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When an electric current passes through a metal wire, a magnetic
field forms around that wire (see diagram at right). Likewise, a
wire passing through a magnetic field creates an electric current
within the wire. This is the basic principle that allows electric
motors and generators to operate.
In the Earth (see image below right), the liquid metal that makes up
the outer core passes through a magnetic field, which causes an
electric current to flow within the liquid metal. The electric
current, in turn, creates its own magnetic field—one that is
stronger than the field that created it in the first place. As
liquid metal passes through the stronger field, more current flows,
which increases the field still further. This self-sustaining loop
is known as the geomagnetic dynamo.
Energy is needed to keep the dynamo running. This energy comes from
the release of heat from the surface of the solid inner core.
Although it may seem counterintuitive, material from the liquid
outer core slowly "freezes" onto the inner core, releasing heat as
it does so. (High pressures within the Earth cause material to
freeze at high temperatures.) This heat drives convection cells
within the liquid core, which keeps the liquid metal moving through
the magnetic field.
The so-called Coriolis force also plays a role in sustaining the
geomagnetic dynamo. Our planet's spinning motion causes the moving
liquid metal to spiral, in a way similar to how it affects weather
systems on the Earth's surface. These spiraling eddies allow
separate magnetic fields to align (more or less) and combine forces.
Without the effects caused by the spinning Earth, the magnetic
fields generated within the liquid core would cancel one another out
and result in no distinct north or south magnetic poles.
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