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Equipotential Apparatus on the cheap?

  1. Oct 30, 2015 #1
    Hi, I recently stumbled across the wonderful equipotential apparatus, which allows one to create equipotentials by hand for educational purposes.
    equipotential.jpg
    After browsing around and finding sales prices ranging between 150 (pasco) and 500 dollars, I'd like to just make one.

    It does not appear at all complicated, but I had a question about the special conductive paper used in those high dollar kits. In one video, the narrator says "semi-conducting paper" which makes me think that simple carbon paper might not be a good substitute for my home-made version. (Pasco sells 50 sheets of the semiconductive paper for $45 dollars..)

    The manual for the mapper kit freely available on Pasco's website says the paper has a resistance of 5k ohms per square (cm I assume). I am looking for a cheap substitute because I already have everything else needed to make this great apparatus. Any ideas?

    Mentioned Video (apparatus shown 2:30 onwards)
     
  2. jcsd
  3. Oct 31, 2015 #2

    mfb

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    No cm. The units for surface resistance are just Ohm. If you increase the size of a square with current flow, you increase the length of the current flow but also the width where current flows. Both effects cancel and you get the same resistance - 5 kOhm.

    I don't know where you can get conductive surfaces within the right resistance range, but I would expect some cheap solution to exist.
     
  4. Oct 31, 2015 #3

    anorlunda

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  5. Oct 31, 2015 #4
    Hm, so Teledeltos paper is higher surface resistance than carbon paper? Is there a way to make Teledeltos paper yourself?
     
  6. Oct 31, 2015 #5

    anorlunda

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    You haven't given any specifications for quality, uniformity, or ruggedness. Perhaps carbon paper would do. Perhaps you could just shade a piece of paper with a graphite pencil if your standards are low enough. Perhaps you could dust sticky paper with lamp black soot.

    I think your best option is to contact other institutions and ask them to give you a single sheet of Teledeltos paper free.
     
  7. Oct 31, 2015 #6
    Well, here is my prototype. The problem I am having with the carbon paper is that the reading fluctuates to the point of being unusable. In the pic it shows a nail head I was using, but I tried various things like small copper squares, galvanized cylinders, etc. It might be more usable with a analog multimeter. The bars are aluminum, and hardware is galvanized. The bars are tightly compressed against the carbon paper. The voltage source is an ancient laptop charger rated at 12 V and 3A. Readings of resistance on the paper with probes roughly 1 cm apart are upwards of 22 Mohms.

    As far as what I'm shooting for, I would like for high school students who know nothing about vector calculus to be able to sketch equipotential maps from real fields in real space. Afterwards, they could compare their plots to theoretical constructions I prepare with Mathematica. So it can be crude and inaccurate as long as the general shapes are there. I was hoping they might be able to drag around the probe and see how the reading does not change on an equipotential. As mentioned though, its unusable due to sporadic readings.

    IMG_1634.jpg
     
  8. Oct 31, 2015 #7

    mfb

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    Your multimeter might draw too much current for a reliable measurement. Do you know its resistance?
     
  9. Oct 31, 2015 #8

    meBigGuy

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  10. Oct 31, 2015 #9
    My multimeter is 10 Mohm.

    I've got some graphite powder, Ill try that conductive paint mixture.
     
  11. Oct 31, 2015 #10

    mfb

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    Then it will influence the measurement significantly. You can use it in zero-current-mode (compare to a fixed voltage at the other lead) to find equipotential lines, but getting a sheet with better conductivity is probably easier.
     
  12. Nov 1, 2015 #11
    For zero-current-mode, do you mean putting one probe on + and the other on the paper?

    Would a cheap 5-10 dollar analog multimeter have a lower resistance?
     
  13. Nov 1, 2015 #12

    anorlunda

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    I don't understand. What high school can't afford $3/sheet for teledeltos paper? How much do you think it will cost to make your own?
     
  14. Nov 1, 2015 #13

    mfb

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    Put the other probe to the potential where you want to measure the equipotential line. Can be externally, can be on the sheet itself.

    10 MOhm is not bad for a multimeter, getting a better one is probably more expensive than buying the paper discussed above.
     
  15. Nov 1, 2015 #14

    NascentOxygen

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    The rolls of coated fax paper that preceded today's "plain paper" fax/copiers---is that a conductive coating? If so, it might be usable here even if it does have a much lower resistance.

    I have none at hand, else I'd try it to see.
     
  16. Nov 1, 2015 #15
    Well, we're a title 1 high school with a graduating class of about 20-30 a year. My budget is typically 600-700 dollars annually for chemistry, astronomy, physics, programming, and electronics. This year we started a new engineering program for upwards of $30,000, which was a huge deal. That was just a few months ago and I don't really want to ask my admin for more money at this point. That said, you're right that teledeltos paper really isn't unreasonable. I don't know how one would make teledeltos paper or the cost, the manufacturing process is unknown to me.

    I have a small regulated power supply, Ill give that a try.
     
  17. Nov 1, 2015 #16

    anorlunda

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    I'm thinking that you design the experiment so that the teledeltos paper does not get marked or destroyed. The probe has a spike on top. Move the probe to the equipotential point, and lower a piece of plain paper down to make a hole in the plain paper. Then connect the dots.

    One $3 sheet of teledeltos could serve you for many years. Beg a single sheet from a nearby university.
     
  18. Nov 1, 2015 #17
    I seem to be in business. Turns out I had some conductive paint kicking around. Decided to spread it out on a piece of acrylic, and cut holes for the conductors. The readings were completely steady with one lead at 0 and the other tapping the surface. I found some of my wife's ironing beads and was able to slip the bead onto the probe and scan around the surface. When I reached the voltage I was looking for, I lifted the probe. Here is an example with brown 10 V, turquiose 7V, purple 5 V and green 3 V. The left conductor is 12.3 V and the right is 0.

    IMG_1642.jpg
    This is obviously more expensive than getting the teledeltos paper, but I had these things on hand and it seems very durable.
     
    Last edited: Nov 1, 2015
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