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Year 11 High School Student, Electrostatics Help !Please!

  1. Jul 20, 2007 #1
    EDIT: I'm extremelly sorry, i just noticed the sticky above my post saying not to post homework qustions in the forum.

    Dear Physics Forums,

    I am a 16 year old high school student currently doing year 11 physics. To do this weekend we were givin an exercise sheet with various questions about electrostatics. Unfortunatly I have found this area of physics hard to comprehend and am unable properly understand my text books. I have had to continually ask the teacher for help in class but have not been given the explainations I need to help me understand. I'm afraid if i ask the teacher to clarify what he means or continually ask him for help i'll be seen as a bit of a dumbarse to everyone else. I know I shouldn't care about this but I would feel much more comfortable getting help from somewhere such as this where i'm more likely to be around more mature people and would no longer feel discouraged to seek help.

    I was wondering if anyone here woudl be able to help me find answers to the following questions... I dont want to be todl the answer outright, I want to make sure i know exacyly why that's the answer, else my end of semester exams are goin to be hell.

    I have 6 questions to do and thought it best to post a new question only after i can understand the previous questions answer.

    The first questions i'm farely comfident about but thought it best to ask just incase i've gone wrong.

    Question 1
    In each of the following cases indicate whether the object has a net negative charge, a net postive charge, or is uncharged. A plastic ruler when rubbed on your jeans becomes negatively charged.
    (a) What is the charge on the part of your jeans you rubbed?
    For this i wrote net postive charge, but then thought the jeans would likely be earthed so i wrote uncharged.
    (b) The charged ruler is brought close to two isolated conducting spheres (A and B) which are in contact. What is the charge on sphere A.
    In a picture provided the ruler is shown to be on the side of A opposite to where B is touching.
    I wrote the charge on A would be postive. Because electrons are replelled onto sphere B.
    (C) While the ruler is kep in the same position, B is removed. What is the charge on B?
    I wrote negative because of the excessive electrons that had been pushed onto the sphere.
    (D) If, instead, the ruler is removed, then sphres A and B are seperated. What is the charge on A?
    I wrote no charge. Because when the ruler is removed the elctrons redistirbute themselves out over the sphres again, meaning when the spheres are seperated they both are uncharged.

    Thankyou for your time.
     
    Last edited: Jul 20, 2007
  2. jcsd
  3. Jul 20, 2007 #2
    You appear right for all of them - but make sure you elaborate on your answer to part a, and argue to merry hell if it's marked wrong - because you're right, in a manner of speaking, your jeans would earth themselves.

    However, you're holding the ruler, are you not? And you're touching the same surface that your jeans earth themselves through. Why then, wouldn't the ruler earth as well? The answer is that it would, but it might take a while.

    All your other answers seem fine.
     
  4. Jul 20, 2007 #3
    thankyou very much for your quick reply Sojourner01. Wondering now tho weither inf act the ruler woudl be earthed. It doesn't actually specify ur holding the ruler. So i think it'd be best to assume that the ruler liek the spheres is unearthed.

    z-Component is what i done above ok, not breaking any rules i mean. The questions further along start t get me completely confused so in some cases i wont be able to show what work i've done sicne there isn't any, but any explainations I cna be given within the rules would be gladly appreciated.

    Question 2 asks me to match various circumstances with gold leaf electroscope diagrams. There are 7 diagrams, labelled A-G respectively. Each with +, - or ' ' to indicate charges.

    A - shows an electrosope with no charges
    B - " " with 2 + on the top plate and 2 + on the two leaves causing the to seperate.
    C - " " with 4 + on the top plate and 4 - on the leaves causing thme to sperate.
    D - " " with 4 - on the top plate and 4 + on the leaves causing thme to seperate.
    E - " " with 2 - on the top plate and 2 - on the two leaves, causing them to seperate.
    F - " " with 4 + on the top plate and no charges on the two leaves.
    G - " " with 4 - on the top plate and no charge on the two leaves.

    I wasn't sure weither the number of + or - mattered. I assuem so to indicate a stronger or weaker charge.

    (a) A positively charges glass rod is broght near the cap of the GLE.
    I picked D because electrons would be attracted to the plate of the GLE giving it a negative charge and leaving the leaves with a postive.
    (b) The GLE is placed inside a hollow metal sphere which carries a postive charge.
    I picked E, becuse i thought th electrons would be drawn to the outhermost parts of the GLE giving negative hcarges to the leaves and cap.
    (c) While a negatively charged strip of plastic is heald near the cap of the GLE, the cap is earthed.
    I picked C becuse the strip would repel electrons to the leaves giving thme a negative charge and leaving the cap in a positive charged state. Tho when is the cap is earthed tho i'd thought electrons woudl flow from earth to the cpa and cancel out the charge on the cap. So i'm thinking maybe A. but would the electrons pushed down to the leaves go to earth even when they're bein pushed away from the earthed part?
    (d) Following the procedure in (c), the earth lead is removed, and then the piece of plastic is removed.
    I was unable to answer this because i was not sure on (c).

    I hope this was easy enoguh to understand. I'm sorry i dont have a scanner to show u the diagrams of the GLE's properly, tho i did draw them as best i could http://img293.imageshack.us/img293/6887/untitledno4.jpg.

    Thank you
     
    Last edited: Jul 20, 2007
  5. Jul 20, 2007 #4
    Hi,

    I don't think you will be considered correct for writing the jeans is uncharged. Firstly, the question never mentions that the jeans is earthed (it is never mentioned that you are actually wearing the jeans at the time of the experiment). Secondly, and more importantly, earthing an insulator (as opposed to a conductor) does not make it neutral. Charge moves extremely slowly through dielectrics if it does move at all. Touching the jeans to an earthed conductor will make only the touching portion neutral, not the rest.

    You have mentioned that in general you are finding electrostatics hard to understand. I find this sad, as electrodynamics is among my favourite parts of physics. You may be a victim of a poor textbook. Try Physics by Resnick and Halliday. Not the new one co-written by Walker, that's bogus. The old edition (mine is 1967, considered a bible in India) is one of the best elementary expositions on electrodynamics. Get the second volume. You can also try Griffith's Introduction to Electrodynamics if you know your calculus and vectors very well.

    Molu
     
  6. Jul 20, 2007 #5
    very true:approve:
     
  7. Jul 20, 2007 #6
    Thankyou Loom91. I looked around two local libraries to see if they had to text book you mentioned but no luck. I think i'll be better off finding a site on the net to explain electrostatics to me. I agree that the more correct answer about the jeans question would in fact not be uncharged since liek u said it is not mentioned to be earthed and also I do recall the teacher now saying that only an earthed area of a non conducting material would becoem neutral when earthed.

    Can anyone tell me if my answers for question 2 are correct?
     
  8. Jul 21, 2007 #7

    andrevdh

    User Avatar
    Homework Helper

    Q1;
    (a) Both the ruler and the jeans are nonconductive materials. So they should not lose their charge obtained by the rubbing, even if you would to earth them since the charge would not flow along the material.

    I agree with your answers to b,c,and d.

    Q2:
    a) could also be G - the rod attracted all the excess charge to the top, causing the leaves to drop.
    b) No electric field exists inside a conductor - this means that the charge separation on the GLE cannot come from the sphere. C,D,F and G is therefore not possible.
    c) connecting it to earth - a very large reservior of charge - has the effect of removing all excess charge, except that induced by the strip.
    d) the induced charge remains - that how charge by induction is effected.
     
  9. Jul 22, 2007 #8
    Thankyou andrevd. Ok rethinking my asnwers for Q2. I forgot to put in the original question that the GLE is in an uncharge state to begin with.

    a) D
    b) A because all charge resides on the outside of the sphere (making a Faraday cage), therefore the GLE is unaffected.
    c) F, beacuse rather then the electrons being repelled down to the leaves they are repelled down to earth, and leave the cap in a postively charged state and the leaves uncharged.
    d) B, once the earth connection and strip are removed the electrons from the uncharged leaves flow up to try an neutralize the positve charge on the plate, leaving a net postive charge on the whole GLE.

    Are these now correct?
     
    Last edited: Jul 22, 2007
  10. Jul 22, 2007 #9

    andrevdh

    User Avatar
    Homework Helper

    Yes, you have got them all correct now.

    I think that it might be allowed (check with your teacher) to reason that positive charges flow and redistribute themselves in some cases.
     
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