Download Section 22.1 - CPO Science

ELECTRICITY AND MAGNETISM 22.1
Chapter Twenty-Two: Electricity
and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
Chapter 22.1 Learning Goals
Identify properties of magnetic
materials and use interactions
between magnets to explain
attraction and repulsion.
Describe the source of Earth’s
magnetism.
Explain how a compass works.
Investigation 22A
Magnetism
Key Question:
How do magnets and compasses work?
22.1 Properties of Magnets
If a material is magnetic, it has the ability
to exert forces on magnets or other
magnetic materials nearby.
A permanent magnet is a material that
keeps its magnetic properties.
22.1 Properties of Magnets
All magnets have two
opposite magnetic
poles, called the north
pole and south pole.
If a magnet is cut in
half, each half will
have its own north
and south poles.
22.1 Properties of Magnets
Whether the two magnets attract or
repel depends on which poles face
each other.
22.1 Properties of Magnets
Magnetic forces can pass through
many materials with no apparent
decrease in strength.
22.1 Properties of Magnets
Magnetic forces are used
in many applications
because they are relatively
easy to create and can be
very strong.
Large magnets create
forces strong enough to
lift a car or a moving train.
22.1 Magnetic fields
The force from a
magnet gets weaker
as it gets farther
away.
Separating a pair of
magnets by twice the
distance reduces the
force by 8 times or
more.
22.1 Magnetic fields
A special kind of
diagram is used to
map the magnetic
field.
The force points
away from the north
pole and towards the
south pole.
22.1 Magnetic fields
You can actually see
the pattern of the
magnetic field lines
by sprinkling
magnetic iron filings
on cardboard with a
magnet underneath.
22.1 Magnetic field lines
A compass needle is a
magnet that is free to
spin.
Because the needle
aligns with the local
magnetic field, a
compass is a great
way to “see” magnetic
field lines.
22.1 Geographic and magnetic
poles
The planet Earth
has a magnetic
field that comes
from the core of
the planet itself.
22.1 Geographic and magnetic
poles
The names of
Earth’s poles were
decided long before
people understood
how a compass
needle worked.
The compass needle’s “north” end is actually
attracted to Earth’s “south” magnetic pole!
22.1 Declination and “true north”
Because Earth’s geographic north pole
(true north) and magnetic south pole
are not located at the exact same
place, a compass will not point
directly to the geographic north pole.
The difference between the direction
a compass points and the direction of
true north is called magnetic
declination.
22.1 Declination and “true north”
Magnetic declination is measured in
degrees and is indicated on
topographical maps.
22.1 Declination and “true north”
Magnetic declination is
measured in degrees and
is indicated on
topographical maps.
Most good compasses
contain an adjustable
ring with a degree scale
used compensate for
declination.
22.1 Earth’s magnetism
Studies of
earthquake waves
reveal that the Earth’s
core is made of hot,
dense molten metals.
Huge electric
currents flowing in
the molten iron
produce the Earth’s
magnetic field.
22.1 Earth’s magnetism
The gauss is a unit used to measure
the strength of a magnetic field.
The magnetic field of Earth (.5 G) is
weak compared to the field near the
ceramic magnets you have in your
classroom. (300- 1,000 G).
For this reason you cannot trust a
compass to point north if any other
magnets are close by.
22.1 Earth’s magnetism
Today, Earth’s
magnetic field is
losing approximately
7 percent of its
strength every 100
years.
If this trend
continues, the
magnetic poles will
reverse sometime in
the next 2,000 years.