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Rotating magnetic field effect on aqueous mixing (opinions?)

  1. Feb 9, 2018 #1
    This is question that can't really get a precise answer without significant modeling or experimentation, but I'm wondering what folks' intuition might be on an answer to the following setup + question:

    Suppose there is a steel tank made of steel pipe with walls about 0.25 cm thick, internal diameter 10 cm, and height 30 cm, filled with an aqueous basic solution, say NaOH(aq), being stirred at a constant rate by with a "disc turbine" impeller or similar mechanical stirring device. Attached to the shaft of the motor which drives the stirrer, on the portion of this shaft outside the steel tank, in atmospheric air, there is a concentric disc which has a thumbnail-sized permanent magnet fixed to it, so that the magnet will pass in front of a Hall Effect sensor each time the shaft rotates (an RPM sensor). (The shaft enters the tank via a leak-free rotary seal mechanism.) Suppose the volume of NaOH(aq) being stirred inside the tank is 2 liters. Furthermore suppose that a few grams of micron-scale Al powder is dropped into the stirred NaOH(aq) periodically, resulting in its (somewhat fast) corrosion and the release of hydrogen gas. ("Somewhat fast" meaning complete reaction within 10 s, but obviously much slower than say, an acid-base neutralization.)
    • For this system, will the magnetic field caused by the presence of the rotating thumbnail-sized magnet cause any increase in the state of "mixedness" of the NaOH(aq)? I assume that in any case, its presence would definitely not decrease the mixedness. I also assume/define the state of mixedness in a binary way: Suppose that with no rotating magnet, the Al corrosion proceeds at the maximum rate as limited by some transport process (molecular diffusion and momentum "diffusion"). Then, if putting the rotating magnet in place causes the Al corrosion to now proceed at the maximum rate as limited purely by chemical kinetics, all else being equal, we'll say that the magnet does have an effect on the mixedness. What do you think- is this likely to be the case?
     
  2. jcsd
  3. Feb 9, 2018 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF. :smile:

    Unless I misunderstand the setup, the steel tank will shield the effects of the external magnet from the contents of the tank.

    Can you post a sketch or picture of the setup? Use the UPLOAD button at the lower right to attach a PDF or JPEG file of the setup.
     
  4. Feb 9, 2018 #3
    No you are right, you didn't misunderstand it, I remember now that it would be shield by steel/ferric materials. Glad to see it is an elementary question... At the time I posted I wasn't totally sure if iron shields as in dissipates/absorbs the magnetic field energy, or if it simply alters the field so that it doesn't behave as predicted or intended before the iron object was brought into its presence.
    Thanks a lot!
     
  5. Feb 9, 2018 #4

    berkeman

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    Staff: Mentor

    The ferrous material has a much lower "reluctance" to the magnetic B-field than free space, so it deflects the magnetic flux into it and away from the initial free-space path. It's kind of like when you have a low-resistance resistor in parallel with higher resistance resistors, the low resistance resistor steals most of the current from the other resistors.

    Sorry for the dorky picture -- it's all I could find with a quick Google Images search...

    https://lessemf.com/images/faq10.gif
    faq10.gif
     
  6. Feb 9, 2018 #5
    OK thanks much, the resistance analogy helps a lot (the picture too, only the guy inside should relax b/c he's safe now :)
     
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