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Pressure at Earth's Core
Name: Jonathan
Status: Student
Grade: Other
Location: TX
Country: United States
Date: Spring 2010
Question:
I am trying to understand what causes the pressure at the
Earth's core. If it is weight (a force), then what is the source of
that force? Is it caused by the Earth's rotation? Or gravity? And
why is the force so much greater the deeper you go inside the Earth?
It seems as though the Earth would be more like an Orange, which
sustains its own weight, else why does it not collapse within itself?
Replies:
Jonathan,
Gravity as we understand it is a property of objects that have mass.
This means that just by having mass, matter attract each other.
Imagine that you were in zero - g space (the point in space where
the gravity of the Moon and the Earth cancel out). If you could then
release two dust particles (in such a way that you did not give
either particle any inertia), then those two dust particles would
eventually attract each other, move toward each other, and eventually connect.
Now imagine you could measure the force between these two particles
(by maybe inserting a very sensitive probe in between them), you
would measure a force due to the gravitational attraction between
these two particles. Now suppose you were to add another dust
particle (imagine them forming a triangle), this would mean that p1
attracts p2 and p3, p2 attracts p1 and p3, and so on. These
attractions tend to pull all the particles inward, so the strongest
force measurement will still be in the middle of the triangle.
Adding more particles will just continue to increase the force felt
in the middle.
So it is with the Earth (or any body of matter). The highest
intensity of the force due to gravitational attraction will be in
the middle of the mass.
As to why an orange does not collapse onto itself (or for that
matter, why the Earth does not collapse onto itself), gravitational
force is very weak. Think of the fact that you can lift metal
objects with a magnet - the magnet's attractive force is able to
combat the force of gravity from the entire Earth! Because
gravitational forces are very weak, matter tends not to collapse
under that force. The only time we note that there is so much matter
that the combined gravitational forces collapses the object is in
incredibly massive objects like black holes and stars that have burned out.
Greg (Roberto Gregorius)
Canisius College
Hi Jonathan,
Gravity is the main force that holds the earth together, and explains
much of the pressure underground. The earth is massive enough to stay
fairly tightly bound by gravity, but the forces are not sufficient to
cause the rock and metal that make up the earth to collapse into
another form of matter. Unlike gaseous stars or planets, Earth's
materials are relatively strong and stable, preventing any kind of
overwhelming collapse.
Hope this helps,
Burr Zimmerman
Pressure at any point within the Earth is caused by the weight of
Earth materials above that point. The Earth materials all attract
each other by gravity. Anything between these interacting bodies
will experience a pressure, because the materials on all sides will
press in as they attract each other.
The pressure is greater closer to the center of Earth because there
is more stuff on all sides pressing inward. A the surface of Earth,
most of Earth's mass is below you, and only the atmosphere is above
you. The mutual attraction is fairly small: only about 1
bar. (That is "atmospheric pressure.") Deeper down, more mass on
both sides presses inward more firmly. The greatest pressure will
be at Earth's center of mass, into which the greatest amount of mass
presses in all directions.
An orange does not hold itself together by gravity. The mutual
attraction of the different parts of an orange is ridiculously
weak. It does not collapse in on itself because no force acting on
the orange is strong enough to squish it very far. Earth has much
more mass, and much more mutual gravitational attraction, than an orange.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
Pressure at the Earth's core is caused by the weight of all of the stuff on
top of it.
The source of the force of weight is gravity.
Earth's rotation would cause a centrifugal force which is directed outward
from the center of the earth. See:
http://en.wikipedia.org/wiki/Centrifugal_force
So Earth's rotation causing centrifugal force would diminish the weight
force.
Pressure is greater the deeper you go because the deeper you go the more
stuff is on top to weight down.
If the earth were mushy inside like an orange then it would collapse in on
itself until the matter is compacted enough to bear the weight of the stuff
weight in on top of it.
Instead, the Earth's core is layered as described at this URL.
http://en.wikipedia.org/wiki/Structure_of_the_Earth
Here is a site that has diagrams:
http://www.google.com/images?hl=en&q=Earth%27s+core&um=1&ie=UTF-8&source=uni
v&ei=vAHiS7-oGIaBlAf12cCRAg&sa=X&oi=image_result_group&ct=title&resnum=1&ved
=0CDEQsAQwAA
Sincere regards,
Mike Stewart
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Update: June 2012
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