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Question about capacitor

  1. Jun 24, 2015 #1
    I just started learning about capacitors in Electrical and Electronics Principles and I thought of a situation which I couldn't get a satisfactory answer/explanation from my lecturer.

    What happens if after charging a capacitor to 99%, I reverse the polarity of my power supply (or flip the capacitor)?
    Also, is it dangerous to do this and why?

    I'm assuming the surplus electrons on the side of the capacitor will flow to the other side of the capacitor, thus changing the polarity of the charged capacitor, i.e. discharge then charge again.
     
  2. jcsd
  3. Jun 24, 2015 #2

    anorlunda

    Staff: Mentor

    You have it right. The polarity of the charge will flip after a surge in the current.

    Dangerous depends on how big the voltages and currents are. High voltages and large currents should always be considered dangerous.

    If you are playing with a 1.5 v charge on a 1 μf capacitor, there is no danger to humans.
     
  4. Jun 24, 2015 #3

    jim hardy

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    2016 Award

    An experiment for you.
    Every new student should try this.

    Try it with an ohm meter and capacitor.

    While you're at it
    take two multimeters . Label one "Measuror" and the other "Measurree".

    Connect their red leads together.
    Connect their black leads together.

    Set "Measuror" to DC volts, a high scale.
    Set "Measuree" to Ohms, highest scale.
    In following steps, write down meter readings.

    Measuror should report a volt or two.
    Measuree should report a few megohms.

    Switch Measuror to DC milliamps.
    It should report a fraction of a microamp.

    Did you notice we have just measured the Thevenin circuit of Measuree set for ohms, open(well ,almost open) circuit voltage and short circuit current??



    DMM's work by applying a controlled current and measuring the voltage that current produces across the unknown. They are limited to a volt or two open circuit so as to not hurt anything.

    Now set Measuree to progressively lower ohm scales and note what current it applies to Measuror . You'll have to set Measuror for progressively higher current scales.

    So
    now when you connect the DMM set for ohms to a capacitor
    you'll actually see that capacitor being charged by current from the meter
    and you'll see Mr Capacitor hold a charge while you swap the leads
    and you'll see how much more slowly it charges with small current from higher ohm ranges.
    And you'll see polarity reverse when you swap the leads - meter reports voltage stored in the capacitor.

    DMM's were rare and exotic idea in the 1950's.
    We used analog meters with a needle and they did not have a capacitance scale, just DC ohms.
    We became skilled at estimating the value of a capacitor by using the meter set for ohms.and watching how far the needle deflected when first touching it to the capacitor.


    Reason i recommend this is -

    1. One should know what is inside his test equipment. That way it's less apt to fool him.
    2. For me, the visceral experience of doing something plants the concept 100X stronger than just reading about it.
    You don't learn to walk from a book...

    old jim
     
    Last edited: Jun 24, 2015
  5. Jun 26, 2015 #4

    meBigGuy

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    of course you don't want to do that with a polarized capacitor. That's easy for some to forget.

    You are correct that it will discharge the capacitor and charge it to the new polarity.

    You can use standard formulas to compute the surge currents and rate of voltage change given your estimates for the power supply impedance and ESR of the capacitor.
     
  6. Jun 26, 2015 #5
    Welcome DMS!

    A capacitor can be fundamentally thought of as energy storage - you have charged it, and now reverse the polarity of the source - think of the condition JUST before you make the last connection... what is the V across the open circuit point? What does make caps dangerous - is they do not have the controls or limits of a power supply - while not truly unlimited - a charged capacitor is pretty much the best source for high current..

    Oh -- an do Jim's experiment.
     
  7. Jun 26, 2015 #6

    Averagesupernova

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    One of the first things we did in school was build our own test equipment so we would actually understand what is inside. We actually built a very small CRT scope. I thought it really gave a better understanding of electronics in general.
     
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