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Engineering ball sorting project, static electricity

  1. Mar 22, 2012 #1
    Hello all,

    I am an engineering student currently undertaking a project at univeristy.

    It involves the sorting of 4 different balls of which I have deduced how to sort 2 of them leaving just 2 types of balls to sort.....

    The remaining two are of identical size, 10mm in radius, and are Nylon 66 (density = 1.1g/cm^2, weight = 4.608g) and Acetal (density = 1.4g/cm^2, weight = 5.864g).

    After researching the two, I have found that they are both pretty similar. Nylon 66 has got static properties and Acetal is made from polar molecules...

    I was thinking maybe if both balls were passed through a tube lined with a sutable material, the nylon balls would become negatively (statically) charged with the acetal balls remaining neutral (being polar). Then the balls could be made to pass through something which produces charge like a capacitor, therefore the negatively charged nylon balls would be attracted to the positive plate of the capacitor therefore changing their paths. The paths of the acetal balls would be unaffected.

    Having no experience with that sort of stuff I was wondering whether or not this theory seems realistic or whether anyone has attempted a similar feat? I'm not sure my assumption that the path of the acetal balls would be unchanged was realistic? and I'm not sure if the capacitor could be made with a gap approx 20mm or whether or not that was too large?

    The other options would be to do something involving centripetal force or a counter ballanced see saw.

    Many thanks :smile:

  2. jcsd
  3. Apr 9, 2012 #2
    The simplest solution to this problem would be to utilize their weights. They are of the same size but they differ in weight (even though not by much).

    You could perhaps use brush hairs carefully woven and fixed internal to a 11mm radius hole. The idea being that the heavier (Acetal) ball will sink through the brush hairs but the lighter nylon ball will pass over.

    I have no idea how your contraption is configured and I don't know how fast these balls are going to be moving, so this idea might not work, since the difference in weight is too small.

  4. Apr 9, 2012 #3


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    Going off weight/density is an ok way to go, you could design a precision linkage that drops for a specific weight or the like. What are the other balls you're sorting made of? Is the idea to sort four seemingly (visually) identical balls by material?

    Another method could be to find a liquid (for example Brine or Glycerine, see here) that has a density between the two materials, say around 1.25 g/cm^3. One type of ball would float in the fluid, the other would sink. The trick is getting them out of the fluid once they're in there I guess (if we're talking a production application). If you're doing a proof-of-concept science experiment, all you need is a little tank of the stuff.

    If you wanted to do a really cool experiment to sort all four balls, find four fluids with properly-spaced densities which don't mix (or react) and drop them into the tank; the balls will drop to their respective fluid layers based on density, kind of like a galileo thermometer.
    Last edited: Apr 9, 2012
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