
~not all objects made from them have magnetic
properties
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Even if a material made from iron, cobalt or nickel is not
permanently magnetic, you can sometimes
temporarily magnetize it.
-when the magnetic field produced by atoms comes in
contact with other atoms, the groups of atoms can
align their magnetic poles so that they all point in the
same direction
magnetic domain: groups of atoms with aligned
magnetic poles
-each domain contains an enormous number of atoms
-since all atoms are aligned, the domain itself can
behave like a magnet with north and south poles 9
How do we line up these domains to
create a magnet?
-only when the domains all line up
in the same direction will it create
a magnet
-if the domains do not line up in
the same direction, it will not
create a magnet
~notice in the nail the domains do
not line up (in Fig 7, p 229)
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If you place a magnet against the
same nail, the atoms in the
domains orient themselves in the
direction of the nearby magnetic
field.
-this forms a temporary magnet
-as soon as the external
magnetic field (from the magnet)
is removed, the nail is no longer
magnetized and the magnetic
domains will unalign
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Permanent magnets can be made by placing a magnetic
material in a strong magnetic field, forcing a large
number of magnetic domains to line up.
-the magnetic fields all add together and create a
strong magnetic field inside the material, which
prevents the constant motion of the atoms from
bumping the domains out of alignment.
However, they can lose their magnetic properties if they
are heated, which causes the atoms to move faster,
knocking the domains out of alignment.
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Magnetism Review 1 (Write Q and A)
1. What does the force between 2 magnets depend
on?
2. How do magnetic poles interact with each other?
3. Which way would a compass needle point if Earth’s
magnetic poles switched places?
4. How does a compass needle move when placed in a
magnetic field?
5. Why are certain materials magnetic and others are
not?
6. How does heating a bar magnet change its magnetic
field?
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Electricity and Magnetism
When a compass was placed near an electric current,
the current affected the direction of the compass
needle.
-it was determined that an electric current produces
a magnetic field around the wire
-the magnetic field
around the wire
changes direction as
the direction of current
changes.
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As the strength of the current increases, the strength of
the magnetic field increases.
-as you move farther away from the wire, the magnetic
field decreases
electromagnet: temporary magnet made by wrapping a
wire coil carrying a current around an iron core
-a magnetic field is only present when current is
flowing through the wire coil
-adding more turns to the coil or increasing the current
increases the strength of the magnetic field
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The magnetic fields
around the different
parts of the wire loop
combine to form the field
inside the loop.
Electromagnets convert
electrical energy into
mechanical energy to do
work
-stereo speakers and
electric motors
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Electromagnets and Sound
How does musical information stored on a CD become
sound you can hear?
-sound is produced by a loudspeaker that contains an
electromagnet connected to a flexible speaker cone
that is usually made from paper, plastic, or metal.
-the mechanical energy vibrates the speaker cone to
produce sound
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Galvonometers
How does a change in the amount of gasoline in a tank
or the water temperature in the engine make a needle
move in a gauge on the dashboard?
-these gauges are galvanometers, which are devices
that use an electromagnet to measure electric current.
-changing the current in the electromagnet causes the
needle to rotate to different positions on the scale.
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Electric Motors
electric motor: device that changes electrical energy
into mechanical energy
-contains an electromagnet that is free to rotate
between the poles of a permanent, fixed magnet.
-a coil is connected to a source of electric current,
such as a battery
-also contains
~brushes: conducting pads
connected to the battery
~commutators: conducting
metal ring that is split and
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rotates with the coil
A magnetic field is produced in the coil when a current
flows through the electromagnet
-changing the direction of this current causes the coil
to rotate
-the rotation speed of the motor can be controlled by
varying the amount of current
~turns faster when more current flows because the
electromagnets magnetic field
becomes stronger, increasing
the force between the coil and
magnet
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Magnetism Review 2 (Q and A)
1. What does the force between 2 magnets depend on?
2. How do magnetic poles interact with each other?
3. How does a magnetic field change when the current
changes direction?
4. What is an electromagnet?
5. How does a stereo speaker use an electromagnet to
produce sound?
6. How can you change the strength of a magnetic field
produced by an electromagnet?
7. List 3 devices that contain electric motors.
8. What does an electric motor do?
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Producing Electric Current
electromagnetic induction: production of an electric
current by
1. moving a wire through a magnetic field
2. moving a magnet through a wire loop
Most of the electrical energy you use comes from a
generator, a device the produces electric current by
using electromagnetic induction to rotate a coil of wire
in a magnetic field
a. wire coil is wrapped around an iron core and placed
between the poles of a permanent magnet
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generator:
b. the coil is rotated by an external 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. the direction of current
in the generator’s coil
changes twice for each
revolution
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What are uses of electric generators?
1. alternator in your car
-provides electrical energy to operate lights and
other accessories
-spark plugs in your car use this electrical energy to
ignite the fuel in the cylinders
2. generating electricity
-uses a fixed coil and permanent magnet that
rotates, producing electric current in the fixed coil
and mechanical energy is used to rotate the
magnet, which is connected to a turbine
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turbine: large wheel that rotates when pushed by water,
wind or steam
Because power outages sometimes occur, some
electrical devices use batteries as a backup source of
electrical energy.
-however, the current produced by a battery is
different than the current from an electric generator.
~batteries produce direct current, current that flows
in one direction
~generators produce alternating current, which
reverses the flow of current in a regular pattern 25
In the US, generators produce AC current at a frequency
of 60 cycles/sec (60Hz)
-changes direction 120 times each second
A device that uses batteries as backup usually require
direct current, however when plugged up to an outlet,
it would be receiving alternating current
-there are electronic components in the device that
converts the alternating current to direct current and
reduces the voltage of the alternating current
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Sometimes we need to increase or decrease the voltage
of an alternating current:
-transformer: device that increases or decreases the
voltage of alternating current
~step-up transformer: increases voltage by putting
more turns of wire on the secondary coil
~step-down transformer: decreases voltage by
placing fewer turns of wire on the secondary coil
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Magnetism Review 3 (Q and A)
1. What is the difference between direct current and
alternating current?
2. What is a generator’s purpose?
3. What is a problem with using batteries as backup
power for electronic devices?
4.Tell the difference between the two types of
transformers.
5. How is a generator different from an electric motor?
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Final Review
p 250-251
# 1-14, 18-20
P 252
# 1-6
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