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Why needs plate capacitor high voltage

  1. Apr 9, 2009 #1
    Hi, I tried to charche a plate capacitor with some 20 Volts but when disconnecting the cables from DC supply the voltage goes down to zero. Why is this? Cables too long? Didnt I wait enough or does a plate cap need high voltage (5000 Volts) and why does it? I am a theoretical physicist and have no idea about such stuff. I also fear somewhat using high voltage over non protected plates (if I touch the plates, I'm dead right?) so I would be very glad if it was possible to charge it with low voltage.
    Last edited: Apr 9, 2009
  2. jcsd
  3. Apr 9, 2009 #2
    You should not be working with high voltage electricity alone.
  4. Apr 9, 2009 #3
    I agree with Bob S. If you "have no idea about such stuff", you should not be experimenting with high voltage. I'm curious as to what you mean by exposed plates. Normal capacitors (variable capacitors excluded) do not have exposed plates. In my experience the term "plate capacitor*" refers to a high voltage capacitor normally used in the power supplies of vacuum tube transmitters and amplifiers. There could be several reasons why you are not seeing the charge. 1) the capacitor is defective 2) you're using a voltmeter with a high ohms per volt rating - this causes the capacitor to discharge through the meter before you have time to read it 3) the capacitance value is too low 4) the capacitor is electrolytic and is bleeding off through the dielectric. 5) any combination of 2,3, and 4.

    *Edit: I had that term confused with "plate transformer" which of course is not a capacitor. However, capacitors of this high voltage are commonly used in vacuum tube transmitters and amplifiers.

    Are you talking about a parallel plate capacitor (with exposed plates) of the type that might be used for demonstration purposes? Please be more specific.
    Last edited: Apr 10, 2009
  5. Apr 10, 2009 #4
    Yes its a parallel plate capacitor. The plates are squares of about 20cm. I would like to show how a charched metalic sphere is attracted to the charged plates. This rises another question: how to charge the sphere? By touching shortly one plate? The former teacher said me that he connected the power supply without resistance between them. The problem is how to discharge it again. Is this possible with the power supply by turning the voltage button slowly to zero?
  6. Apr 10, 2009 #5

    Vanadium 50

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    Step 1 - make sure your life insurance is paid up.

    Step 2 - make sure your will is up to date.

    You don't understand electricity - even the most basic things like Q = CV. While there's nothing wrong with that, and lots of people fall into that category, people in that category should not be handling high voltage. If you keep this up, it is very likely that you will die.
  7. Apr 10, 2009 #6
    You are joking, of course I understand the formulas, even this one I=Io Exp(-t/RC), this is the current that will go through a body of resistance R if one touches a plate. Not to recommend! I just need help about experimental stuff that theorie alone cannot learn me, like handling devices. Maybe you can tell me if a voltmeter can usually be adjusted to not discharge quickly the plates.
    Last edited: Apr 10, 2009
  8. Apr 10, 2009 #7
    Since you are using metal plates it seems like your capacitance(C=epsilon*A/d)was small.
    A good thing to use for your sphere would be a table tennis ball painted with aquadag or any other conducting paint.Because of your lack of practical experience you are not advised to use standard E.H.T. supplies but a small demonsration Van de Graff should do the job you want.
  9. Apr 10, 2009 #8
    Thanks for the clarification. I have not worked with this type of equipment so I cannot advise you on it's use. However, I can tell you the reason you cannot measure the 20 volts on your capacitor. C is very small. So the very small amount of energy stored there will dissipate quickly through the voltmeter.
    Sorry, I do not know of any voltmeter sensitive enough to do this. A storage oscilloscope maybe. It would have to be quick enough to capture the voltage before it discharged through the instrument.
  10. Apr 10, 2009 #9
    All research laboratories in the United States have a Two Man Rule when working around exposed electrical terminals that present s dangerous shock hazard. The rule even applies to personnel who have had very extensive electrical training, both academic and professional employment. Do not work around exposed electricity alone, unless there is absolutely no personnel hazard. I have seen two cases where people were knocked unconcious by electricity. It is not a pretty scene.
  11. Apr 10, 2009 #10
    An electrostatic voltmeter may work for you. Try google "electrostatic voltmeter" and "peak hold". But you may still have to charge the capacitor with high voltage and have some distance between the plates.
    Last edited: Apr 10, 2009
  12. Apr 10, 2009 #11
    Thanks for your replies. I am the only physics teacher at the school so nobody can help me. All I want to do is to take this parallel plate capacitor, distancing the plates from each other at about 20cm (8 inches). I place a little conducting hollow sphere as suggested by TurtleMeister in the middle of the plates pending at a isolating thread. Then I connect it to the 5000Volt supply without resistor, the buttons are on zero voltage. I put power on, turn the voltage button slowly on about 200Volts. I take a long isolating stick and touch the sphere with one of the plates such that it will be charged positively or negatively, I guess it doesnt matter. Then I continue to turn the voltage button at a safe distance from the plates. The sphere will then hopefully tend to one of the plates (of course it must not touch it). Thats all! To discharge all, I turn the button slowly on zero. To discharge the sphere, I touch it with one of the plates using the stick. The system should then be free of charge. To be sure, I may short-circuit the plates with the connection cables before touching them. Do you think that this can work or is it still too risky? I guess there is no E-field that comes back into the plates after a while or anything else.
  13. Apr 10, 2009 #12

    Vanadium 50

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    Here you are trying to charge a capacitor with substantially less voltage, and questioning why it works less well.

    From the above question, it sounds like you don't. You can surely see how one can draw this conclusion.

    As many people have said, this is a dangerous thing you are doing, and even experts are not allowed to work alone doing this. I wouldn't touch this without someone who could show me (not tell me) how to handle it safely.
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