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Direction positive and negative charges will movebetween electrodes?

  1. Feb 10, 2014 #1
    1. The problem statement, all variables and given/known data

    If a small positive and negative charge are created at rest at a point between electrodes as shown in the figure. In which direction will they move if one would neglect gravity?

    Ignore those weird arrows, those are for later questions.

    jenNUByl.jpg

    2. Relevant equations



    3. The attempt at a solution

    I'm not sure what Vb means. A different question indicates that it is the "biased electrode." Does this mean it is the charged electrode? If so, I think that the positive charge would move towards the negative plate and the negative charge would move toward the grounded plate.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 10, 2014 #2

    Simon Bridge

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    Va is the voltage on plate "a" and Vb is the voltage on plate "b".

    It may do - but it does not have to. You'll notice that it says Va=0 ... this means that all voltages are measured with the negative terminal of the voltmeter on plate "a".

    Are you told that pate "a" is grounded?

    To determine the charge on plate "b" you need to know if Vb is greater than or less than zero don't you?

    But it may be that both plates are positively charged - in which case Vb > Va if plate b has more positive charges than plate a. Or maybe they are both negatively charged - or are oppositely charged.

    (Note: that b is "biased" wrt a suggests Vb > Va ... if Vb < Va then we'd say that b is "negatively biased" wrt a.)

    Do you know how the electric field relates to the voltage?
    Do you know how the force on a charge relates to the electric field?
     
    Last edited: Feb 10, 2014
  4. Feb 10, 2014 #3
    Yes, it says that plate "a" is grounded.

    It does not say whether or not Vb is greater than or less than zero, only that these charges are between electrodes as seen in the figure.


    E = V/d and F=kQ1Q2/R^2

    I'm thinking that the electric field can overcome the forces that the point charges exert on one another and the positive charge would move to Va and the negative charge would move towards Vb.
     
  5. Feb 10, 2014 #4

    Simon Bridge

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    These don't really answer the questions I put though do they?

    The first is my fault - I did not specify the voltage that I meant.
    So: Can you rewrite E=V/d in terms of Va and Vb?

    But the electric field is a vector. This is not a vector equation.

    Can you sketch the electric field lines on your diagram? What would they look like? (Describe them.)
    How do the electric field lines relate to the motion of the charges?

    The second equation does not answer the question at all ... I asked about the force in relation to a charge q and an electric field. So the equation you need has an "F", a "q", and an "E" in it. Your's has only one of these.

    ... that is fair, otherwise the question makes no sense ;)

    You will have to decide, from the information you are given, which is more reasonable: Vb > Va or Vb < Va or Vb = Va.

    Note: there are only two possibilities - if the first was wrong then the other must be correct ... but was the first one wrong?
    ... it is your reasoning that counts here not the final answer.
     
  6. Feb 10, 2014 #5
    In terms of Va and Vb, Vb-Va = 0, assuming that Vb > Va.

    The electric field lines would go from positive to negative, so assuming Vb is positively charged, they would flow from Vb to Va. The positive charge would follow these field lines and go away from the positive plate and to the negative plate. The negative charge would do the opposite and go towards the positive plate.

    The equation you must be talking about is F=qE
    Force = charge * electric field
    This says a positive particle will be accelerated in the direction of an electric field while a negative charge will be accelerated in the opposite direction of an electric field.

    It seems as though the correct answer is that the positive charge will move towards Va and the negative charge will move towards Vb.
     
  7. Feb 10, 2014 #6

    Simon Bridge

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    Surely Vb-Va=0 only when Vb=Va? But you said that Vb > Va ?!

    Well done - you have answered your question.

    Not only have you figured out the answer, but you also eliminated your need to have someone tell you it's correct :) well done.
     
  8. Feb 10, 2014 #7
    Thanks for the help!
     
  9. Feb 10, 2014 #8

    Simon Bridge

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    No worries. It's all your work - I just asked annoying questions ;)
     
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