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Homework Help: Electrostatics test on Tuesday - helpless

  1. Jul 15, 2007 #1
    Electrostatics test on Tuesday -- helpless

    Hi all,

    I have a bit of an awkward problem: it is the end of the year for the
    2-year "get your A-levels as an adult"-school I'm currently visiting,
    and the only test left is in Electrostatics. Unfortunately Physics is a
    major subject and thus you can't pass the year with mark 6 (marks
    range from 1 to 6 here, with 6 being the worst). Right now I'm a 5
    but in this test I could well go to 6.

    I've already been trying to learn yesterday and the day before (when
    I was told I would have to take the test on Tuesday) by looking up
    things on the internet, but that doesn't seem to be working :frown:

    So maybe somebody in here could help me get up to speed... it's
    only a 5 I need so I don't have to become an Electrostatics expert
    in two days time...

    1. The problem statement, all variables and given/known data

    How do Electrostatics work, from resistors to electroscopes to
    capacitors. What is current, voltage, charge, influence, electric work
    and electric power? What do the electrons do when all that stuff
    happens? What is there to know about streamlines of electrical fields?

    That should be about it.

    I will have to be able to explain one or two basic concepts and there's
    likely to be some questions on what happens when you do some stuff
    to an electroscope and why. The major part tho will probably involve
    doing calculations of one of these things:

    * Resistors in series or parallel
    * Capacitors in series or parallel
    * How far does something move how fast into what direction because
    of some electromechanical occurence

    2. Relevant equations

    Hopefully not too many :uhh: ... I will be allowed to use a formulary
    anyways, so I won't have to know too many formulas by heart.
    What I should add though is that I'm not very good in basic maths
    but will be allowed to using my bad-ass TI Voyage 200 calculator
    which I also know how to use pretty well (something at least :smile:)

    3. The attempt at a solution

    Like I wrote above, I tried to find the necessary information on the
    web which proved not too successful. On some websites I wasn't
    even sure if I could trust the explanations :grumpy:

    I'm willing to work for this tho, the remaining one day-and-a-half
    so I'll do all exercises and homework that I am given by you. I'd be
    grateful for any help whatsoever possible!

    Best regards from Germany,
  2. jcsd
  3. Jul 15, 2007 #2

    Chi Meson

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    Start here for tutorial information. Click the sticky link "Introductory Physics tutorials" for an index of basic on-line tutorials.
    https://www.physicsforums.com/forumdisplay.php?s=&daysprune=&f=160 [Broken]
    We generally don't give lectures, nor assign homework. If you have specific questions, or need a clarification, we'll be glad to help.
    Last edited by a moderator: May 3, 2017
  4. Jul 16, 2007 #3


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    Last edited by a moderator: Apr 22, 2017
  5. Jul 16, 2007 #4
    Hi! Thanks for the links ... I think I'm making good progress so far, I've read about Ohm's Law and parallel/serial circuits with resistors and how they behave in them and did some exercises about that. I also read about plate-type capacitors, how to calculate their electrical field and field force and about parallel/serial circuits with capacitors and how these behave in the various circuits and did exercises about that.

    What I still have the most problems with is the electroscope and I would like to ask a specific question about that: when you come close to a electroscope's top with a charged sphere for example, the needle deflects because of influence. But I couldn't understand any of the explanations from the links above WHY it does that (e.g., what happens _in_ the electroscope, with its electrons) and why it doesn't matter whether the sphere is charged positively and negatively?
  6. Jul 16, 2007 #5


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    The charged sphere will either attract or repel electrons in its vicinity (the conductive top of the electroscope). If the electroscope started out uncharged then this displacement of electrons relative to the top (either pushed down towards the needle or pulled up from it) will also cause a charge imbalance at the needle (it now has more or less electrons than it had before the sphere approached it). If you look carefully at the construction of the electroscope you will see that there also is a metallic plate next to one half of the needle (usually the top pivoting half). This plate is also connected to the top plate of the electroscope, so it will obtain the same charge imbalance as the needle. The (top half of the) needle is therefore deflected away from this plate since they have the same charge. So it does not matter what charge is on the sphere, the needle will always be pushed away from the plate next to it since they acquire the same charge due to proximity of the charged sphere.
    Last edited: Jul 16, 2007
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