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Basic(?) Electroscope/Electrometer Question

  1. Feb 14, 2015 #1
    Hi folks; apologies in advance if this is covered elsewhere or posted in the wrong place.

    I am a high school physics teacher, and I've been tasked with designing some new labs. I'm apparently confused by something rather simple here, so try not to be too hard on me. We have electroscopes that have angle markings for the deflection, 0 through 90 degrees (yes, I'm aware, this technically makes them electrometers, but no one seems to call them that). Of course, we can use this to make basic comparisons about the amount of charge various objects have, and also naturally use them to give students instructions (e.g. transfer enough charge that the deflection is about 45 degrees). However, every manufacturer of these electroscopes claims they can be used for quantitative measurements of charge amounts and relate to calculations of the coulomb force. I tried a rather naive experiment on my own time of using a single charged rod and seeing what distance from the electroscope produced what deflection angle. While of course I saw that further away was less deflection, I couldn't readily discern any sort of inverse-square-like pattern; in fact, it rather clearly was not.

    Is there a standard experiment that anyone is aware of that utilizes gathering the deflection angle data, even if for fairly basic or "hand-wavy" use? I'd like ideally to have them learn something quantitative that isn't too far over their heads. This is a standard introductory physics course (not honors or AP, which also exist in the building), so the math can't be too tedious (though I'd consider giving them a derivation and a formula to use if it were something plausible for students to follow at this level). If there is a way to salvage some sort of sense out of the type of procedure I outlined above (hold a charged rod at various distances, look at angle measurements), that would be the ideal sort of experiment I'd like to see them get some sort of intuition out of. Any thoughts anyone has would be much appreciated.

    Below are images of the sorts of electroscopes we have, though the foil type we have are particularly terrible, and the vane/bar/needle/whatever-you-call-it type are quite good, so I'd like to use those for this. I am willing to sacrifice one to mass the particular components, if that data would be needed, though I'd prefer to save an experiment of that type for the AP courses (I presently do one with holding up charged pieces of tape, measuring the angle between them, massing the tape, and eventually finding an estimation of the number of extra/missing electrons on each piece of tape).

    static_detection_pro_electroscope.jpg m-9593.jpg

    P.S. I'd dearly love to avoid any pith-ball experiments, as the kids are amazingly capable of breaking them instantly in my experience.
  2. jcsd
  3. Feb 15, 2015 #2


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    Staff: Mentor

    I've never tried to use electroscopes like these for actual charge measurements. Instead, as part of our college-level intro physics electrostatics lab we hang two equal-mass balloons on long strings from a single point on the ceiling, then charge them up (approximately) equally by rubbing them with fur (or a sweater), which makes them repel each other. Measure the angle of deflection, the balloon mass, and the string length, and you can calculate the product of the two charges with the help of a free-body diagram. Then if you assume the two charges are actually equal, just take the square root.
  4. Feb 15, 2015 #3
    To calibrate such electroscope for charge measurements is rather difficult.
    But connecting it to a HV source you can calibrate for voltage measurement.
  5. Feb 15, 2015 #4
    Hmm... I'm curious if you can clarify what the difficulties are? Thanks for your reply.
  6. Feb 15, 2015 #5
    This is similar to the scotch tape procedure I was mentioning with the AP courses. I'm not necessarily interested in making actual measurements of the charges, I'm more interested in getting them to see a general pattern, such as an inverse-square relationship between force and distance, and that this is true regardless of the sign of the charges. Something more quantitative would be great, but not necessary for a basic activity for students at this level. Reinforcing some general concepts and trying to build some basic intuition is a big step for them.
  7. Feb 15, 2015 #6
    Problems with small capacity and capacitive couplings are sufficient to cause troubles.
  8. Feb 15, 2015 #7
    Here is an experiment that could work. It's 38 minutes into the video.
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