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Chapter 21
Magnetism
21.1 Magnets and
Magnetic Fields
Magnetic Forces
Magnetic force is the force a magnet
exerts on another magnet, on iron or a
similar metal, or on moving charges.
• Magnetic forces, like electric forces, act
over a distance.
• Magnetic force, like electric force,
varies with distance.
Magnetic Forces
All magnets have two magnetic poles,
regions where the magnet’s force is
strongest.
• One end of a magnet is its north pole.
• The other end is its south pole.
A magnetic field surrounds a magnet and
can exert magnetic forces. Magnetic field
lines begin near the north pole and extend
toward the south pole.
• The arrows on the field lines indicate
what direction a compass needle would
point at each point in space.
• Where lines are close together, the
field is strong.
• Where lines are more spread out, the
field is weak.
Magnetic Fields
Magnetic Field Around Earth
Earth is like a giant magnet
surrounded by a magnetic field. The
area surrounding Earth that is
influenced by this field is the
magnetosphere.
A compass points north because it
aligns with Earth’s magnetic field.
Magnetic Materials
A property of electrons called “spin”
causes electrons to act like tiny magnets.
• In many materials, each electron is
paired with another having an opposite
spin so magnetic effects mostly cancel
each other.
• Unpaired electrons in some materials
produce magnetic fields that don’t
combine because of the arrangement
of the atoms.
Magnetic Materials
In a few materials, such as iron, nickel,
and cobalt, the unpaired electrons
make a strong magnetic field.
• The fields combine to form
magnetic domains.
• A ferromagnetic material, such
as iron, can be magnetized
because it contains magnetic
Magnetic Materials
Magnetized Materials
If you place a nonmagnetized ferromagnetic
material in a magnetic field, it will become a
magnet when the domains are aligned.
• Magnetization can be temporary. If the
material is moved away from the magnet,
the magnetic domains become random.
• In some ferromagnetic materials, the
domains stay aligned for a long time. These
materials are called permanent magnets.
Magnetic Materials
If you cut a magnet in half,
each half will have its own
north pole and south pole
because the domains will still
be aligned.
A magnet can never have just
a north pole or just a south
pole.
Assessment Questions
1.
Where does the magnetic field of a
magnet have the strongest effect
on another magnet?
a. the north pole
b. the south pole
c. both poles equally
d. midway between the two poles
2.
How are the magnetic field lines drawn to
show the interaction of two bar magnets that
are lined up with their north poles near one
another?
a. Field lines begin at the north pole of each
magnet and extend to the south pole of
the other magnet.
b. Field lines begin at each magnet’s north
pole and extend toward its south pole.
c. Field lines extend from the north pole of
one magnet to the north pole of the other
magnet.
d. Field lines cannot be drawn because the
magnetic forces cancel one another.
3.
Why does a compass not point exactly
toward the geographic north pole?
a. Earth’s magnetic field is constantly
changing due to effects of the solar wind.
b. The magnetic pole is near but not exactly
at the geographic pole.
c. Earth’s magnetic field lines are too broad
for a compass point exactly toward the
pole.
d. Daily variations in the magnetic field
mean that compasses are not very
accurate.
4.
What happens to a permanent magnet
if its magnetic domains lose their
alignment?
a. The magnetic field reverses
direction.
b. It loses its magnetic field.
c. It has several north poles and
several south poles.
d. It is no longer a ferromagnetic
material.
Chapter 21
Magnetism
21.2
Electromagnetism
In 1820 Hans
Oersted
discovered how
magnetism and
electricity are
connected. A unit
of measure of
magnetic field
strength, the
oersted, is
named after him.
Electricity and Magnetism
Electricity and magnetism are different
aspects of a single force known as the
electromagnetic force.
• The electric force results from
charged particles.
• The magnetic force usually results
from the movement of electrons in
an atom.
Electricity and Magnetism
Magnetic Fields Around Moving
Charges
Moving charges create a magnetic
field.
• Magnetic field lines form circles
around a straight wire carrying a
current.
If you point the thumb of your right
hand in the direction of the current,
your fingers curve in the direction of
the magnetic field.
Direction of
current
Direction of
electron flow
Current-carrying
wire
Direction of
magnetic field
Forces Acting on Moving Charges
A magnetic field exerts a force on a
moving charge.
• A charge moving in a magnetic field is
deflected in a direction perpendicular to
both the field and to the velocity of the
charge.
• A current-carrying wire in a magnetic
field will be pushed in a direction
perpendicular to both the field and the
direction of the current.
Electricity and Magnetism
Reversing the direction of
the current will still cause
the wire to be deflected, but
in the opposite direction.
• If the current is parallel to
the magnetic field, the force
is zero and there is no
deflection.
•
Solenoids and Electromagnets
If a current-carrying wire has a
loop in it, the magnetic field in
the center of the loop points
right to left through the loop.
Multiple loops in the wire make
a coil. The magnetic fields of
the loops combine so that the
coiled wire acts like a bar
magnet.
Solenoids and Electromagnets
The field through the center
of the coil is the sum of the
fields from all the turns of the
wire.
A coil of current-carrying
wire that produces a
magnetic field is called a
solenoid.
If a ferromagnetic material, such as an iron
rod is placed inside the coil of a solenoid,
the strength of the magnetic field
increases.
• The magnetic field produced by the
current causes the iron rod to become
a magnet.
• An electromagnet is a solenoid with a
ferromagnetic core.
• The current can be used to turn the
magnetic field on and off.
Solenoids and Electromagnets
The strength of an electromagnet depends
on the current in the solenoid, the number
of loops in the coil, and the type of core.
The strength of an electromagnet can be
increased using the following methods.
• Increase the current flowing through
the solenoid.
• Increase the number of turns.
• Use cores that are easily magnetized.
Electromagnetic Devices
Electromagnets can convert electrical
energy into motion that can do work.
• A galvanometer measures current in a
wire through the deflection of a
solenoid in an external magnetic field.
• An electric motor uses a rotating
electromagnet to turn an axle.
• A loudspeaker uses a solenoid to
convert electrical signals into sound
waves.
Electromagnetic Devices
Galvanometers
A galvanometer is a device that
uses a solenoid to measure
small amounts of current.
Electromagnetic Devices
Electric Motors
An electric motor is a device that
uses an electromagnet to turn an axle.
• A motor has many loops of wire
around a central iron core.
• In the motor of an electric
appliance, the wire is connected to
an electrical circuit in a building.
Electromagnetic Devices
Loudspeakers
A loudspeaker contains a
solenoid placed around one
pole of a permanent magnet.
The current in the wires
entering the loudspeaker
changes direction and
increases or decreases.
Assessment Questions
1.
A charged particle is moving across a plane
from left to right as it enters a magnetic field
that runs from top to bottom. How will the
motion of the particle be changed as it
enters the magnetic field?
a. It will accelerate.
b. It will deflect either up or down on the
plane.
c. It will deflect perpendicular to the plane.
d. Its motion will not be affected.
Assessment Questions
2.
Which change will increase the
strength of an electromagnet made by
wrapping a conductive wire around an
iron nail?
a. reversing the direction of current flow
b. replacing the nail with a wooden
dowel
c. increasing the number of coils of wire
around the nail
d. using a longer nail
Assessment Questions
3.
A loudspeaker uses a magnet to cause
which energy conversion?
a. mechanical energy to magnetic
energy
b. electrical energy to mechanical
energy
c. electrical energy to magnetic energy
d. mechanical energy to electrical
energy
Assessment Questions
4. The motion of an electric
charge creates an electrical
field.
True
False
Chapter 21
Magnetism
21.3 Electrical Energy
Generation and
Transmission
A magnetic field can be used to produce
an electric current.
• Electromagnetic induction is the
process of generating a current by
moving an electrical conductor relative
to a magnetic field.
• Changing the magnetic field through a
coil of wire induces a voltage in the
coil.
• A current results if the coil is part of a
complete circuit.
Generators
Most of the electrical energy used in homes
and businesses is produced at large power
plants using generators.
• A generator is a device that converts
mechanical energy into electrical energy
by rotating a coil of wire in a magnetic
field.
• Electric current is generated by the
relative motion of a conducting coil in a
magnetic field.
Generators
AC Generators
An AC generator produces alternating
current, in which charges flow first in
one direction and then in the other
direction.
The generator looks very similar to an
electric motor. While a motor converts
electrical energy into mechanical
energy, a generator does the opposite.
Generators
DC Generators
A DC generator produces a direct current.
• Its design is very much like the design
of an AC generator except that a
commutator replaces the slip rings.
• As opposite sides of the commutator
touch the brush, the current that leaves
the generator flows in only one
direction.
Transformers
The number of turns in the primary
and secondary coils determines the
voltage and current.
• To calculate the voltage, divide
the number of turns in the
secondary coil by the number of
turns in the primary coil.
• The result is the ratio of the
output voltage to the input
voltage.
Transformers
Types of Transformers
A step-down transformer
decreases voltage and
increases current.
Transformers
A step-up transformer
increases voltage and
decreases current.
Electrical Energy for Your Home
A turbine is a device with fanlike blades that
turn when pushed, for example, by water or
steam.
• Burning fossil fuels or nuclear reactions can
heat water to produce steam that spins a
turbine.
• Water from a reservoir behind a dam can
also turn a turbine.
• To produce electrical energy, the turbine
may turn the coils of a generator, or it may
spin magnets around the coils of wire.
A power plant transmits electrical energy at
hundreds of thousands of volts.
• After the current passes travels through
high-voltage transmission lines, the voltage
is stepped down at a substation, to a few
thousand volts.
• The electrical energy is then distributed and
stepped down to between 220 and 240
volts.
• Appliances like an electric stove use 240volt circuits. Most other appliances in the
home use 120 volts.
Assessment Questions
1.
In a DC generator, the commutator
a. generates an electric current.
b. converts an alternating current to
a direct current.
c. reduces the voltage.
d. reverses the direction of the
direct current.
Assessment Questions
2.
A transformer has 400 turns on the
primary coil and 1600 turns on the
secondary coil. What is the output
voltage if the input is 1,000 volts?
a. 250 V
b. 500 V
c. 2,000 V
d. 4,000 V
Assessment Questions
3. Which property
would you want to
increase in transmitting electrical
energy as efficiently as possible
over long distances?
a. current
b. voltage
c. resistance
d. insulation