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Can capacitor be made with 2 pennies + paper?

Discussion in 'General Electronics Discussion' started by NuLED, Jun 26, 2013.

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  1. NuLED

    NuLED

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    Jan 7, 2012
    I might try it but I am wondering what kinds of voltages do I need to have to get this working somewhat?

    I might try 12V but wonder if that will do anything?

    I also need to figure out how to hold the pennies together properly.
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    yes that will make a capacitor

    you realise putting 12V across a capacitor doesn't do much ?
    what were you expecting to happen/observe ?

    Dave
     
  3. NuLED

    NuLED

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    Jan 7, 2012
    dunno. anything! Haha

    I am in experimental learning mode all the time. old school :)

    I can't seem to be able to put theory to practice until I actually fool around hands on, then it all comes together

    this is why I am not using circuit simulators much right now and instead try to attach DMMs to components all around and see what happens. (I realize there is a little bit of voltage being introduced by the DMMs).
     
  4. john monks

    john monks

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    Mar 9, 2012
    If you hold two pennies together with a piece of paper between them you will get a capacitor of about 20 picofarads or 2E-11 Farads. You would get better results if you rolled two pieces of aluminum wrap separated with a piece of paper. And if you emersed this into water or oil the capacitance would would increase about 80 fold. But the results may still be disappointing The capacitance you get will be small and the voltages you need to get meaningful results can be dangerous. But if you stick with a 9 volt battery and a voltmeter you should be able to get some good results. So these are the things to remember:
    1. Air as a dielectric constant similar to empty space.
    2. The capacitance of two plates of one square meter separated by 0.001 meters is about 885 picofarads.
    3. The capacitance is inversely proportional to the space between the plates.
    4. The capacitance is proportional to the size of the plates.
    Have fun!
     
  5. BobK

    BobK

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    Jan 5, 2010
    When I took physics in college, the unit of capacitance was cm, which, I thought sounded funny. That would be a 100000 cm capacitor. (i.e. 10000 cm sqared / 0.1 cm)

    Bob
     
  6. john monks

    john monks

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    Mar 9, 2012
    Bobk, you might be right. I studied this in physics and what I remember is that the capacitance = (dielectric constant) * area / distance and I don't remember all the units. And when in school I saved all the relevant formulas in my TI-83. And at the end of the course I accidentally erased all my formulas and I thought, "What a disaster" Actually this is the best thing that ever happened because now I only use the concepts that I will never forget after spending many many years in school. So when NuLED's question came up all as I had to do is look up one capacitor example and rattle off some figures. In the end I sincerely believe that it is much more profitable to understand the underlying physics than it is to memorize formulas because the underlying physics is much easier to remember than formulas.
     
    Last edited: Jun 26, 2013
  7. poor mystic

    poor mystic

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    Apr 8, 2011
    Interesting.
    Training as a technicians, we were shown that capacitance needs to be measured in terms of charge per volt, i.e. a charge of 1 coulomb for a potential of 1 Volt is the same as a capacitance of 1 farad.

    I just looked at the Wikipedia entry for capacitance but my curiosity about BobK's "cm" remains unsatisfied. However I recommend the entry to NuLED.

    Tiny capacitors like NuLED's 2 coins separated by paper could change the resonant frequency of a high frequency tank such as the front end of an fm radio. Maybe we could think of a simple and quick way to let him see how that works in practice.
     
  8. NuLED

    NuLED

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    Jan 7, 2012
    It sounds like it's pretty hard to get significant capacitance without a modern factory if one has to achieve either high surface area and/or very short distances.

    What might have been the capacitance of those good old Leyden jars? They simply measured the attractive force, right? (Along the lines of "amber" static electricity, which has high voltage but no current?)
     
  9. NuLED

    NuLED

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    Jan 7, 2012
    Ha! I started wondering about this stuff and then went to the kitchen to get a large roll of foil and was thinking maybe I will use some olive oil as the dielectric... and then I realized HECK I don't even have a high enough resolution DMM to measure anything! (Not if it is at the picofarad scale).

    So, at least I didn't use up the foil. We need it this week to bake something. :)
     
  10. NuLED

    NuLED

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    Jan 7, 2012
    I think I tried reading it before. Unfortunately, I have found Wikipedia to often, though not always, confuse me more than anything. The entries usually are just a laundry list of all terminology related to the concept, and do not explain anything from the ground up or even provide at least a big picture framework so you can get oriented. Wikipedia is probably more useful as a memory jogger than as a resource for learning from scratch.
     
  11. duke37

    duke37

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    Jan 9, 2011
    Wikipedia says capacitance is
    C=8.854E-12 * e * A / d
    where e = dielectric constant (air = 1)
    measurements in meters

    so yes, you need a large area and small separation. An interesting exercise is to unwind a plastic insulated capacitor. There will be a very large tape with a very thin polyester or polypropylene dielectric.

    If you are rich, you can get a vacuum variable capacitor, the vacuum enables smaller spacing to be used with less chance of arc over.
     
  12. BobK

    BobK

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    Jan 5, 2010
    Here is a link to the system on units in which capacitance is measured in centimeters:

    http://en.wikipedia.org/wiki/Centimetre–gram–second_system_of_units

    1F = (10**-9 * c**2) cm

    The system of units is called cgs for cm / gram / seconds. All units are expressed only in a combination of those 3 fundamental units.

    The unit of resistance in that system is s / cm
    The unif of inductance is s**2 / cm

    It took some getting used to since I was already familiar with standard electrical units.

    Bob
     
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