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Movement of Dust Particles

  1. Jun 14, 2012 #1
    I am a curious (or perhaps bored!) fellow, and as such I noticed something peculiar recently. I was sitting in a car in a garage. The car was stationary and off, and it was facing the east. The sun was beginning to set in the west and a light shone through the back window shield. In the rays of light, I could see particles of dust slowly moving around. The thing that struck me is that, although they also had a degree of random (Brownian?) motion to them, they all appeared to be moving to the west, in the direction of the sun. The impression I'm under is that these particles should all be moving randomly, period, and should have no general direction.

    I've only taken two awesome semesters of calculus-based physics. In spite of my knowledge could not come up with an explanation for what I saw. I spoke with a friend about it and she suggested that the earth's magnetic field may have had something to do with it. And while I like to keep an open mind, I feel that that cannot be the case since I live in California, and the magnetic field would cause these particles to move either north or south (and if these particles were moving according to that notion, wouldn't that make them monopoles!?).

    Is there a physical phenomena to explain my observation?
  2. jcsd
  3. Jun 14, 2012 #2


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    I think there was a very slight "wind" moving the particles. The side towards the sun is hot, the other side cold. This will set up a circulation, heated air rising on the sun side,moving backwards across the top of the car, down the back, and forwards along the lower part of the car. You might have been looking at the lower air currents. Take an incense stick or cigarette and hold it in different places and watch which way the smoke goes after it stops rising, see if that theory is correct.
  4. Jun 14, 2012 #3


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    Nothing magnetic is likely, nor electrostatic. Just plain old convection currents.

    Brownian motion is much smaller scale than you can see under those circs. I think. What you could see would have been turbulence and any apparent mixed motion of particles going in different directions would probably be an illusion due to mass motion of particles at different distances from your eye.
    You need a microscope to see Brownian Motion, afaik.
  5. Jun 14, 2012 #4
    Ah, I didn't even think of heat conduction. I guess that makes sense why I didn't though: thermodynamics was my worst section!
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