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Capacitor building

  1. Mar 16, 2012 #1
    I have been taking some foil and various "dielectrics" in attempt to build a capacitor. The best I have done so far is an extremely low capacitance (potential difference drops fast even on reading voltage with great resistance) where I will get about 3v for a few milliseconds after having charged with a 19 v battery pack.

    Really though, no matter the capacitance, I should be able to get the same potential difference.

    I am thinking this is not the case because the plate separation to plate area ratio is too large. If I were to decrease the distance, would I then be able to get a closer potential difference to that of the supplier (battery)? Or that is; does the potential -> battery potential as separation distance -> 0?

    Also, would saran rap be a good or bad dialectic material considering it stores charge quite well?
     
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  3. Mar 17, 2012 #2

    phinds

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    I don't know how saran wrap would be as a dielectric, but certainly the closer you can get the plates without dielectric breakdown, the more capacitance you will have.

    If you are getting less charge than your source voltage, pretty much regardless of the size of the capacitor, I suspect a leak or an impedance connection of some kind.
     
  4. Mar 17, 2012 #3

    sophiecentaur

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    Have you looked up the formula for the capacity of a basic parallel plate capacitor? Any answer you get from applying the formula will tend to be over-optimistic because of irregularities in your assembly - making the spacing more than you hoped. What capacity are you aiming for?

    Any capacitor you may make will probably be huge (physically), compared with one you can buy. These days the capacitors are harder to get inside and study, without destroying the structure but it may be worth while 'getting at' one with a scalpel to see how it's made.
     
  5. Mar 17, 2012 #4
    My advice is be VERY careful when doing such things as building capacitors (I would actually recommend not doing it). I speak from experience; when I was about 14 years old I tried to build a capacitor with foil and a plastic tube. It ended up with a small explosion and blown fuses (stupid me :uhh:), and my daddy got very angry... nothing worse happened though.

    But if you insist on experimenting with this, my firm advice is to use safety goggles and keep water close by to extinguish any possible fire.
     
    Last edited: Mar 17, 2012
  6. Mar 17, 2012 #5

    NascentOxygen

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    The current drawn by your voltmeter is discharging the capacitor. To gauge how well your capacitor is storing charge, wait for 2 mins and then connect your voltmeter--does it give much the same reading as when you connect it immediately after charging the capacitor? You need to do this in a very dry atmosphere, otherwise charge leaks away through humidity. Frosty winter's days with clear skies are usually of low humidity.
    I don't know.
     
  7. Mar 17, 2012 #6
    Yeah, I wonder about this leakage.

    Yes, though I have not attempted to derive a formula for the capacitance for plates being rolled up in tube form. Does this rolling up increase capacitance given a finite sheet size and separation? My original guess was yes, but I am unsure now thinking about it. That is, the E fields should be only between the plates so any wrap around would have no reinforcement of charge - just makes the capacitor more compact. But given some finite plate separation other than zero would mean then that the E fields would slightly diverge radially to the outer plate. So I really don't know. Nonetheless, it is not so much capacitance that I am trying to get, but rather a potential difference reading over the plates "equal" to the potential put over it to charge it.

    Yeah, I usually wear safety glasses. I am not too worried about my pathetic capacitors doing too much, but nevertheless, I will try to keep it safe.

    This was one of my theories. I may simply need to increase the capacitance before I can measure such a potential difference. I tried also charging the capacitor while the voltmeter was connected and as soon as I disconnected the battery, it dropped. Well, maybe a bit slower than without the capacitor in parallel.

    Ill try seeing how well it stores charge. Though I imagine it is a bit humid right now. This winter has been pathetic were I live lol.
     
  8. Mar 17, 2012 #7

    NascentOxygen

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    If you are using two flat sheets of foil and one of plastic film, then rolling into a tight cylinder will more than double the capacitance. You are squeezing the air out so reducing the spacing, and now making use of both sides of the metal foil.
     
  9. Mar 18, 2012 #8

    sophiecentaur

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    You really should be doing a rough calculation of capacity (same as for a parallel plate capacitor - just twice the value because both sides of the foil count). To find the thickness of the dielectric, measure the thickness of ten or more sheets and find the mean (better for accuracy). Don't forget to convert your dimensions to metres.


    There is no point in using high volts for this, initially. You can't hurt yourself if you only use an AA cell to charge it with for a start and you should still be able to record a constant voltage over time - dabbing the meter on at the end and watching the volts drop noticeably slower than without the C.
    The time constant (time to 1/e of starting value) will be RC so, if you manage to produce 0.1μF and the meter has a resistance of 10MΩ, the decay time would be more than a second. I should imagine your C will be quite a bit less than that, though (which is why you need to calculate what you expect) and the decay will be quite a bit faster. If you can set your multimeter to non-auto ranging, the display will be a lot less confusing.
    Good luck and use a hair dryer to keep the air nice and dry around your C whilst you are building it.
     
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