Download Chapter 21 Magnetism

Chapter 21
Magnetism
Section 1 - What is Magnetism?
Pages 550 - 554
Early Uses of Magnetism
• Thousands of years ago, people noticed that a
mineral called magnetite attracted other
pieces of magnetite and bits of iron.
• When pieces of iron were free to turn, one
end pointed north
north.
• These might have been the first compasses
compasses.
• The compass was an important development
for navigation and exploration
exploration.
• Ancient writing suggests that 2,000 years ago,
Egyptians used magnets to make statues float!
Magnets
• Magnets attract objects containing iron,
nickel, or cobalt, such as nails and paper clips.
• Magnets can also attract or repel other
magnets.
• Every magnet has two ends or poles
poles….a north
pole and a south pole.
• Like poles repel each other and opposite
poles attract each other.
The Magnetic Field
• Remember a force is a push or pull that
causes an object to move
move.
• A magnetic force can be exerted even when
objects are not touching
(just like gravity
gravity).
• The magnetic force
becomes weaker as the
magnets get farther apart.
apart
• The magnetic force is
exerted through a magnetic field.
field
Field Lines
• Magnetic field lines begin at a magnet’s north
pole and end on the south pole.
• The field lines are close together where the
field is strong and get farther apart as the field
gets weaker
weaker.
• According to the diagram,
where is the magnetic field
the strongest?.
poles.
• Near the poles
Attraction and Repulsion
• Field lines that curve toward each other show
attraction.
attraction
• Field lines that curve away from each other
show repulsion
repulsion.
Making Magnetic Fields
• A moving electric charge, such as a moving
electron, creates a magnetic field.
electron
• A group of atoms, with their fields pointing in
the same direction, is called a magnetic
domain.
domain
Magnetic Domains
• Microscopic sections of
iron and steel act as tiny
magnets.
• Normally, these domains
are oriented randomly and
their magnetic fields cancel
each other.
• When a strong magnet is
brought near the material,
the domains line up,
up and
their magnetic fields add
together.
Earth’s Magnetic Field
• Earth has a magnetic field similar to the field
of a bar magnet.
• The region of space affected by Earth’s
magnetic field is called the magnetosphere
magnetosphere.
• One theory is the shifting
of molten iron in the outer
core of the Earth caused
Earth’s magnetic field.
Nature’s Magnets
• Some animals use natural magnets to detect
Earth’s magnetic field to help them navigate.
• Instead of using compasses, honeybees,
rainbow trout, and homing pigeons have little
pieces of magnetite in their bodies.
Earth’s Changing Magnetic Field
• Earth’s magnetic poles do not stay in one place.
place
• The magnetic pole in the north is in a different
location than where it was 20 years ago.
ago
• Sometimes Earth’s entire magnetic field reverses
direction! (North Pole become South)
direction
• This has happened more that 70 times
in the past 20 million years.
The Compass
• A compass needle is a
small bar magnet with a
north and south
magnetic pole.
• The north pole of the
compass needle points
toward Earth’s north
magnetic pole.
pole