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Jumping cookie crumb, static charge?

  1. Apr 3, 2009 #1
    A whole pack of Oreo cookies was finished, the clear plastic wrapper was sitting on a open notebook. I observed the most curious phenomenon.

    Between the plastic wrapper and the notebook was a black cookie flake about two mm in length and quite flat.The black cookie flake was, I'm guessing, statically attracted to the under side of the plastic. It hung vertically touching at a point. After a short while it dropped a distance of about 3 mm momentarily touched the notebook paper and then "jumped" back up and was again statically attracted to the plastic. Another 5 seconds would elapse and the process would repeat it self, drop, jump, wait, drop.

    Sitting back amused I became dejected at the thought the Feynman would have quickly figured out what was happening while I was blank.

    Any Feynman out there care to explain the physics to me?

    Thank you for any thoughts.
     
  2. jcsd
  3. Apr 3, 2009 #2

    turin

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    It was a flea. I hope you didn't get worms or something.
     
  4. Apr 4, 2009 #3
    How small a flake are we talking? It could have just been wind.
     
  5. Apr 4, 2009 #4
    It was in a basement office, very little air movement. The flake was flat, about 2 mm long and maybe a half a mm in thickness.
     
  6. Apr 4, 2009 #5

    DaveC426913

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    Cool.

    I'm just guessing, but it sounds like it's discharging its static charge when it drops to the notebook, then is electrostatically attracted to the plastic again.

    My assistant, Bob_S, will explain it better.
     
    Last edited: Apr 4, 2009
  7. Apr 4, 2009 #6
    Yes, it is static electricity. If you paint a ping pong ball with a conductive coating and place it between two high voltage electrodes, it will toggle back and forth between the two electrodes. Easily explained using classical E & M.
     
  8. Apr 4, 2009 #7
    I'm still not sure it's not air currents. If you see something that looks like a horse, your first guess should be horse, not zebra.
     
  9. Apr 4, 2009 #8

    DaveC426913

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    :confused: But it doesn't behave anything like air currents.
     
  10. Apr 4, 2009 #9

    turin

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    I don't think the comparison to the ping pong ball is quite relevant. The notebook paper and cookie bag are hardly good enough conductors to transfer charge as two electrodes would.
     
  11. Apr 4, 2009 #10

    DaveC426913

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    And yet ... it's the best theory we have going.

    The observed phenomenon is almost assuredly electrostatic in nature, and it is behaving to outward appearances, just like the ping pong balls.

    Unless we have some other serious contenders, I'm sayin' if it walks like a duck and talks like a duck...
     
  12. Apr 4, 2009 #11
    " But it doesn't behave anything like air currents. "
    Little crumbs moving around... sounds like air movement to me.

    "And yet ... it's the best theory we have going."
    According to whom?

    You still have some interesting questions to answer, including:
    1.) How the heck do the paper and chip bag transfer charge?
    2.) How do they have such an imbalance of charge in the first place to permit this?
    3.) How is the electrostatic force going to be any bigger than gravity?

    My theory is that small vibrations or displacements in air (as can be caused when you blink) could have moved such a small flake enough to cause the observed behavior. The sticking could be friction.

    If it walks like a duck, and it looks like a duck, I agree it is generally a duck, unless of course you take "walks like a duck" to mean "moves" and looks like a duck to mean "occupies space", in which case everything starts looking more or less like a duck.
     
  13. Apr 5, 2009 #12

    DaveC426913

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    That is a so ridiculously over-simplified mangling of the observation that it does not deserve comment. And it kind of destroys your credibility.
     
  14. Apr 5, 2009 #13
    "That is a so ridiculously over-simplified mangling of the observation that it does not deserve comment. And it kind of destroys your credibility."

    A little touchy, huh? Like any of us have any credibility on an internet forum. Get over yourself.

    Seriously, though, I doubt that a simple order-of-magnitude calculation of the forces involved would permit the electrostatic behavior you're talking about. If you estimate the mass of the crumb, and the distance it was jumping, it should be relatively straightforward to ballpark the the voltages involved using elementary physics. How high could the potential difference be? Not very, I imagine.

    To pigheadedly say it couldn't be air moving it around is just that - pigheadedness. It could be static electricity. I don't see it. The only fair way to test this would be to set the thing up in a vacuum chamber. Shielding it by putting it in a basement would so obviously not work that it destroys any credibility *you* had... which was none anyway.
     
  15. Apr 5, 2009 #14

    DaveC426913

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    It's not about a person's credibility, it's about an internally-consistent argument that anyone could follow. Reducing the OP's account to "crumbs moving around" indicates that you're dismissing the empirical evidence and thus don't understand the scientific method.


    You've so quickly turned inward to examine the theory that you're ignoring what actually happened.

    The crumbs drop to the surface then jump back up again, pause 5 seconds, dangle by a point (something electrosticaly charged particles are often seen doing yet particles carried by air current do not), then repeat several times.

    While you're supposing electrostatic forces could or couldn't do something, you're not acknowledging that the crumbs undeniably did something that needs explaining.


    Have you never seen cookie crumbs stick to their plastic container from static forces? I have. Maybe that's why it makes sense to me. It's common.
     
    Last edited: Apr 5, 2009
  16. Apr 5, 2009 #15

    turin

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    Let me first come out and say that I have no idea what physical principles are responsible for the observed phenomenon.

    While I have seen cookie crumbs sticking to all sort of things, I have never once personally verified that triboelectricity was responsible for the sticking. I just assumed that the crumbs were sticky (like gooey sticky) - crumbs have never before invoked enough curiosity in me for further investigation, as they have in the OP. I suppose a materials or condensed matter scientist might argue that gooeyness is just a form of triboelectricity - not my field.

    At any rate, I am not at all disputing the consistency of the static part of the description. What I am disputing is the dynamic part. How could the crumb become unattracted to one surface (or at least more attracted to the other surface) after some time? And even more puzzling to me, how could this process repeat multiple times? This is something completely unfamiliar to me when only insulating materials are involved. Perhaps the assumption that all materials involved are insulators is false? Perhaps air currents provided the charge transport :)~

    EDIT: I just thought of an alternative explanation:

    Perhaps the crumb was sticky to surface A from which it hangs, but not sticky to surface B onto which it drops. When it hits surface B, the impact causes surface B to depress, and then surfact B pops back out, launching the crumb back to surface A, where it sticks for a while. Repeat as necessary.

    That is just qualitative. I suspect that the numbers will not work out.
     
    Last edited: Apr 5, 2009
  17. Apr 5, 2009 #16
    The effect was recreated. Note book opened up, another bag of Oreos eaten, smallish various crumbs scattered on note book, clear plastic wrapper "stroked" several times with roll of paper towels, plastic lowered onto note book.

    Some of the crumbs just get attracted to the bottom of the plastic and stay there while some jump, hang, drop, jump, ect.

    With the plastic wrapper removed the crumbs just sit there on the note book. Air currents not a factor.

    Thank you to all who responded.
     
  18. Apr 5, 2009 #17

    DaveC426913

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    In post 6, Bob S describes this exact behavior under other circumstances. It is merely a question of whether the OP's circumstance is plausibly similar. If it's wiithn the realm of plausibilty, then we have an answer. If not, we don't. (I grant that there are definitely differences. The question is whether there are big enough differences to rule it out.)
     
  19. Apr 5, 2009 #18

    DaveC426913

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    Cool. So we've repeated the experiment, ruling out spurious phenomena. But does this bring us any closer to an answer?
     
  20. Apr 6, 2009 #19

    turin

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    I have just repeated the experiment, but with some slight modifications. Since I don't have bags of cookies, I had to replace the cookie bag with a ziplock type (generic) baggie. I repeated the experiment several times with various bottom surface materials:
    - notebook
    - single sheet of paper on counter top
    - plastic
    - glass
    - metal
    - wood
    and various jumping particle materials:
    - cookie crumbs
    - coffee grounds
    - sugar
    - tea leaf particles

    Procedure:

    I made no attempt at rigor nor precision. This was just an observational phase.

    I placed the bottom surface on the kitchen countertop. I scattered several candidate particles onto the bottom surface. (I found that the observation becomes increasingly confusing with particle density, so I tried to have no more than a few particles per cm2.) Sitting at the countertop, I draped the baggie over my leg and scraped it thoroughly with a paper towel. After doing this, the baggie was readily clinging to itself, and so apparently statically charged. I carefully lowered the baggie by hand over the particles on the bottom surface, trying to keep the baggie level. At the point where I start seeing particles jumping up to the baggie, I tried to hold the baggie steady, and simply observe. Occassionally I would not observe any significant jumping, and would then rescrape the baggie with the paper towel and repeat.

    I also conducted two variations. In one variation, I lightly tapped the baggie with a spare finger from above, while trying to otherwise hold it steadily in position over the bottom surface. In the other variation, I tried to hold the baggie level at elevation while I scanned it back and forth over the bottom surface.

    Initial results and hypotheses:

    There is no doubt in my mind that there is a triboelectric element of this phenomenon. Basically every combination of materials that I listed above, with the exception of a plastic bottom surface, leads to jumping particles. In the original procedure, I did not make any conclusive obervation of a particle jumping repeatedly; they always apparently jumped up to the baggie a single time, and then stayed there. However, the tapping procedure readily produced an apparently unlimited number of multiple jumpings. The scanning procedure occasionally produced multiple jumpings, but it was much less reliable than the tapping procedure.

    One interesting comment, at least a personal observation. In the tapping procedure, the particles resembled to me tiny fruit flies flying around. If the baggie was sufficiently elevated above the bottom surface, then the tapping would dislodge some of them, but they appear to fly back up to the baggie, like tiny little flies attracted to the baggie. It was pretty neat.

    My initial hypothesis is that there must be some perturbation of the cookie bag that causes the crumbs to dislodge and fall to the notebook. The tapping procedure probably dislodges the particles by a simple excess of effective inertial force. The scanning procedure probably works because the charge distribution on the baggie and bottom surface is nonuniform, and so the system becomes reconfigured with varying degrees of relative attraction to the baggie and bottom surface. I think that gravity has a relatively insignificant effect.

    Outlook:

    I don't plan to pursue this investigation any further.
     
    Last edited: Apr 6, 2009
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