Magnets and Magnetic Field
Magnetism in History and Real Life
• The effects of magnetism have been known since
antiquity. For example, a piece of naturally occurring
iron-oxide mineral known as lodestone can behave just
like a manufactured magnet.
• Your first direct experience with magnetism was
probably a playful exploration of bar magnets and their
properties. From such experiences, you know that the
two ends of a magnet are different.
• Specifically, you learned that a bar magnet attracts or
repels another bar magnet depending on which ends
of the magnet are brought together.
Magnets
• One end of a magnet is referred to as its north
pole and is labeled “N”.
– The end of a bar magnet that points toward the
north geographic pole of the Earth is the “northseeking” pole, or simply the north pole.
• The other end of a magnet is its south pole,
which is labeled “S”.
– It points towards the Earth’s south geographic
pole so it is the “south-seeking” pole, or simply
the south pole.
Magnets and Poles
• Magnets always have two
poles
• Breaking a magnet in half
produces two new poles
• This is different from
electricity where the two
types of charge (positive
and negative) can exist
separately.
– A “magnetic monopole”
has never been found.
Magnetic Force
Magnetic Field
• The area around a
magnet where it can
influence the behavior
of other magnetic
things
• Lines that represent the
field go from the north
pole of the magnet to
the south pole of the
magnet
Magnetic Field
Earth’s Magnetic Field
• The Earth produces a
magnetic field that
resembles a giant bar
magnet
• Because the “north
seeking” pole of a magnet
(the north pole of the
magnet) points towards
the north geographic
pole, that is actually the
south magnetic pole of
the Earth
Earth’s Magnetic Field
• Earth's magnetic field
reverses direction over
geological time periods.
• These reversals have
left a permanent record
in the rocks of the
ocean floors
Why are things magnetic?
• All moving charges cause a magnetic field
– All of the electrons within an object moving create
their own small magnetic fields
– The movement of protons within the nucleus of
the atom creates a small magnetic field
– The “electron spin” also produces a tiny magnetic
field
• The combination of these effects creates areas
within the magnet called “domains” that have
a locally strong magnetic field
What are things magnetic?
In most materials, the fields
created in each domain are at
random angles and essentially
cancel each other out.
When a strong magnetic field
(from another magnet) comes
close, the fields in each domain
begin to “line up” with each
other. The combined effect of all
of the domains results in
something becoming magnetic