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Homework Help: Gold leaf electroscope problem

  1. Aug 27, 2015 #1
    1. The problem statement, all variables and given/known data
    http://www.physicstutorials.org/home/electrostatics/electroscope
    Negatively charged electroscope and X-Y plates are placed like in the figure given below. If the distances between electroscope and conductor plates are equals; find the types of charges of plates for given situations.
    1)If the leaves are closed a little bit
    2) If the leaves rise a little bit

    2. Relevant equations
    ___________

    3. The attempt at a solution
    Answers of 1) is
    1. Both of the plates can be positively charged and have different amounts of charges. They both attract electrons of the leaves.
    2. They can have different charges; however, amount of positive charge must be larger than the negative charge. Since, there must be net positive charge to attract electrons from the leaves.

    Should not there be third possibility i.e
    3)Both of the plates can be positively charged and have same amount of charge

    similarly Answers of 2) is
    1.Both of the plates can be negatively charged and have different amounts of charges. They repel electrons from top of the electroscope to the leaves.

    2.They can have different types of charges, but amount of negative charge must be larger than the amount of positive charge to repel electrons from top of the electroscope to the leaves.


    Should not there be third possibility i.e

    3)Both of the plates can be negatively charged charged and have same amount of charge
     
    Last edited: Aug 27, 2015
  2. jcsd
  3. Aug 27, 2015 #2
  4. Aug 27, 2015 #3

    BvU

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    Hello, good you rang, because I indeed missed this thread.

    I have little to add, but I am quite sure you have a question. What is the question ?
     
  5. Aug 27, 2015 #4
    Hi !
     
  6. Aug 27, 2015 #5
     
  7. Aug 27, 2015 #6

    BvU

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    Sorry for overlooking that ! Let me check.
     
  8. Aug 27, 2015 #7

    BvU

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    I fully agree with you.
     
  9. Aug 27, 2015 #8
    And what about
    quite obvious ?
     
  10. Aug 27, 2015 #9
    upload_2015-8-27_19-13-9.png

    In the third point,it is written that "During this process, leaves of electroscope are closed and after completion of charge sharing they rise again".
    Why the leaves of electroscope were closed earlier?
     
  11. Aug 27, 2015 #10

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    Same thing. They reason correctly that some electrons move towards the leaves. The only thing that is needed for that is some negative charge near the tip sphere of the electroscope. One negative, two negative (equal or not equal), one negative and one lesser charged positive, it doesn't matter.

    It looks to me as if they wanted to ask and explain a little too much for this example.

    Can't find an e-mail address for this club, otherwise you could point this out to them !
     
  12. Aug 27, 2015 #11
    Similarly i have found another mistake on this particular page
     
  13. Aug 27, 2015 #12
     

    Attached Files:

  14. Aug 27, 2015 #13
    upload_2015-8-27_19-18-36.png
    Opposite charges repel?opposite !
     
  15. Aug 27, 2015 #14
    it's also correct,right?
     
  16. Aug 27, 2015 #15

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    Can't keep up with you! posts cross: e.g. my #10 was in reply to your #8. and I missed your #9 (& 11, 12, 13, 14 !) For #14: yes. I thought we've had that settled already before.
    For the others, give me time to read the stufff ....o0)
     
  17. Aug 27, 2015 #16
    Don't take tension .#(11,12 13) are same
     
  18. Aug 27, 2015 #17

    BvU

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    For me (no expert in didactics, just a student long time ago) this is again not a very helpful exercise. But what it says is correct.
    One can argue about details (formally Y is + and Z is - goes too far. All that can be concluded is that Y is more + than Z).

    Opposite charges repel. That's correct. But the sentence
    is gibberish.

    My account:
    If the leaves spread, positive charge must be pushed down the electroscope. So Y has become more postitive than before. YZ is on an insulator, so the total charge on YZ is constant. THerefore Z must have become less positive (= more negative) than before. That means T has pushed away positive charge from Z. Since pushing away is repelling that means T is positively charged.
     
  19. Aug 27, 2015 #18
    I thought Opposite charges attract.
     
  20. Aug 27, 2015 #19

    BvU

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    See ! You did manage to confuse me. Or rather, the PhysTut site managed that.

    Of course you are right!

    What happens is "YZ is on an insulator, so the total charge on YZ is constant. Therefore Z must have become less positive if Y has become more positive."

    You're better than I in this, so keep up the good work !
     
  21. Aug 27, 2015 #20
    Not possible.Teachers always lead irrespective of the context.
     
  22. Aug 27, 2015 #21
    PhysTut site is the culprit.Don't blame me !:smile:
     
  23. Aug 27, 2015 #22

    BvU

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    Don't blame you. And you must be a natural (teacher, that is) :smile:
     
  24. Aug 27, 2015 #23
    Sir,please answer my post #9.
     
  25. Aug 27, 2015 #24

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    No sirring in PF (one of the nice features of such a forum) ! Will look. After all I admitted I missed it.

    - behavior of leaves when we touch charged object A to the charged electroscope
    - A has +4q charge and electroscope has -2q
    - capacities of a and scope are same

    A far away: leaves apart

    A closer: A is pushing + charge down the electroscope (equal charges repel, remember :wink:), so leaves come closer

    A touching: -2q from electroscope and +2q from A get married and happily live ever after. Whole lot now has a total charge of +2q, nicely divided over a and the scope, probably the +1q of the scope sits aas far away as it can, in the leaves.
    The +1q makes the leaves of the electroscope spread out again. But -if I'm right- not as much as the -2q at the very start.

    A no longer touching: A is still pushing away the + charge from the top sphere on the scope

    A far away again: scope evenly distributes +1q and spread reduces

    ---

    I've been thinking this up from what I know. My last experience with these things was when my teacher demonstrated, a very, very long time ago. So I hope you or other bright readers correct me if I went wrong at some point !

    :nb)
     
  26. Aug 27, 2015 #25

    andrevdh

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    There is a flow of electrons from the electroscope to the rod up to the point where
    the rod is charged +2q. At this stage the electroscope has lost all of its charge
    and the leaves are closed all the way down. Now the rod draws more electrons
    off the electroscope and the leaves rise again until both are charged +q.

    Note this can only happen if the rod is made of an conductive material.
     
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