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Molecular motion and Heat

  1. Nov 2, 2011 #1
    Is the molecular kinetic energy associated with an increase of heat equally distributed to molecular vibrational, rotational, and translational energies?

    What is then the exact link to Brownian motion?
  2. jcsd
  3. Nov 2, 2011 #2
    Origin of Brownian Motion

    Are there any theories existent for the cause of Brownian Motion? Or is it just generally accepted as is?

    Have any experiments been conducted to attempt to perturb the seemingly random motion of molecules (i.e., cause it to have a biased probability distribution)? Have any single-tracking-particle studies been done?
  4. Nov 2, 2011 #3


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    Re: Origin of Brownian Motion

    No theories are ever accepted "as is". Every theory will have quantitative predictions and results that have to be verified by experiments.



    Last edited by a moderator: Apr 26, 2017
  5. Nov 2, 2011 #4
    Thank you very much for the articles.

    I should have been more precise, I meant to say: is Brownian motion considered a physical fact without an (as of yet) underlying explanation, or is there a theory behind its causation.
  6. Nov 2, 2011 #5


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    I thought the theory that atoms exist explained it perfectly. I believe it was one of the things discovered by Einstein.
  7. Nov 2, 2011 #6
    I thought this was explained 100+ years ago by Einstein?

    EDIT: Sorry, didn't see Drakkith's post.
  8. Nov 2, 2011 #7
    It seems I am fundamentally confused.

    I take it from what is said that Brownian motion refers to atomic motion caused by collisions from neighboring particles.

    I originally thought Brownian motion also referred to the inherent vibrational motion of particles that exists even if collisions do not take place. I also thought that subatomic particles (e.g., nuclei) were perpetually vibrating. The previous two statements might be falsely referring to phenomena, which would be a source of confusion.

    EDIT: From what I'm reading it seems atomic vibrations are primarily due to atomic bonds. Is this the only source of atomic vibrations?

    So more specifically:

    1. What exactly is Brownian motion? Is it a complete description to say the: translational, rotational, and vibrational motion acquired by particles as a result of collisions with other particles?

    2. Is any motion an inherent property of an atom (no existing bonds) such that it would be in motion even if no collisions were occurring? That is, even if it were all by itself?

    3. Why are they moving in the first place, and why don't they ever run out of energy?
    Last edited: Nov 3, 2011
  9. Nov 3, 2011 #8
    Yes , you are correct.

    You are also correct , precisely.

    Huh ? Brownian motion is the random motion or drifting of particles in a particular solution or a solvent or let's call it fluid.
    Wikipedia defines it as follows : Brownian motion (named after the botanist Robert Brown) or pedesis is the presumably random drifting of particles suspended in a fluid (a liquid or a gas) or the mathematical model used to describe such random movements, which is often called a particle theory.

    It is more frequent in http://www.edinformatics.com/math_science/suspensions_colloids.htm" [Broken].
    There can be many causes of brownian motion. Some solutions like colloidal solutions and suspensions give cause to brownian motion without any cause because they have particles drifting and colliding. Though the basic cause is collision.

    There can be many and many and many theories behind the causation of Brownian motion.
    It can be heat , percussion etc.

    Call it particle motion instead of atomic motion caused by collisions from neighboring particles. Yes, we can also take this at atomic level but you just cannot say that the distilled water have brownian motion. Can you ?

    1. I have already given the definition. It can be either translational, rotational, or vibrational motion or all of these motion acquired by particles as a result of collisions with other particles. Brownian motion is just the random drifting of particles in a fluid. Particles can behave in any motion they like. You can refer the brownian motion - zigzag.

    2. There must be some collision in the system. Will the particle in a still distilled water randomly drift ? No! But if you heat the water , the particle will gain kinetic energy leading to its collision with the neighbouring molecules. We know that after every collision the momentum of it will be conserved and so is its kinetic energy at a constant temperature.

    p1=p2=p and K.E. or Ek = p2/2m .

    3. They run out of energy. You heat water to θo C such that : Q=MCθ
    That heat energy is utilized in increasing potential energy. MCθ = Mgh , where g is intermolecular force of attraction. Since Mgh = Mv2/2
    so Mv2/2 = MCθ
    Now θ keeps on decreasing and so is kinetic energy and so are the collisions and so is the movement of a particle ! When molecules have momentum = Mv during collision they transfer that momentum to particle = mV such that Mv=mV (conservation of momentum)and hence particle gains different kinetic energy.
    Last edited by a moderator: May 5, 2017
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