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Homework Help: Electrostatic Charge Experiment

  1. Apr 29, 2012 #1
    Hello, I needed some help for my physics homework. The aim is to design an experiment that answers the question: Is the number of rubs of a piece of mica (that bag where you put documents to file them or to protect them, often made of plastic. Here is a Pic: http://images.segundamano.com.mx/2011/03/01/6057833/19405116.jpg [Broken] ) against a piece of wool directly proportional to the charge that it gains? The only thing I'm missing is a way to measure the charge, Could you give me a hand?
    Sincerely Gabriel De la Torre
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. May 1, 2012 #2

    rude man

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    Homework Helper
    Gold Member

    Don't know if this would be sufficiently sensitive, but:
    get an oscilloscope with a high input impedance (standard probes are Z = 10MΩ), charge your mica, set the sweep speed to something like 1 us/division, and record the trace: it should look like V0e-t/RC where R = 'scope input Z, C = capacitance between your mica and ground; then V0 = Q/C where Q is the charge you initially put on the mica. All units SI.

    You find RC at the time point when the voltage is about 0.37V0.

    Example: Z = 10MΩ, C = 1 pF, then RC = 10us = t at 0.37V0, and then Q = 1e-12/V0.

    What I don't have a good feel for is how much charge you can get onto the mica. In order for the above to be feasible you would need V0 at least 10 mV (probably; depends on your scope sensitivity), so 1e-2V = Q/1e-12F or Q should be at least about 1e-14 C. That's about 62,000 electrons.

    Good luck!
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