Gravity versus electromagnetism ...
27 comments
this doesn't really comapre the two forces qualitatively I think..
Suppose you did the same experiment on Jupiter; you would fall a lot faster but the speed of decelleration wouldn't be affected. Perhaps on something like a neutron star the two times would even be similar, leading you to conclude the two forces are similar in magnitude...
Incidentally, it's for a large part statistical mechanics, not electromagnetism which would halt you fall - the hardness of solid material is due in a significant part to the electrons not wanting to share orbitals, and not just to them repelling each other.
Suppose you did the same experiment on Jupiter; you would fall a lot faster but the speed of decelleration wouldn't be affected. Perhaps on something like a neutron star the two times would even be similar, leading you to conclude the two forces are similar in magnitude...
Incidentally, it's for a large part statistical mechanics, not electromagnetism which would halt you fall - the hardness of solid material is due in a significant part to the electrons not wanting to share orbitals, and not just to them repelling each other.
The Pauli exclusion principle pertains, does it not, to electrons (etc) not wanting to duplicate QM states, but is it not still the case that one atom not passing through another atom is becuase of the repulsion of the orbitals?
Electrons in a single atom is a different question from electrons in different atoms.
Of course, I am probably wrong, and look forward to your pointers to further reading to help me understand more.
Electrons in a single atom is a different question from electrons in different atoms.
Of course, I am probably wrong, and look forward to your pointers to further reading to help me understand more.
Most detailed explanation in the blogosphere is "Why Stuff is Hard" by Jim Pivarski,
http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-har...
http://cornellmath.wordpress.com/2007/08/11/why-everything-i...
http://cornellmath.wordpress.com/2007/08/11/why-everything-i...
Complete argument is the Dyson, Lieb and Thirring's proof of stability of matter, http://projecteuclid.org/euclid.bams/1183555452 .
http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-har...
http://cornellmath.wordpress.com/2007/08/11/why-everything-i...
http://cornellmath.wordpress.com/2007/08/11/why-everything-i...
Complete argument is the Dyson, Lieb and Thirring's proof of stability of matter, http://projecteuclid.org/euclid.bams/1183555452 .
Many thanks for that last link.
But if one atom "tries" to pass through another, the electrons from both atoms will be spatially very close to each other, and consequently, they will be occupying very similar states. Coulombic repulsion certainly also plays a role, but it's not the only force at work here. I don't know the relative magnitudes for single atoms bumping into each other, but for bulk material, I believe it's about 40% exclusion, 60% coulomb.
Why would electrons in different atoms interact differently than electrons in the same atom? They don't, as far as I know. Furthermore, if electrons from different atoms only interacted Coulombically (repelling each other), molecules and solids would never get formed.
These are difficult questions, not addressed in the clear-cut cases often presented in introductory QM textbooks. I can't think of any must-read references right now, but if you're interested in this kind of thing I know of two books on molecular quantum mechanics; one by McWeeney (methods of molecular quantum mechanics) and one by Atkins (molecular quantum mechanics), which I haven't read myself, but I hear it's good. I don't know of any free online sources for this kind of stuff, but it's probably out there, I just haven't really looked yet.
Why would electrons in different atoms interact differently than electrons in the same atom? They don't, as far as I know. Furthermore, if electrons from different atoms only interacted Coulombically (repelling each other), molecules and solids would never get formed.
These are difficult questions, not addressed in the clear-cut cases often presented in introductory QM textbooks. I can't think of any must-read references right now, but if you're interested in this kind of thing I know of two books on molecular quantum mechanics; one by McWeeney (methods of molecular quantum mechanics) and one by Atkins (molecular quantum mechanics), which I haven't read myself, but I hear it's good. I don't know of any free online sources for this kind of stuff, but it's probably out there, I just haven't really looked yet.
Not just electrons. Take a look at Fermions: http://en.wikipedia.org/wiki/Fermion
Obviously the structure of an atom is affected by electrons not wanting to share orbitals. But isn't it electromagnetism that keeps atoms from passing through each other? And isn't it also electromagnetism that causes groups of atoms to clump in molecules, then the molecules to stick together in a crystalline pattern? (Update, the answers respectively seem to be "not entirely" and "yes".)
If that were true, why doesn't all matter stick together in one giant gob?
If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference?
In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible.
If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference?
In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible.
If that were true, why doesn't all matter stick together in one giant gob?
A typical atom has a net charge of 0 with minor local variations. Therefore its attractive force drops off much faster than 1/r^2. And so electromagnetic attraction between atoms effectively disappears at any significant distance.
If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference?
It is possible for both rigid and non-rigid materials to satisfy the Pauli exclusion principle. Therefore the rigidity of things around us cannot be caused by the Pauli exclusion principle.
In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible.
Are you talking about volume or hardness?
You are right that the exclusion principle results in volume. See http://en.wikipedia.org/wiki/Pauli_exclusion_principle#Stabi... for verification. (That effect is much bigger than I had realized.) However it does not necessarily result in rigid things that are hard.
Consider, for example, dropping a bucket of fresh water into salt water. Does the fresh water stop quickly upon impact? Obviously not. Now consider dropping a rock in the dirt. Does the rock stop quickly upon impact? Obviously it does.
What is different between the two scenarios? The answer is the structure of the materials, which are held together by electromagnetic forces. Therefore it really is electromagnetic forces that quickly stop falling solid objects that hit solid things.
A typical atom has a net charge of 0 with minor local variations. Therefore its attractive force drops off much faster than 1/r^2. And so electromagnetic attraction between atoms effectively disappears at any significant distance.
If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference?
It is possible for both rigid and non-rigid materials to satisfy the Pauli exclusion principle. Therefore the rigidity of things around us cannot be caused by the Pauli exclusion principle.
In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible.
Are you talking about volume or hardness?
You are right that the exclusion principle results in volume. See http://en.wikipedia.org/wiki/Pauli_exclusion_principle#Stabi... for verification. (That effect is much bigger than I had realized.) However it does not necessarily result in rigid things that are hard.
Consider, for example, dropping a bucket of fresh water into salt water. Does the fresh water stop quickly upon impact? Obviously not. Now consider dropping a rock in the dirt. Does the rock stop quickly upon impact? Obviously it does.
What is different between the two scenarios? The answer is the structure of the materials, which are held together by electromagnetic forces. Therefore it really is electromagnetic forces that quickly stop falling solid objects that hit solid things.
You say electromagnetic interactions dissapear at any significant distance, yet they are responsible for holding materials together?
Therefore the rigidity of things around us cannot be caused by the Pauli exclusion principle.
Not true. The necessary condition for rigidity (disregarding amorphous solids) is crystalline microstructure. Subject to that condition, the Pauli exclusion force is responsible for a significant fraction of the rigidity of the material. In non-rigid materials, the atoms (or molecules) have more freedom to get out of each other's way under application of a force, all the while obeying the Pauli principle.
As for volume vs. hardness, for rigid materials, if you accept that the Pauli force is making it much more voluminous than it would otherwide be, I would say that the same force is also making it hard.
Therefore the rigidity of things around us cannot be caused by the Pauli exclusion principle.
Not true. The necessary condition for rigidity (disregarding amorphous solids) is crystalline microstructure. Subject to that condition, the Pauli exclusion force is responsible for a significant fraction of the rigidity of the material. In non-rigid materials, the atoms (or molecules) have more freedom to get out of each other's way under application of a force, all the while obeying the Pauli principle.
As for volume vs. hardness, for rigid materials, if you accept that the Pauli force is making it much more voluminous than it would otherwide be, I would say that the same force is also making it hard.
I'm getting really tired of your using the Socratic method to get me to explain the obvious. But last round.
You say electromagnetic interactions dissapear at any significant distance, yet they are responsible for holding materials together?
There is essentially no direct attraction between distant atoms, but nothing stops you from having a line of atoms, each of which attracts the next.
Consider the case of a piece of glass. I am saying that electromagnetic forces hold the glass together. But those forces only exist between atoms that are in close proximity. However one atom holds on to the next holds on to the next through the whole glass, and it acts like a single rigid object.
However if you take that piece of glass and shatter it, you've separated the atoms along the cracks and they do not attract each other. In theory you should be able to put the pieces together and the crack will mend. In practice you simply can't put the pieces of glass closely enough together. (But The Feynman Lectures on Physics explains how sliding a piece of wet glass past a piece of wet glass does result in small portions mending then breaking, resulting in scratches on the glass. The water is necessary to lift surface impurities to allow pure connections to form.)
The necessary condition for rigidity (disregarding amorphous solids) is crystalline microstructure.
And what causes that crystalline microstructure other than the pattern of positive and negative charges on the atoms involved, resulting in adjacent atoms being attracted to each other?
Anyways as I've said, I've had enough of your drawing out rounds with asking questions that I am sure you know the answer to. Therefore if your next response isn't rather extraordinary, I'll be leaving this conversation.
You say electromagnetic interactions dissapear at any significant distance, yet they are responsible for holding materials together?
There is essentially no direct attraction between distant atoms, but nothing stops you from having a line of atoms, each of which attracts the next.
Consider the case of a piece of glass. I am saying that electromagnetic forces hold the glass together. But those forces only exist between atoms that are in close proximity. However one atom holds on to the next holds on to the next through the whole glass, and it acts like a single rigid object.
However if you take that piece of glass and shatter it, you've separated the atoms along the cracks and they do not attract each other. In theory you should be able to put the pieces together and the crack will mend. In practice you simply can't put the pieces of glass closely enough together. (But The Feynman Lectures on Physics explains how sliding a piece of wet glass past a piece of wet glass does result in small portions mending then breaking, resulting in scratches on the glass. The water is necessary to lift surface impurities to allow pure connections to form.)
The necessary condition for rigidity (disregarding amorphous solids) is crystalline microstructure.
And what causes that crystalline microstructure other than the pattern of positive and negative charges on the atoms involved, resulting in adjacent atoms being attracted to each other?
Anyways as I've said, I've had enough of your drawing out rounds with asking questions that I am sure you know the answer to. Therefore if your next response isn't rather extraordinary, I'll be leaving this conversation.
My apologies if I've annoyed you, that really wasn't my intention. I've enjoyed this exchange, and would be unhappy to have it end on this note. My questions were genuinely meant to structure the conversation and probe your thinking, not to exhaust you into submission.
One last comment then:
And what causes that crystalline microstructure other than the pattern of positive and negative charges on the atoms involved, resulting in adjacent atoms being attracted to each other?
You must be thinking of an ionic solid such as NaCl, but there are many solids (iron and silicon come to mind) in which the atoms have no net charge, yet the material is a crystalline solid. My point is that there is more at work here than just electromagnetic forces, namely quantum mechanics. Similarly, refering to our original discussion, there is more than just electrostatics that is responsible for the hardness of crystalline solids.
One last comment then:
And what causes that crystalline microstructure other than the pattern of positive and negative charges on the atoms involved, resulting in adjacent atoms being attracted to each other?
You must be thinking of an ionic solid such as NaCl, but there are many solids (iron and silicon come to mind) in which the atoms have no net charge, yet the material is a crystalline solid. My point is that there is more at work here than just electromagnetic forces, namely quantum mechanics. Similarly, refering to our original discussion, there is more than just electrostatics that is responsible for the hardness of crystalline solids.
I'm sorry for misinterpreting your intention.
And no, I was not just thinking of ionic solids. For instance take ice. A molecule of H2O has no net charge, but locally there is a pattern of positive and negative charges on the surface which results in a predictable crystal structure. Most molecules aren't that strongly polarized, but no matter what there is always some pattern of charges, and atoms will tend to line up so that positive meets negative and they attract. They will do this even when passing by quickly, hence the Van der Waals forces.
And no, I was not just thinking of ionic solids. For instance take ice. A molecule of H2O has no net charge, but locally there is a pattern of positive and negative charges on the surface which results in a predictable crystal structure. Most molecules aren't that strongly polarized, but no matter what there is always some pattern of charges, and atoms will tend to line up so that positive meets negative and they attract. They will do this even when passing by quickly, hence the Van der Waals forces.
You are right about ice, but metals (which are comprised of neutral atoms which lack an intrinsic dipole moment like that of H2O, and nearly always have crystalline structure) are not held together by Van der Waals or similar forces - one needs to invoke quantum mechanics to get even a qualitative picture of why they stick together.
Consider for instance Sodium. If there were no force other than Van der Waals holding the atoms together in the crystal, there is no explanation for the fact that Argon, a significantly heavier atom, has much lower melting and boiling points.
Consider for instance Sodium. If there were no force other than Van der Waals holding the atoms together in the crystal, there is no explanation for the fact that Argon, a significantly heavier atom, has much lower melting and boiling points.
If you did the same experiment on Jupiter, then the speed of deceleration absolutely would be affected. When you hit the ground, you stop because there is a large force accelerating you upwards. On Jupiter, there is still the same upwards force on you, but a stronger downwards force from gravity, and therefore you will decelerate somewhat more slowly. Not a lot more slowly, since the upward force from the ground is much larger than the gravitational force, but it still makes a difference.
On the other hand, if you're on a planet with more gravity, the ground is likely to be harder since there's more force compressing it together, but that's a whole different issue.
On the other hand, if you're on a planet with more gravity, the ground is likely to be harder since there's more force compressing it together, but that's a whole different issue.
Well, I don't know...
If you postulate beforehand that the ground won't deform, that means it will match any force applied to it, including that of a plummeting experimentalist. On jupiter, the force of gravity is greater, so the force applied to the ground by the experimenter's face will also be greater, but since the ground always matches it, the decelleration will occur in the same timespan (which is in fact zero provided neither the ground nor the experimenter's body deform).
If you do allow deformation, your point is valid. However, the thought experiment is still not accurate as two people of different weight performing the experiment will take different times to decellerate, due to the heavier one having gained more momentum. However, since they would have reached the ground in the same time (Gallileo's finding), they would arrive at different conclusions regarding the relative magnitudes of the Gravitational and Electromagnetic forces.
If you postulate beforehand that the ground won't deform, that means it will match any force applied to it, including that of a plummeting experimentalist. On jupiter, the force of gravity is greater, so the force applied to the ground by the experimenter's face will also be greater, but since the ground always matches it, the decelleration will occur in the same timespan (which is in fact zero provided neither the ground nor the experimenter's body deform).
If you do allow deformation, your point is valid. However, the thought experiment is still not accurate as two people of different weight performing the experiment will take different times to decellerate, due to the heavier one having gained more momentum. However, since they would have reached the ground in the same time (Gallileo's finding), they would arrive at different conclusions regarding the relative magnitudes of the Gravitational and Electromagnetic forces.
And why do electrons not want to share orbitals?
[deleted]
Because they are fermions, not bosons. Which has to do with their spin, not their charge.
See http://en.wikipedia.org/wiki/Pauli_exclusion_principle for more.
See http://en.wikipedia.org/wiki/Pauli_exclusion_principle for more.
Because that's just the way they are :-)
More precisely, no two electrons can ever occupy exactly the same quantum mechanical state. http://en.wikipedia.org/wiki/Pauli_exclusion
More precisely, no two electrons can ever occupy exactly the same quantum mechanical state. http://en.wikipedia.org/wiki/Pauli_exclusion
Why the thing that keeps them from occupying the same quantum mechanical state by holding them apart (like in neutron stars) is not called force (the fifth one)?
That's actually quite funny that we call 'fundamental forces' only the forces that we can't explain the source of.
That's actually quite funny that we call 'fundamental forces' only the forces that we can't explain the source of.
It's a nice example. Another one I like is "It takes the whole earth to attract this small iron object, but only this small magnet to overcome that pull"
The easiest way to get a sense for the difference between the laws is to look at their mathematical statements. From the sheer magnitude of the forces induced, electrostatic repulsion of like charges (Coloumb's law) is ~10^23 times more strong between two electrons than the attractive force between their mass as given by the law of gravity; for protons (which have more mass & opposite but equal charge) you lose a few orders of magnitude, but the margin is still ~10^20. But gravity always attracts, while electrostatics may attract or repulse, so when you consider a real situation then things like your falling example get more complicated. I prefer just to remember the difference in their magnitudes.
It's visualisations like that that help me keep otherwise apparently random facts straight.
Are there any similarly nice visualisations for biological features? How do bacteria, viruses, cells, DNA, mitochondria, molecules, proteins, fatty and amino acids compare in size? (Without resorting to rote SI unit memorisation, I mean)
Are there any similarly nice visualisations for biological features? How do bacteria, viruses, cells, DNA, mitochondria, molecules, proteins, fatty and amino acids compare in size? (Without resorting to rote SI unit memorisation, I mean)
Yes, there are!
http://news.ycombinator.com/item?id=909618
http://news.ycombinator.com/item?id=909618
It is often said that gravity is an incredibly weak force. But how weak?
Consider ...
Suppose you fall from a tall building. It takes ages to accelerate up to even a moderate speed, and yet you stop effectively in an instant. It's gravity that's making you fall, but electromagnetism that makes you stop.
It's visualisations like that that help me keep otherwise apparently random facts straight.
Hope you find that interesting, enlightening, or otherwise diverting.