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Zeno
2005-Sep-06, 02:08 AM
Why does matter compress when pressure is applied ?

and more importantly why does it expand when pressure is relieved ?

Matthew
2005-Sep-06, 07:47 AM
Lets look at gas first, as they behave most apparently with changes in pressure. Gas molecules have a high velocity, because there are no bonds between one molecule and another, and so their mass is very small. When a pressure is applied you are typically decreasing the volume taken up by the gas. So there is less room and thus the shape changes. The pressure required to compress gas depends on the number of molecules, the energy of the particles (temperature), and the volume.

Liquids can also be compressed, though to perform any amount of compression a great deal of pressure needs to be applied. This is because they are already close together and have stronger "repelling" bonds. Solids can be compressed, but again require a great deal of pressure.

When pressure is relieved the molecules spread to their initial positions. In gas due to entropy the gas molecules wil try and fill any vacuum, which is created if pressure is suddenly relieved.

jfribrg
2005-Sep-06, 01:03 PM
The quick answer is electromagnetism. The electrons in a molecule repel the electrons in other molecules. Increasing the pressure forces the molecules to be closer to each other than they otherwise would be. Releasing the pressure allows the electrons' repulsive force to push the molecules farther away. Thing are actually far more complicated that the above statement, because of the different types of chemical bonding, and the quantum behavior of matter at that electron scale, but the general idea is that the electrons repel each other.

Dr Nigel
2005-Sep-06, 08:09 PM
Some solids are easily compressible (such as rubber). This is because they have a dosordered molecular structure which can be rearranged by applying pressure. The weak intermolecular interactions (mostly Van der Waals forces) allow the molecules to adopt different arrangements. This, however, causes the molecules to adopt an energetically unfavourable configuration. When the pressure is relieved, the repulsive forces of the electron clouds in close proximity to one another cause the molecules to move apart again. In a disordered structure such as rubber, this is not always to the same shape.

Zarkov
2005-Sep-07, 12:09 AM
>> The quick answer is electromagnetism. The electrons in a molecule repel the electrons in other molecules. >>

so in fact it is the pressure that keeps matter together..

or without pressure matter pushes itself apart.

matter attracting matter is a myth.

grant hutchison
2005-Sep-07, 12:21 AM
so in fact it is the pressure that keeps matter together..Not exactly. Chemical bonds will hold atoms together into molecules, and hydrogen bonds and van der Waals' forces will produce a certain "stickiness" between molecules. But they strongly resist being shoved closer together than the equilibrium separations established by these various bonds.

Grant Hutchison

the_shaggy_one
2005-Sep-07, 12:23 AM
Matter attracting matter a myth? Now how could that be? We know matter attracts matter, we see it in the planets, stars, and galaxies. It's why the Earth orbits the Sun.

It's simply, over small scales, the force of electromagnetism is much greater than gravity. At the larger scales, most objects are electrically neutral and there are no electromagnetic effects, so gravity becomes the major player.

Zarkov
2005-Sep-07, 12:27 AM
>> We know matter attracts matter, we see it in the planets, stars, and galaxies. It's why the Earth orbits the Sun. >>

LOL

Is that right ?

I think not.

Ricimer
2005-Sep-07, 01:32 AM
sooo...what keeps satelites in orbit about earth?

Zarkov
2005-Sep-07, 01:36 AM
>> sooo...what keeps satelites in orbit about earth?


or anywhere else in the cosmos.

pressure, magnetic and electric.

"Gravity" via differential field spin.

jfribrg
2005-Sep-07, 05:31 PM
matter attracting matter is a myth.

Of course not. How things behave at the subatomic level cannot be extrapolated to the planetary scale. The electromagnetic force is far far stronger than the gravitational force. IIRC, it is about 40 orders of magnitude stronger. However, with the EM force, the positive and negative charges can cancel each other so that at a large scale, the EM force is negligible. With gravity, there is nothing to cancel it with, and so as tiny as the gravitational force is at the small scale, it becomes very substantial at a large scale because the tiny gravitational force of each atom accumulates to become strong enough to keep us in orbit around the sun.

Ricimer
2005-Sep-07, 09:44 PM
>> sooo...what keeps satelites in orbit about earth?


or anywhere else in the cosmos.

pressure, magnetic and electric.

"Gravity" via differential field spin.


So, the fact that the satellites are neutral/minor electrical charge, and the earth has almost no net charge, thus only a very minor electrostatic attraction means nothing?

Also the fact that the measured magnetic field at those locations (heck, anywhere on earth) is not enough to allow a satellite to orbit means nothing?

Those forces are measured, and are found extremely inadequate.

Zarkov
2005-Sep-08, 03:34 AM
Electrostatic charge has little (??, I am still investigation this aspect) to do with this problem

It is an electrodynamic problem whereby the motion of magnetic field lines induce an opposing magnetic field via Lenz's law, producing an equilibrium.

the motion is due to differentially driven field spin ..or standing wave drift, however uou like to view it.... see Schumann resonance....

The Poynting vector in Pointicare 3-space is the source of all gravity.

This is a model, and very accurate at that

If you would like to hone the fine tuning re "correct" english language words, please feel free.