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Homework Help: Uncharged object between two charged plates

  1. Apr 29, 2016 #1
    1. The problem statement, all variables and given/known data
    Imagine if you have two charged plates which have the same magnitude but opposite in charges.
    If you place an uncharged object near of the positive plate what will happen?

    2. Relevant equations
    I don't think it depends on equations

    3. The attempt at a solution
    Okay, so If you rub a balloon with your hair, it will gain a negative charge. If you place that balloon really close to really small pieces of paper, It will attract them because the balloon will repel the electrons away from it and the protons don't move so the object will be polarized ( I am pretty sure that I cant say that this effect split the electrons and protons into two different sides equally , can I?)
    So the negative charge will attract the protons more than it will attract the electrons because the protons are much closer to it than the electrons. So that will result in a net force to the balloon.

    Now back to the question
    If I place the uncharged object near the positive plate, the opposite of the above will happen. However, The negative charged plate will attract the protons and the positive plate will repel the protons in the same direction
    and the same thing will happen to the electrons but opposite.

    The answer in my book was that the uncharged object wont move. But what I think if we assume that it wont move we are just saying that the (distance) between the repelled electrons and attracted protons is just zero which proves that the first idea is wrong.

    So, What is really happening here?

    Thanks in advance.
    Last edited: Apr 29, 2016
  2. jcsd
  3. Apr 29, 2016 #2


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    There's a difference between the field from a charged sphere and the field between parallel plates. As you say, the electrons in the object will be attracted towards the positive plate, but what does that mean regarding the interaction with the negative plate?
  4. Apr 29, 2016 #3
    Sorry, I didn't get your question. Could you explain what you mean?

    The positive plate will attract electrons and the negative plate will repel electrons, But also I have to take into account that the positive plate will repel protons and the negative plate will also attract protons. If that is what you mean

    The force of the negative plate will be weaker (because it increases the distance) than the force of the positive plate but they are in the same direction
    Last edited: Apr 29, 2016
  5. Apr 29, 2016 #4


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    If the plates are parallel and close to each other, the electric field is homogeneous between them.
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