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Construction of capacitor

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  1. Jan 26, 2015 #1
    Hello,

    I have made a capacitor with two aluminium foils separated with paper dielectric with,

    dielectric constant (K) = 3.8 for paper material
    paper thickness, D = 0.1mm = 1e-4 m
    plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m
    air permitivity, Eo = 8.85e-12 F/m

    Calculated capacitance = (K Eo A)/d = 3.3 nano Farad

    When I measured the capacitance i got,

    Measured capacitance = 125 micro Farad

    Can any body explain why this huge error is existing? I have tried with different foil areas but could not find the reason of error.

    I m looking forward, Cheers.
     
  2. jcsd
  3. Jan 26, 2015 #2
    !
    It's not just huge error, It's a huge value of capacity for described capacitor. My guess is you made a serious error in measurement procedure, readings or misuse of the instruments .
     
  4. Jan 26, 2015 #3

    berkeman

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    Staff: Mentor

    For one thing, this is incorrect: "plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m" Convert the two lengths to meters before multiplying, and your final area units need to be m^2.

    Also, how did you measure the capacitance? What type of meter are you using? What do you measure for the resistance between the plates? How are you holding the plate stack together? Can you post a picture?
     
  5. Jan 26, 2015 #4

    Svein

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    Just by looking at your numbers, my feeling (based on 50 years of electronics experience) is that your calculated value is in the correct range. There is absolutely no chance at all that you can create a capacitor yourself with a value of 125μF.
     
  6. Jan 26, 2015 #5
    I'm actually not exactly sure how devices measure capacitance (RC circuit? Or LC?), but I wouldn't be surprised if they don't show the same accuracy across all ranges (kinda how you have to dial in the right resistance range to get an accurate reading of a resistor).
     
  7. Jan 26, 2015 #6

    berkeman

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    Staff: Mentor

    My personal inexpensive B&K LCR meter definitely needs to be put on the capacitance range that is closest to (but not less than) the capacitor to be measured. It also has a D (= 1/Q} measurement setting to let you know if the capacitor you are measuring is very lossy. If it is too lossy, that messes up the capacitance measurement. That's why I asked the resistance question in my earlier post... :-)
     
  8. Jan 27, 2015 #7

    Svein

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    I once built an LCR meter based on a Wien bridge. If anybody is interested, I could try to dig up the schematics.
     
  9. Feb 9, 2015 #8
    hi,

    0.010043 is in meter-sq not in meters. the unit was wrong sorry for that. I measuring capacitance with LCR meter.
     

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