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Understanding electometer

  1. Jun 4, 2007 #1

    I need help to understand electometer. I can not understand the reason of opened leaves which grounded.

    Suppose an electrometer charged negative then its metal knob connected with metal case with a conductor. Ok, now both leaves and metal body has the same potential. Now, lets disconnect metal case and metal know without loosing charges. They are already the same pottential. Thus, leaves closed, since pottential difference is zero.

    Now, lets ground the leaves by touching metal knob and then cut the ground connection. I did not observed this, but i suppose leaves must open. Because there is a pottential difference with metal body and leaves. I can approve this.

    But, if leaves open then must be charged.

    Is it? If so,
    Which charge?
    Whence come?

    Thank you
  2. jcsd
  3. Jun 4, 2007 #2


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    The leaves of an electrometer will part if they become charged. The electrometer does not measure the potential difference between the leaves and the case. The amount of separation of the leaves depends only on the static charge they receive.

    If you didn't observe this, how can you be sure it will happen ?
  4. Jun 5, 2007 #3
    Thank you Mentz114, but as i know electrometer measure the potential difference between metal case and leaves.

    Ok i didnt use this instrument but i read many articles which explains how it works. Electrometer has one connection point on case this ties a charged matter or ground. This charges the case and then we touch a charged matter to knob too.

    Most of the time the case of electrometer is grounded thus, leaves potential and zero(ground) potential difference is measure. But some cases the case ties another static charged matter in this case it measures the difference of two different potential.

    In this explains i assume Braun Electrometer type of electrometer. This one looking like a gold leaf electroscope.
  5. Jun 5, 2007 #4


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    Hi Volcano. We are talking about diferent instruments. The machine I know works like this - "When the electrode is charged by induction or by contact, the leaves acquire similar electric charges and repel each other due to the Coulomb force. Their separation is a direct indication of the net charge stored on them." ( from the Wiki article).

    I can see that if you have a conducting case the PD will affect the issue.

    Not necessarily. There are two ways the leaves can open,

    1. They are charged and repel each other.
    2. there is an electric field between the leaves and the case due to a potential difference, and the leaves are attracted to the case.
  6. Jun 7, 2007 #5
    ok. In my example, electrometer leaves grounded so it is uncharged. Than case charged negative. So, what is the charge of leaves and knob? Uncharged leaves may charge due to charged case? How? And did you try this experiment?
  7. Jun 7, 2007 #6


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    I think with the leaves grounded and a -ve voltage on the case, there will be an excess of +ve charge on the leaves and 2. above applies.
  8. Jun 8, 2007 #7
    Thank you. I suppose you mean leaves charged by inducting and knob is pozitive charge in this case.

    If this is the explanation, than i have one more question; i suppose there must not be an electric field inside case. How this occur?

    Thank you for all your helps
  9. Jun 8, 2007 #8


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    Yes, that's what I mean.

    If you connect the case to the leaves, there will be no potential difference and no field between leaves and case.

    I'm glad if I've helped, all the best.
  10. Jun 9, 2007 #9

    Thank you very much. With my best wishes.
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