4 ms·

You say electromagnetic interactions dissapear at any significant distance, yet they are responsible for holding materials together? Therefore the rigidity of

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.

btilly 17y ago
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.
- graphene 17y ago
  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.
  - btilly 17y ago
    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.
    - graphene 17y ago
      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.

btilly 17y ago

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.

graphene 17y ago
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.
- btilly 17y ago
  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.
  - graphene 17y ago
    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.