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Difficulties in understanding Brownian motion.?

  1. May 9, 2013 #1
    I hope you can bear with me as I warm up to my question regarding Brownian motion. I am currently studying physics on my own and am watching a series of lectures on Quantum Theory. Obviously I am just some guy with an interest in physics and I have no clue.

    Apparently, Brownian motion is the movement of atoms. We all know the story of Robert Brown's discovery and Einstein's explanation of it. But I am not satisfied with merely asserting that it is because the atoms absorb photons (that is, “heat”) and thus they vibrate faster. So I began thinking about the “Brownian motion” of a single isolated atom. I made some of my own assumptions in order to help me clarify my thoughts.

    I thought only electrons absorbed photons, and as a result the electron moves into a higher quantum energy state. I made the assumption that the electron/photon interaction has little effect on the nucleus of the atom because the nucleus is shielded from that interaction. I wondered how that could translate into the “increased vibration” of the atom? Why does the atom move in the first place when all that’s happened is that the electron has absorbed (destroyed?) a photon and moved up one or more quantum energy levels in a higher electron shell. I could not believe nor imagine a massless particle like a photon having enough kinetic energy to physically move an atom in space. It would be like hoping to move the Earth by smashing a grain of sand against it. Even at the speed of light the effect would be too small to notice.

    I think many of us learn at a basic level that Brownian motion is caused by atoms ricocheting off each other like some tiny billiard game, but what really causes it and what keeps it in motion? I can imagine the motion of mechanical kinetic energy being transferred chaotically like when you pour water into a glass, but that has to die down sometime due to friction and gravity. So I am left with only the additional of more heat as the cause of Brownian motion. I see only two kinds of motion relevant to this question, that of translational movement like a billiard ball hitting another ball and transferring kinetic energy, and the other movement is some sort of internal vibrational energy like the ringing of a bell.

    So to sum up what I want to know: How does heat make an atom move faster, and by movement I am assuming the only movement involved is some sort of ringing effect like maybe when you strike an immovable bell with a hammer, and not a translational movement in some random direction. If a vibrating atom wandering around somehow collides with another one, would not the kinetic energy eventually be consumed? How then is Brownian motion sustained in the absence of kinetic energy from outside the system other than heat energy?
  2. jcsd
  3. May 10, 2013 #2

    Simon Bridge

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    Welcome to PF;
    Brownian motion is a stochastic process. In early observations it was seen in pollen grains suspended in oil.

    It is well modeled by systems involving lots of small, random, collisions.

    Anything that feels the electromagnetic interaction can absorb or release photons.

    The nucleus interacts with electrons electromagnetically - through being positively charged to the electron's negative. You know this. This is how we can move objects around by pushing on them.

    Your questions are not going to be answerable in the way you appear to want because you are making bad assumptions and definitions.

    * Some kind of "ringing like a bell" is not the only movement involved.
    * energy does not get consumed - it changes form.
    * Heat energy is external kinetic energy.

    And so on and on.

    What I recommend to you is that you actually conduct a Brownian motion experiment.
    Suspend some grains in oil and look at them under a microscope - see the motion for yourself. Very un-bell-like.
    You can also find many Brownian motion examples on youtube - all involving particles much bigger than atoms.
    Go look.

    Then come back and revisit these questions.
  4. May 10, 2013 #3
    OK, so the collision of a photon with an atom does make it move through the electromagnetic effect. That's good to know, now I'm a little less ignorant. I understand that stochastic means, more or less, random. I know that energy changes form and does not get consumed. So energy gets transformed by collisions which translate into motion and by collisions with the container which is a link to the larger external system. Does the motion continue because there is always an imbalance of energy between the system and the outside world? I think that would imply that one could see the slowing down of the Brownian motion by lowering the temperature. Thanks for your reply and I will look into the videos on You Tube but I cannot do the microscope thing as I do not have one.

    I think I need to understand a lot more about how atoms move before I can revisit this. I think that maybe the absorption of a photon which brings about a more excited state of an electron is a totally different process unrelated to Brownian motion. A lot more reading is in my future.
  5. May 10, 2013 #4


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    One important clarification you need to understand. Brownian motion describes the motion of tiny particles (usually visible) like dust in air or water. The motion is caused by (Einstein explanation) molecules of air or water bouncing against these dust particles.

    Brownian motion is NOT a description of the motion of the molecules themselves.
  6. May 10, 2013 #5
    mathman: Thanks! This is indeed an important distinction which I had not carefully considered. The observed motion of these tiny particles was evidence supporting the atomic theory or so I'm told. And it makes sense. The particles are huge compared to water molecules or gas molecules. The small particles must themselves be a large conglomerate of molecules with a relatively huge mass compared to the mass of a water or gas molecule. Which leads me to speculate that a fairly large number of water molecules impinge upon the small particle from the same general direction allowing their combined masses to actually move the small particle suspended in the water. Although the motion of the water molecules is completely random, undoubtedly the movement of the particle in a certain direction is the result of the vector sum of the number of water molecules hitting it at any given moment. Totally chaotic.
  7. May 11, 2013 #6


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