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PHYSICAL SCIENCE
Magnetism
Part 1: Magnets and Magnetic
Fields
14.1 Magnets and Magnetic Fields
Objectives
• Recognize that like magnetic poles repel
and unlike poles attract.
• Describe the magnetic field around a
permanent magnet.
• Explain how compasses work.
• Describe the orientation of Earth’s
magnetic field.
Magnets
• Naturally occurring magnetic rocks, called
loadstones, were first found in Magnesia (part
of modern-day Greece) almost 3000 years
ago.
• A loadstone is composed of an iron-rich
mineral called magnetite.
• Permanent magnets are magnetic all of the
time.
• The magnetism of a permanent magnet can
be weakened or removed by hammering or
heat.
Lodestone and Ancient Compass
Magnets
• Some materials retain their magnetism longer than
others do.
• Soft magnets are easier to magnetize than hard
magnets, but they also loose their magnetism easier
that hard magnets.
• A magnetic pole is an area of a magnet where the
magnetic force appears to be the strongest.
• Two like poles of magnets repel each other; two
opposite poles attract one another.
• If a magnet is cut, each piece will still have two poles.
Magnetic Forces
Magnetic Fields
• A magnetic field is a region where a magnetic force
can be detected.
• All magnets produce a magnetic field; some are
stronger than others.
• The strength of a magnetic field depends on the
material from which the magnet is made and the
degree to which it has been magnetized.
• Magnetic field lines are used to represent magnetic
fields.
• A magnetic field gets weaker as the distance from the
magnet increases.
Bar and Horseshoe Magnets
Magnetic Fields
• Magnetic field lines that are close together
indicate a strong magnetic field; lines that are
farther apart indicate a weaker field.
• A compass is a magnet suspended on top of a
pivot so that the magnet can rotate freely.
• A compass aligns with the Earth’s magnetic
field.
• The first compasses were made by placing a
lodestone on a small plank of wood floated in
calm water.
Map and Compass
Magnetic Fields
• A compass can be used to determine direction
because of Earth’s magnetic field.
• The source of the Earth’s magnetism is under debate.
• The Earth’s magnetic field has changed directions
throughout its geologic history – more than 20 time in
the past 5 million years alone.
• Evidence for magnetic reversals can be seen in the
alignment of the minerals in the seafloor rocks as they
progress outward from a spreading center.
• The Earth’s magnetic poles are NOT in the same
place as the Earth’s geographic poles.
The Earth’s Magnetic Field
Magnetic Fields
• The magnetic pole in Antarctica is actually a
magnetic N pole and the magnetic pole near
the Boothia Peninsula in Canada is actually a
magnetic S pole.
• The poles of magnets are named for the
geographic pole they point toward for historical
reasons; thus, the end of the magnet labeled
N is a “north-seeking” pole, and the end of the
magnet labeled S is a “south-seeking” pole.