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Thread: The dumb questions thread

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    Quote Originally Posted by steve_bank View Post
    You have a long metal rod. You hit one end with a hammer and force is transmitted down the rod. It can not be instantaneous.

    At the atomic-molecular scale how is energy transferred down the rod?
    Compression wave.

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    Quote Originally Posted by bigfield View Post
    Quote Originally Posted by steve_bank View Post
    You have a long metal rod. You hit one end with a hammer and force is transmitted down the rod. It can not be instantaneous.

    At the atomic-molecular scale how is energy transferred down the rod?
    Compression wave.
    The techical term is longitudinal wave, but that does not explain the physics.

    I was wondering how the energy transfer occurs at the atomic scale. I do not know off the top of my head..

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    Compression wave transmitted by kinetic energy as molecules are forced together by the initial impact?

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    Quote Originally Posted by steve_bank View Post
    Quote Originally Posted by bigfield View Post
    Quote Originally Posted by steve_bank View Post
    You have a long metal rod. You hit one end with a hammer and force is transmitted down the rod. It can not be instantaneous.

    At the atomic-molecular scale how is energy transferred down the rod?
    Compression wave.
    The techical term is longitudinal wave, but that does not explain the physics.

    I was wondering how the energy transfer occurs at the atomic scale. I do not know off the top of my head..
    At the atomic scale, atoms don't like to get close to each other due to electrostatic forces (i.e. like charges repel, and atomic nuclei are all positively charged). If you push the atoms in your hammer into the atoms in the metal rod, they will push back against the hammer and against the atoms below them in the rod. This reaction happens over and over again, all the way down the rod. The atoms in the metal move at a finite speed, which is why the energy isn't transmitted instantaneously.

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    Fair dinkum thinkum bilby's Avatar
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    Quote Originally Posted by bigfield View Post
    Quote Originally Posted by steve_bank View Post

    The techical term is longitudinal wave, but that does not explain the physics.

    I was wondering how the energy transfer occurs at the atomic scale. I do not know off the top of my head..
    At the atomic scale, atoms don't like to get close to each other due to electrostatic forces (i.e. like charges repel, and atomic nuclei are all positively charged). If you push the atoms in your hammer into the atoms in the metal rod, they will push back against the hammer and against the atoms below them in the rod. This reaction happens over and over again, all the way down the rod. The atoms in the metal move at a finite speed, which is why the energy isn't transmitted instantaneously.
    Yup. It's electromagnetic forces between electrons, mediated by photons. Just like pretty much EVERY phenomenon in everyday life that's not gravity. Radioactivity and nuclear physics are really the only exceptions; Either it's stuff falling towards a massive object (Earth, Moon or Sun), or it's electromagnetic interactions between electrons, mediated by photons.

    Gravity is too weak for humans to notice unless it involves one of those three objects; And nucleons tend to be buried under a sea of electrons. So the answer to pretty much every question about pretty much every interaction in our lives is electromagnetic forces between electrons mediated by photons.

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    Quote Originally Posted by bigfield View Post
    Quote Originally Posted by steve_bank View Post

    The techical term is longitudinal wave, but that does not explain the physics.

    I was wondering how the energy transfer occurs at the atomic scale. I do not know off the top of my head..
    At the atomic scale, atoms don't like to get close to each other due to electrostatic forces (i.e. like charges repel, and atomic nuclei are all positively charged). If you push the atoms in your hammer into the atoms in the metal rod, they will push back against the hammer and against the atoms below them in the rod. This reaction happens over and over again, all the way down the rod. The atoms in the metal move at a finite speed, which is why the energy isn't transmitted instantaneously.
    That makes sense. I would not have thought of it as charge repulsion.

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    Quote Originally Posted by steve_bank View Post
    That makes sense. I would not have thought of it as charge repulsion.
    It's not really charge repulsion as much as electron degeneracy pressure - a pressure exerted because electrons, being half spin fermions, cannot inhabit identical quantum states.

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    Quote Originally Posted by Derec View Post
    Quote Originally Posted by steve_bank View Post
    That makes sense. I would not have thought of it as charge repulsion.
    It's not really charge repulsion as much as electron degeneracy pressure - a pressure exerted because electrons, being half spin fermions, cannot inhabit identical quantum states.
    Ok, you probably know more thyan I do.

    A metal rod is in equilibrium, atoms are moving around in place. An impulse is inut on ne end.

    Dimensionally it is Newtons in and Newtons out. Atoms are held in place by atomic forces. Electrons are charged particles which should have a field. How is force transmitted through an atom?

    In a ping pong ball there is an elastic deformation. Does the atom physically deform with an equal and opposite reaction force?

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    Quote Originally Posted by steve_bank View Post
    Quote Originally Posted by Derec View Post
    Quote Originally Posted by steve_bank View Post
    That makes sense. I would not have thought of it as charge repulsion.
    It's not really charge repulsion as much as electron degeneracy pressure - a pressure exerted because electrons, being half spin fermions, cannot inhabit identical quantum states.
    Ok, you probably know more thyan I do.

    A metal rod is in equilibrium, atoms are moving around in place. An impulse is inut on ne end.

    Dimensionally it is Newtons in and Newtons out. Atoms are held in place by atomic forces. Electrons are charged particles which should have a field. How is force transmitted through an atom?

    In a ping pong ball there is an elastic deformation. Does the atom physically deform with an equal and opposite reaction force?
    Yes, the atom does physically deform in the sense that the nuclei are closer together during compression and that atoms are cheek by jowl in a metal lattice. But atoms do not have a sharply defined radius or surface like a ping-pong ball would. When the nuclei get closer to each other and repel more, so do nuclei and electrons and they attract each other more. Coulomb forces are therefore a wash. So you need quantum mechanics and the idea of electron degeneracy pressure which is related to the Pauli exclusion principle which says that two electrons (or any other fermions) cannot inhabit the same quantum state.

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