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B When a charged object is brought close to an uncharged one (attraction)

  1. May 3, 2016 #1
    I have always been told that a charged object will attract an uncharged on when brought close (but do not touch) the uncharged object. However, wouldn't this depend on whether the uncharged object is an insulator or an conductor? This is my reasoning, but I am not sure if it is correct:

    When the uncharged object is brought close to an uncharged insulator, there will not be attraction because there are no free electrons in the insulator, and the charges on the insulator will hardly move. But in the case of uncharged conductor, when the charged object is brought close but do not touch the conductor, there will be a force as the opposite charge will be brought to the front while same charge will be pushed to the back.

    Am I correct? Or I am confused,,,.. Any clarification will be appreciated.. Thanks.
     
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  3. May 3, 2016 #2

    BvU

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    Many texts on electrostatics start with rubbing an insulator to charge it and attracting light objects (small pieces of paper) ...
     
  4. May 5, 2016 #3

    Merlin3189

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    An interesting question. I'm sorry you haven't had an answer from the physicists, but I'll have a shot as promised.
    You are quite correct about the mechanism of attraction for an uncharged conductor. This idea is also the basis of electrostatic induction and producing charge on a conductor.
    As BvU says, the common experiment is to attract small pieces of paper to a piece of charged plastic like a comb. So, if paper is an insulator, there is electrostatic attraction for uncharged insulators. I think we can say paper is an insulator, though with electrostatics one has to be careful - what is an insulator for low and extra low voltages (mains and common batteries) may not be able to insulate the high voltages of electrostatics.

    The explanation I was given for this (at school, and the question has not arisen since) is polarization of the molecules of the insulator. Although the electrons are bound to their molecules or atoms and are not free to move to the other side of the object, it may be possible for them to move within the molecule. Under the influence of an electric field, it may also be possible for the conformation of molecules to change - the molecule may slightly change shape - or the molecules may rotate slightly. Any of these will produce the same sort of effect as with a conductor - the like charges move further away and the unlike charges move slightly nearer.
    The clearest reference I've found supporting this is section 3 of a lab exercise.

    It seems to me that, because the charge separation in insulators is much smaller than in conductors, that the magnitude of the attraction should be much smaller, but I can't (so far) find any reference to that and I've not done the experiment yet. I envisage trying a charged balloon hanging on a thread between two large plates, one an insulator and one a conductor. The problem I'm having is what to use as an insulator. I started with a large sheet of polystyrene foam, but it always seems to be charged itself and I don't know how one can discharge an insulator! Anyhow, that is proceeding.

    The other scrap of knowledge (or belief?) that may bear on this, is capacitors. (And BTW the effect of dielectrics on capacitance is explained in Hyperphysics by polarisation of molecules, as described above.) Placing an insulator between the plates of a capacitor increases the capacitance. (As explained in Hyperphysics.) If a capacitor (with air dielectric) is charged with a fixed amount of charge, there will be a certain voltage between the plates and a certain amount of energy stored. Now if we introduce a piece of dielectric material (insulator to you and me) like paper or polythene between the plates, the capacitance increases. So the voltage decreases and the stored energy decreases. (I'l show the sums later if you want.) So work has been done by the electric field. Then when we want to withdraw the insulator, we must do work to pull it out and provide the energy to decrease the capacitance, increase the voltage and stored energy again. All this doing work means forces - the capacitor plates attract the dielectric, pulling it into the space between them and opposing us when we pull it out.

    In capacitor terms this is explained in terms of the permittivity of the insulator material and I make an analogy (physicists please close your eyes for a few moments!) with magnetic permeability. If a piece of iron with high permeability is placed in a magnetic field, it is attracted to where the field is strongest and you have to pull and do work to remove it.

    As I said, I had hoped that a real physicist would come to your aid. But since they haven't, that's my understanding of it.
     
  5. May 5, 2016 #4

    BvU

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    Try playing (move the balloon with your mouse) here. (the wall is non-conducting and polarizable)
    Your
    is quite correct, but if a lot of charges move only a little (which is what we call polarization - as merlin posted so clearly) you still get a force.
     
  6. May 9, 2016 #5
    Wow I didn't expect such a thorough explanation from you.. This explains every doubt I had about insulator and conductor. The reference you provided also clarified things up. You are very helpful, Merlin, my respect to you!
     
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