Electromagnetism Electromagnetism

460
Biology
CONNECTION
Scientists think that birds may use the
Earth’s magnetic field to help them
navigate. Tiny pieces of magnetite have
been found in the brains of birds,
which could help them sense which
direction is north as they fly.
Figure 11 The Earth acts
like a giant magnet.
Chapter 18
North or South? Try this simple experiment. Place a compass
on a bar magnet that has its north and south poles marked.
Which pole of the magnet did the marked end of the needle
of the compass point to? If your compass is working properly,
the marked end should have pointed to the south pole of the
magnet, as shown in Figure 10.
Does that surprise you? Think
about what you have already
learned about magnets.
Figure 10 The marked end
of a compass needle always
points to the south pole of
a magnet.
One property of magnets is that opposite poles attract each
other. That means that the north pole of one magnet is attracted
to the south pole of another magnet. A compass needle is a
small magnet, and the tip that points to the north is the needle’s
north pole. Therefore, the point of a compass needle will
be attracted to the south pole of a bar magnet.
North Is South! So why does the needle of a compass point
north? The answer is that the magnetic pole of Earth that is
closest to the geographic North Pole is actually a magnetic
south pole! So a compass needle points to the north because
its north pole is attracted to a very large magnetic south pole.
The Core of the Matter Although you can think of Earth
as having a giant bar magnet in its center, as shown in
Figure 11, there isn’t really a magnet there. The temperature
of Earth’s core (or center) is so high that atoms
in it move too violently to remain aligned in domains.
Scientists think that the Earth’s magnetic field is
produced by the movement of electric charges in the
Earth’s core. The Earth’s core is made mostly of iron
and nickel. The inner core is solid because it is under
such great pressure. In the outer core, the pressure is
less and the metals are in a liquid state. As Earth
rotates, the liquid in the core flows and causes electric
charges to move, creating a magnetic field.
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