18.1 magnets and magnetic fieldsimages.pcmac.org/sisfiles/schools/ga/houstoncounty/perryhigh...18.1...
TRANSCRIPT
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Physical Science
Chapter 18 Magnetism
Bill Nye-Magnetism Video
• What happens when the poles of two
magnets are thought close together?
• What causes a magnet to attract or repel
another magnet?
• How is Earth’s magnetic field oriented?
18.1 Magnets and
Magnetic Fields
What is an example of how you use magnets?
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Magnets around the House:Headphones
Computer speakers
Telephone receiversPhone ringers
Microwave tubesSeal around refrigerator door
Credit card magnetic strip
TV deflection coilComputer monitor
Computer hard drive
Shower curtain weights / attach to tubPower supply transformers
Magnets inside of motors:
CD and DVD spinner and head positioner
Audio and VHS tape transport
VHS tape loader Microwave stirring fans
Kitchen exhaust fans Garbage disposal motor
Sump pump Furnace blower and exhaust
Garage door opener Bathroom exhaust fan
Electric toothbrush Ceiling fan
Pager or cell phone vibrator
Clocks (not the wind-up type or LCD type)
Computers
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Magnets in your Car:Starter motor
A/C clutch
Interior fan motor
Electric door locks
Windshield wiper motor
Electric window motor
Side-view mirror adjuster motor
CD/tape player motor and playback
Engine speed sensors
Alternator
Starter relay
Windshield washer pump motor
More than 3,000 years ago Greeks discovered deposits of a mineral that was a natural magnet.
The mineral is now called magnetite.
MagnetsMagnets
� These magnets were used by the ancient peoples as compasses to guide sailing vessels.
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• Today, the word magnetism refers to the properties and interactions of magnets.
MagnetsMagnets
� magnet – any material that
attracts iron and materials
that contain iron
� magnetism – the
attraction or repulsion of
magnetic materials
� Magnets attract or repel
other magnets.
Common metals affected by magnetism are iron, nickel, and cobalt
IRON NICKEL
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Magnets
• Nonmagnetic materials, such as plastic, glass
and wood, have electrons that flow in
different directions.
• Magnetic poles are where the magnetic force exerted by the magnet is strongest.
• the two ends on any magnet, no matter what its shape
• All magnets have a north pole and a south pole.
Magnetic Poles
• magnetic force – the attraction or repulsion between magnetic poles
Magnets
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� Magnetic poles
that are alike
repel each other.
� Magnetic poles that
are unlike attract
each other.
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� every magnet has two poles
� north (N) pole
� south (S) pole
� even if you break a magnet in half, each half will have a north pole and a south pole
Magnetic Poles
Poles- two ends of a magnet
No Monopoles Allowed
It is not possible to end up with a single North pole or a single South
pole, which is a monopole ("mono" means one or single, thus one
pole).
S N
• Recall that even individual atoms of magnetic materials act as tiny magnets.
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Making a Magnet
� materials that are hard to magnetize
will also stay magnetized for a long
time
� a magnet that is hard to magnetize
but tends to keep its magnetism is
called a permanent magnetEx: Cobalt, Nickel, Aluminum
Making a Magnet
� a material that is easily magnetized
tends to lose its magnetism quickly
� a magnet made of this kind of material
is called a temporary magnetEx: Iron nails
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• The two ends of a horseshoe-shaped magnet are the north and south poles.
• A magnet shaped like a disk has opposite poles on the top and bottom of the disk.
Where are the Poles on different shaped magnets?
• Magnetic field lines always connect the north pole and the south pole of a magnet.
Magnetic Fields
A magnet has a
‘magnetic field’
distributed throughout
the surrounding space
Michael Faraday realized that ...
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• A magnet is surrounded by a magnetic field.
• A magnetic field exerts a force on other magnets and objects made of magnetic materials.
• The magnetic field is strongest close to the magnet and weaker far away.
A magnetic field also has a
direction & illustrated by
arrows.
� magnetic field lines – the lines that map
out the magnetic field around a magnet
Magnetic field lines spread out from one pole, curve
around the magnet, and return to the other pole
forming a closed loop.
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Strong
Field
Weak
Field
The density of field
lines indicates the
strength of the field
Magnetic Field
Video
Field Lines Around Magnets
Bar Sphere
Repelling Attracting
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� magnetic domain – a
grouping of billions of atoms that have their
magnetic fields lined up in the same
direction
� Atom’s electrons
spin around when
placed near a
magnetic field.
An unmagnetized substance looks
like this…
While a magnetized substance looks
like this…
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Earth as a Magnet• Earth has a magnetic field with two main poles.
• This magnetic field enables us to use compasses
• Our planet's rotation
causes molten iron-
nickel in its outer core
to circulate
• creating electrical
currents and a
magnetic field.
Earth as a Magnet
� William Gilbert
� a British scientist
� Showed that the Earth behaves as a magnet
� Earth has north and south poles like a bar magnet
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The Earth’s Magnetic Field
The Earth is surrounded by a magnetic field which extends far into
space. Called the magnetosphere
�Magnetic poles of the Earth are
different from the geographic poles of the Earth
Earth as a Magnet
� the North Magnetic Pole is located near the geographic South Pole
� the South Magnetic Pole is located near the geographic North Pole
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• Earth’s magnetic poles move slowly with time.
Earth’s Magnetic Poles
• Sometimes Earth’s magnetic poles switch places so that Earth’s south magnetic pole is the southern hemisphere near the geographic south pole.
Video
Compasses�Compasses are used to determine direction
�Compass needles are magnetized and respond to the magnetic field of the Earth
�We use the Earth’s magnetic field to find direction.
� Migration of birds
The needle of a compass always
points toward the magnetic
south pole.
The compass needle points
toward the geographic “North”
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Magnetic Effects
The most visible effect of the earths magnetic field is a colorful light display, called an aurora
Collisions between the charged particles and other particles in the upper atmosphere create glowing lights
Video
18.2 Magnetism from Electric
Currents
• What happens to a compass near
a wire that is carrying a current?
• Why are electric motors useful?
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Electromagnetism
• In 1820, Han Christian Oersted, a Danish physics teacher, found that electricity and magnetism are related.
• Oersted hypothesized:
• Electric current must produce a magnetic field around the wire, and the direction of the field changes with the direction of the current.
• When a compass was brought
near electric current, the
compass needle no longer
pointed north
• When an electric current passes through a wire a
magnetic field is formed.
• The magnetic field depends on the direction of
the current in the wire.
Electromagnetism
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• The direction of the magnetic field around the wire reverses when the direction of the current in the wire reverses.
• As the current in the wire increases the strength of the magnetic field increases.
Electromagnetism
• Electromagnetism – relationship between electricity
and magnetism
• Oersted’s discovery is responsible for the invention of
new tools based on the principles of electromagnetism.
Electromagnetism
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When an electric current is passed through a coil of wire wrapped
around a metal core, a very strong magnetic field is produced. This is
called an electromagnet.
The strength depends on the number of turns in the coil, the
amount of current, and the size of the iron core.
Electromagnetism
Electromagnetism
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YOU CAN MAKE AN ELECTROMAGNET
Electromagnets can be turned on and off
You can make an electromagnet by wrapping a wirearound a piece of ironand sending a currentthrough the wire with a battery.
Classroom electromagnet
vs. commercial electromagnet
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• A single wire wrapped into a cylindrical wire coil is called a solenoid.
• A type of electromagnet
• The solenoid’s magnetic field magnetizes the iron core.
Electromagnetism
• The magnetic field inside a solenoid is stronger than the field in a single loop.
• The field inside the solenoid with the iron core can be more than 1,000 times greater than the field inside the solenoid without the iron core.
Electromagnetism
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Properties of Electromagnets
• Electromagnets are temporary magnets because the magnetic field is present only when current is flowing in the solenoid.
• The strength of the magnetic field increases:
• by adding more turns of wire to the solenoid
• by increasing the current passing through the wire.
Electromagnetism
• Electromagnet has a north pole and a south pole.
• If placed in a magnetic field, an electromagnet will align itself along the magnetic field lines, just as a compass needle will.
• An electromagnet also will attract magnetic materials and be attracted or repelled by other magnets.
Properties of Electromagnets
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A galvanometer gives a measurement of
the current.
an electromagnet that interacts with a
permanent magnet.
The greater the current
passing through the wires,
the stronger the
galvanometer interacts
with the permanent
magnet.
Electromagnetic Devices
Galvanometer
Use of a GalvanometerGalvanometers are used as gauges in cars and many other applications.
• Functions as an ammeter to measure current, when other circuit elements are used.
• Functions as a voltmeter to measure voltage.
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Electric Motors
• Electric Motor – device that changes electrical
energy into mechanical energy
• An electric motor is made up of an
electromagnet and a permanent magnet
Electromagnetic Devices
• A fan uses an electric motor
• The motor in a fan turns the fan blades, moving air past your skin to make you feel cooler.
• Almost every appliance in which something moves contains an electric motor.
Electric Motors
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Electric Motor
• A simple electric motor also includes components called brushes and a commutator.
• The brushes carry current to coil using the commutator.
• The commutator is used to change the currents direction.
• Form a closed electric circuit between the battery and the coil.
Video
18.3 Electric Currents from
Magnetism
What happens when a magnet is
moved into or out of a coil of wire?
How are electricity and magnetism
related?
What are the basic components of a
transformer?
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The process of inducing a current by moving a magnetic field through a wire coil without touching it.
Electromagnetic Induction
Causes charges to move within the wire.
• Working independently in 1831, Michael Faraday in Britain and Joseph Henry in the United States both found that:
• Moving a loop of wire through a magnetic field caused an electric current to flow in the wire.
• They also found that moving a magnet through a loop of wire produces a current.
Electromagnetic Induction
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Generators
• electric generator – a
device that converts
mechanical energy into
electrical energy
• A generator uses motion in
a magnetic field to
produce an electric
current
Electromagnetic Induction
How a Generator Works
– a current can be
induced in a loop of
wire by spinning the
loop inside a
magnetic field
– due to the changing
direction of the wire,
an alternating
current is produced
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• The type of generator in a car, is called an alternator.
• The alternator provides electrical energy to operate lights and other accessories.
Using Electric Generators
Electromagnetic Induction
Induced current in a Generator
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• Electrical energy comes from a power plant with huge generators.
• Produces the electrical energy that you use in your home.
• The coils in these generators have many coils of wire wrapped around huge iron cores.
Electromagnetic Induction
• The rotating magnets are connected to a turbine(TUR bine)a large wheel that rotates when pushed by water, wind, or steam.
Electromagnetic Induction
• Thermal energy is then converted to mechanical energy as the steam pushes the turbine blades.
Video
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Electromagnetic Force
• Electromagnetic force: Electricity and
magnetism are two aspects of a single force.
• Electromagnetic energy results
– Light is an example of electromagnetic energy.
EM Waves are made of
both electric and
magnetic fields.
Electric field
Magnetic field
Transformers
• A transformer is a device that increases or
decreases voltage alternating current.
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• a transformer consists of two coils of wire around a magnet
– the first coil, called the primary coil, is connected to the power source
– the second coil, called the secondary coil, is connected to the load
• a load is something that uses electricity, such as a light bulb or a motor
Transformers
Step-down transformer
a transformer that
decreases voltage
Transformers
Two types of transformers:
Step-up transformer
a transformer that
increases voltage