Transcript Unit 3 Lesson 5

Unit 3 Lesson 5 Electromagnetism
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Unit 3 Lesson 5 Electromagnetism
Magnetic Attraction
What is electromagnetism?
• In 1820, Danish physicist Hans Christian Oersted
observed a magnet move in the presence of an
electric current.
• Oersted hypothesized that the electric current had
produced a magnetic field. Later experiments
confirmed his hypothesis, demonstrating the
relationship between electricity and magnetism.
• Electromagnetism results when electric currents
and magnetic fields interact with each other.
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Unit 3 Lesson 5 Electromagnetism
How can you make a magnet using
current?
• A coil of wire that carries an electric current, and
therefore produces a magnetic field, is called a
solenoid. The more loops, the stronger the
magnetic field.
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Unit 3 Lesson 5 Electromagnetism
How can you make a magnet using
current?
• Wrapping a solenoid around an iron core makes
an electromagnet, which combines the magnetic
field of the solenoid with the magnetic field of the
magnetized iron core.
• Adding loops to the solenoid or increasing the
electric current strengthens the electromagnet.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for electric
magnets?
• Electromagnets are useful for lifting and moving
large metal objects containing iron. When current
runs through the solenoid coils, it creates a
magnetic field that attracts the metal objects.
Turning off the current turns off the magnetic field
so that the metal can be easily dropped in a new
place.
• Powerful electromagnets can raise a maglev train
above its track. Just as poles of a bar magnet
repel each other, electromagnets in the train and
track repel each other when the electric current is
turned on.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for electric
magnets?
• A galvanometer is a meter that measures the
strength and direction of an electric current in a
wire.
• A galvanometer contains an electromagnet
between the poles of a permanent magnet. When
current is applied to the electromagnet, the two
magnetic fields interact and cause the
electromagnet to turn.
• The indicator, attached to the electromagnet,
moves to one side of the zero on the scale,
indicting the strength and direction of the current.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for electric
magnets?
• The indicator on a galvanometer shows current
direction and strength.
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Unit 3 Lesson 5 Electromagnetism
A Look Inside
• Magnetic resonance imaging (MRI) machines use
powerful electromagnets and radio waves to
produce detailed images of the body’s interior.
• Doctors use MRI scans to diagnose many
conditions, including broken bones and strained
tendons.
• Some MRI scans help scientists understand how
the brain works.
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Unit 3 Lesson 5 Electromagnetism
Let’s Motor!
How do motors work?
• An electric motor changes electrical energy into
mechanical energy.
• Electric motors range in size from large motors,
used to power Ferris wheels, to small motors used
in computer cooling fans. Almost every time a
device uses electricity to make something move,
there is a motor involved.
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Unit 3 Lesson 5 Electromagnetism
How do motors work?
• A simple motor has a coil or loop of wire called an
armature mounted between the poles of a
magnet. The armature becomes an electromagnet
when current passes through it.
• The armature rotates because its poles are
pushed and pulled by the opposite poles of the
magnet. The armature turns until its north pole is
opposite the magnet’s south pole.
• Then, a device called a commutator reverses the
direction of the current in the wire, causing the
armature to complete its turn.
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Unit 3 Lesson 5 Electromagnetism
How do motors work?
• Electric motors are very similar to galvanometers.
However, in a motor, the electromagnet is made to
rotate all the way around instead of back and
forth in the magnetic field.
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Unit 3 Lesson 5 Electromagnetism
A New Generation
What are some uses for induction?
• Using a magnetic field to create an electric current
in a wire is called electromagnetic induction.
• When electric charges move through a wire, the
wire carries a current. Magnetic force from a
magnet moving inside a coil of wire can make the
electric charges in the wire move. When the
magnet stops moving inside the coil, the electric
current stops.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for induction?
• The electric current increases if you move the
magnet through the coil faster or if you add more
loops of wire. The current can also be induced by
moving the coil over the magnet.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for induction?
• Transformers use induction to increase or
decrease the voltage of alternating current. For
example, transformers on power lines increase
voltage to send current miles away and then
decrease voltage for a single home.
• Most transformers are iron “rings” with two coils
of wire. The current in the primary side makes an
electromagnet. Because the current alternates,
the magnetic field changes. The changing
magnetic field induces a current in the secondary
side.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for induction?
• In a step-up transformer, there are more turns of
wire on the secondary side. In a step-down
transformer, there are more turns of wire on the
primary side.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for induction?
• Electric generators use induction to change
mechanical energy into electrical energy.
• In power plants, mechanical energy is used to
rotate turbines. The turbines turn magnets inside
coils of wire, generating electricity.
• Many power plants use rising steam to turn the
turbines. The steam is produced from burning
fossil fuels or using nuclear reactions to heat
water. Other sources of mechanical energy to turn
turbines are blowing wind, falling water, and
ocean tides and waves.
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Unit 3 Lesson 5 Electromagnetism
What are some uses for induction?
• Generators induce electric current when a magnet
moves in a coil of wire or when a wire moves
between the poles of a magnet.
• In a simple generator, a wire loop at the end of a
rod moves through the magnetic field of a
magnet. In the first half of the turn, one side of
the loop moves downward.
• In the second half of the turn, the part of the loop
that was moving downward now moves upward,
reversing the current. This creates alternating
current.
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