Chapter 16: Electromagnets and Induction

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Electricity and Magnetism
Unit 5: Electricity and Magnetism
Chapter 16: Electromagnets and
Induction
 16.1
Electric Current and Magnetism
 16.2
Electric Motors
 16.3
Electric Generators and Transformers
16.1 Investigation: Electromagnetic Forces
Key Question:
How does an electric motor work?
Objectives:

Build a simple electric motor.

Describe the components required for an electric motor to work.

Test the effects of changing different variables on the function of
an electric motor.
Electric Current and Magnetism
 In
1819, Hans Christian
Oersted, a Danish physicist
and chemist, and a
professor, placed a compass
needle near a wire through
which he could make electric
current flow.
 When the switch was closed,
the compass needle moved
just as if the wire were a
magnet.
Electric current and magnetism
 Electric
current is made of moving charges
(electrons), which creates the magnetic field around
a current-carrying wire
 Magnetism
is created by these moving charges.
The magnetic fields of straight wire
 The
magnetic field lines are concentric circles with
the wire at the center of the circles.
 The
direction of the field depends on the direction of
the current in the wire.
The magnetic fields of straight wire

The strength of the magnetic field near the wire
depends on two factors:
1.
The strength is directly proportional to the current, so
doubling the current doubles the strength of the field.
2.
The field strength is inversely proportional to the distance
from the wire. (Decreasing the distance to the wire by half
doubles the strength of the field.)
The magnetic fields of straight wire

Near a straight wire, the
north pole of a compass
needle feels a force in the
direction of the field lines.

The south pole feels a
force in the opposite
direction.

As a result, the needle
twists to align its northsouth axis along the
circular field lines.
The magnetic fields of loops and coils

The magnetic field around a single wire is too
small to be of much use.

There are two techniques to make strong
magnetic fields from current flowing in wires:
1.
Parallel wires can be bundled together. (10 wires,
each with 1 A of current, create a magnetic field 10X
as strong as 1 wire carrying 1 A).
2.
A wire can be looped into a coil so the magnetic field
is concentrated at the center.
The magnetic fields of loops and coils
 The
most common form of
electromagnetic device is a
coil with many turns called a
solenoid.
 A coil
takes advantage of
these two techniques
(bundling wires and making
bundled wires into coils) for
increasing field strength.
Magnetic forces and electric currents

Two wires carrying electric current exert force on each
other, just like two magnets.

The forces can be attractive or repulsive depending on
the direction of current in both wires.
Unit 5: Electricity and Magnetism
Chapter 16: Electromagnets and
Induction
 16.1
Electric Current and Magnetism
 16.2
Electric Motors
 16.3
Electric Generators and Transformers
16.2 Investigation: Electromagnetic Induction
Key Question:
How does an electric generator work?
Objectives:

Explain how an electric generator works.

Describe the relationship between the voltage output of a
generator and the speed of the rotor.

Modify the design of a generator to test the effects of different
factors, such as the number of magnets and the orientation of
the magnets.
Electric motors

Electric motors convert electrical energy into mechanical
energy.

The disk in the motor is called the rotor because it can
rotate.

The disk will keep spinning as long as the external magnet is
reversed every time the next magnet in the disk passes by.

One or more stationary magnets reverse their poles to push
and pull on a rotating assembly of magnets.
Using magnets to spin a disk
 Reversing
the magnet in your fingers attracts and
repels the magnets in the rotor, making it spin.
Commutation
 The
process of reversing the current in the
electromagnet is called commutation and the
switch that makes it happen is called a
commutator.
Electric Motors

All types of electric motors have three
key parts:
1. A rotating element (rotor) with magnets.
2. A stationary magnet that surrounds the
rotor.
3. A commutator that switches the
electromagnets from north to south at
the right place to keep the rotor
spinning.
AC and DC motors
 Motors
that run on alternating current (AC) electricity
are easier to make because the current switches
direction all by itself—a commutator isn’t needed.
Electric motors
 The
rotating part of the
motor, including the
electromagnets, is called
the armature.
 It
has 3 electromagnets
that correspond to the 3
coils.
Electric motors
 The
permanent magnets
are on the outside, and
they stay fixed in place.
 The
wires from each of
the three coils are
attached to three metal
plates (commutator) at
the end of the armature.
commutator
Electric Motors

As the rotor spins, the three plates come into
contact with the positive and negative brushes.

Electric current flows through the brushes into the
coils.
Unit 5: Electricity and Magnetism
Chapter 16: Electromagnets and
Induction
 16.1
Electric Current and Magnetism
 16.2
Electric Motors
 16.3
Electric Generators and Transformers
16.3 Investigation: Generators and
Transformers
Key Question:

How do electricity and magnetism work
together in generators and transformers?
Objectives:

Apply an understanding of electricity and magnetism to describe
how generators and transformers function.
Electromagnetic Induction
 If
you move a magnet near a coil of wire, a current
will be produced.
 This
process is called electromagnetic induction,
because a moving magnet induces electric current
to flow.
 Moving
electric charge creates magnetism and
conversely, changing magnetic fields also can cause
electric charge to move.
Induction
 Current
is only produced if
the magnet is moving
because a changing
magnetic field is what
creates current.
 If
the magnetic field does
not change, such as when
the magnet is stationary, the
current is zero.
Induction
 If
the magnetic field is increasing, the induced
current is in one direction.
 If
the field is decreasing, the induced current is in
the opposite direction.
Faraday’s law of induction
 A moving
magnet
induces current in a
coil only if the
magnetic field of
the magnet passes
through the coil.
Faraday’s law of induction
 Michael
Faraday (1791–1867), an
English physicist and chemist, was
first to explain how moving magnets
and coils induced voltage.
 Faraday’s
found that the induced
voltage is proportional to the rate of
change of the magnetic field through
the coil.
Faraday’s law of induction
 Faraday’s
law says
the current in a coil is
proportional to the rate
at which the magnetic
field changes.
Faraday's Law
Generators
 A generator
is a device that uses induction to
convert mechanical energy into electrical energy.
Electrical generators
 The
electrical energy created
by a generator is not created
from nothing.
 Energy
must continually be
supplied to keep the rotating
coil or magnetic disk turning.
 In
hydroelectric generators,
falling water turns a turbine
which spins a generator to
produce electricity.
Producing and transporting energy
 Hoover
Dam is called a
hydroelectric plant
because it converts the
energy of falling water into
electricity.
 Using
the potential energy
of water is one way to
produce electricity.
Energy flow
 With
each transformation (green arrows), some
energy is lost to the system in the form of heat (red
arrows).
Electricity from different resources
 A nonrenewable
resource is not replaced as it is
used.
 Any
fossil fuel is an good example of
nonrenewable resource.
 Besides
their growing scarcity, burning fossil fuels
produces sulfur oxide emissions that reduce air
quality and may be accelerating climate change.
Electricity from different resources
 A renewable
resource can
be replaced naturally in a
relatively short period of
time.
 Falling
water, energy from
the Sun, wind energy, and
geothermal energy are
examples of renewable
resources.
Geothermal, biomass and hydroelectric
energy
power plants use Earth’s internal heat in
the form of water or steam, to produce electricity.
 Geothermal
 Biomass,
such as organic material from plants or
animals or municipal waste, can be burned to
produce steam for a turbine.
 Impoundment
and pumped storage hydroelectric
power plants use falling water differently to generate
electricity.
Electrical Power
 Recall
that electrical power (in
watts) is the rate at which
electrical energy is changed into
other forms of energy such as
heat, sound, or light.
 Anything
that “uses” electricity is actually converting
electrical energy into some other type of energy.
 Utility
companies charge customers for the number
of kilowatt-hours (kWh) used each month.
Transformers
 Transformers
are
extremely useful because
they efficiently change
voltage and current, while
providing the same total
power.
 The
transformer uses
electromagnetic
induction, similar to a
generator.
Transformers

Consider the transformer between the outside
power lines and your house:
1. The primary coil is connected to outside power
lines. Current in the primary coil creates a
magnetic field through the secondary coil. The
primary coil’s field is shown by the magnetic field
lines (green arrows)
2. The current in the primary coil changes constantly
because it is alternating current.
Transformers
3.
As the current changes, so does the strength and direction
of the magnetic field through the secondary coil.
4.
The changing magnetic field through the secondary coil
induces current in the secondary coil. The secondary coil
connects to your home’s wiring.

The relationship between
voltages and turns for a
transformer is the result of
two coils having a different
number of turns.
Transformers
 In
the same changing
magnetic field, a coil with
100 turns produces 10
times the voltage of the
induced current as a coil
with 10 turns.
Changing voltage in a transformer
A cell-phone AC adapter reduces the 120 V AC to the 6 V DC
needed by the phone’s battery. If the primary coil has 240 turns,
how many turns must the secondary coil have?
1.
Looking for: …no. of turns of the secondary coil.
2.
Given: …voltage of each coil (120VAC and 6 VDC) and the
no. of turns of the primary coil (240)
3.
Relationships: …V1 = N1 Solve for N2 = V2 x N1
V2
N2
V1
4.
Solution: … 6 V x 240 = 12 turns
120 V
Michael Faraday

Despite little formal schooling,
Michael Faraday rose to become
one of England’s top research
scientists of the nineteenth
century.

He is best known for his
discovery of electromagnetic
induction, which made possible
the large-scale production of
electricity in power plants.
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