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Vibrating molecules?

  1. May 11, 2009 #1
    I've heard molecules are constantly moving or vibrating. What is the mechanism behind this? Where does the energy come from, and go? Is it always a function of temperature/pressure?
    Last edited: May 11, 2009
  2. jcsd
  3. May 12, 2009 #2


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    Generally speaking these oscillations are driven by coulomb forces. For more details you may want to read about quantum oscillator.

    Molecule that is excited just 'has' its energy, and its energy doesn't change. You may say that this energy is stored in vibrations.

    Edit: it is a little bit more complicated, as the energy can be also stored in electrons occupying different orbitals, let's assume that electrons are not moving fom one orbital to another for now. Besides, to excite electron we usually need much more energy than to exctite bond vibrations.

    Molecule can exchange energy with surroundings through collisions or interacting with radiation.

    For a given molecule (to some extent for a given bond) frequency of the possible vibrations is independent on the surroundings. however, which frequencies dominate depends on the molecule energy, which in turn is a function of the temperature.
  4. May 12, 2009 #3


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    It may be useful to consider why molecules can't stay still. This phenomena is somewhat tricky to understand because it arises fully from quantum mechanics.

    First, consider the Heisenberg Uncertainty Principle. This principle states that we cannot know the exact momentum and position of a particle simultaneously. If we know the exact position of a particle at a certain time, then we cannot know its precise momentum (i.e. speed and direction) and vice versa.

    So, consider an unmoving particle. Since it is not moving, we know that it's momentum must be zero. Furthermore, because it is not moving, we would also know it's precise location. Therefore, an unmoving particle violates the Heisenberg Uncertainty principle and cannot exist even at absolute zero. This requirement for particles to have momentum (and therefore kinetic energy) at absolute zero is what is known as the infamous zero point energy.
  5. May 13, 2009 #4


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    Ygggdrasil: No offense, but that's a pretty useless answer.
    It explains why molecules aren't still at Absolute Zero, which is an unattainable temperature. It doesn't really explain the why-and-hows of thermal energy in general.
  6. May 13, 2009 #5


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    Nah, Ygg can be right - it is 100% not clear what the OP asks about. Could be the original question is about the zero point energy. Could be it is not.
  7. May 13, 2009 #6
    Ok, so here's my understanding now. Molecules just have an intrinsic vibrational energy that can be considered stored energy.. This energy can't be utilized, and is just "there"... It has no special name.

    Then there is a bond energy "vibrational", (seperate from above) that is a function of pressure/temperature.

    Then a molecules zero-point-energy only exists when the molecules are at 0K ... Above 0K zero-point energy doesn't exist (or not measurable?)

    Is this accurate?
    Last edited: May 13, 2009
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