Transcript Section 17.1 - CPO Science

UNIT FIVE: Electricity and Magnetism
 Chapter 16 Electricity
 Chapter 17 Magnetism
Chapter Seventeen: Magnetism
 17.1 Properties of Magnets
 17.2 Electromagnets
 17.3 Electric Motors and Generators
 17.4 Generating Electricity
Chapter 17.1 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 17A
Magnetism
 Key Question:
How do magnets and compasses work?
17.1 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.
17.1 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.
17.1 Properties of Magnets
 Whether the two magnets attract or repel
depends on which poles face each other.
17.1 Properties of Magnets
 Magnetic forces can pass through many
materials with no apparent decrease in
strength.
17.1 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.
17.1 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.
17.1 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.
17.1 Magnetic fields
 You can actually see
the pattern of the
magnetic field lines by
sprinkling magnetic
iron filings on
cardboard with a
magnet underneath.
17.1 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.
17.1 Geographic and magnetic poles
 The planet Earth has
a magnetic field that
comes from the core
of the planet itself.
17.1 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!
17.1 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.
17.1 Declination and “true north”
 Magnetic declination is measured in
degrees and is indicated on topographical
maps.
17.1 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.
17.1 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.
17.1 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.
17.1 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.