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So let's say we have a sheet of white paper on the desk. We then put some confetti on top of the white paper. We cause a white plate to be positively charged by friction (rubbing on the desk) and place that plate above the confetti.

The confetti was being attracted to the plate, which makes sense.

If we change that white paper with aluminum foil, things change a bit. The confetti is not being attracted to the plate anymore! In order to make this possible, it makes sense to assume that less electrons were present in the confetti because electrons were transferred from the confetti to the aluminum foil (conduction), which in effect prevented the attraction between the positively charged plate and the confetti.

But what I don't understand is that, before the plate was placed above the confetti, the confetti was neutral as well as the aluminum foil! So there can't be any transfer of electrons between neutral objects! So.. if there is no transfer of e's, then why is it that the confetti is not being attracted to the plate when using a sheet of aluminum foil beneath them?

Please explain at a high-school level! I might not understand some deep stuff

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  • $\begingroup$ Have you seen $for\ yourself$ the effect of replacing the paper with aluminium foil? $\endgroup$ – Philip Wood Sep 12 '18 at 12:09
  • $\begingroup$ @Philip Wood Yes $\endgroup$ – Ke Ke Sep 12 '18 at 13:25
  • $\begingroup$ Well, I've just tried the experiment and found no difference between the way in which my rubbed plastic comb picked up fragments of paper from a paper surface and from a sheet of foil! This is as I would have expected. May I suggest that you try the experiment again? $\endgroup$ – Philip Wood Sep 12 '18 at 14:08
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As I said in my comment, I've just tried the experiment and found no difference between the way in which my rubbed plastic comb picked up fragments of paper from a paper surface and from a sheet of foil! This is as I would have expected. Here is the basic mechanism for the attraction of neutral objects…

Let's assume that the attracting object, a plastic rod (say) is positively charged. If the confetti is assumed to be conducting because it contains free electrons (perhaps not very likely) the electrons will be attracted towards the 'end' of each piece of confetti nearest the rod, leaving the ends furthest away depleted of electrons and positively charged. The negative ends will be attracted to the rod and the positive ends repelled. But the rod's electric field gets weaker as you go further from the rod. Therefore the repulsive force on the far ends of the confetti is weaker than the attractive force on the near ends. So the resultant force on the confetti is towards the rod.

Now suppose instead that the confetti is made of an insulating material. Free electrons can't move through the material. But within each molecule electrons will be displaced slightly towards the rod, and nuclei slightly away from the rod. So the molecule becomes a 'dipole' with its negative end facing towards the rod. [Some sorts of molecules are dipoles in the first place, in which case the rod will tend to orientate the molecules so that their negative ends are closest to the rod.] Inside the piece of confetti the negative ends of dipoles will be next to the positive ends of neighbouring dipoles, so it will be as if there is neutrality. But at the end of the piece of confetti nearest the rod, there will be uncancelled negative dipole ends, and at the furthest end, unconcealed positive dipole ends. So we have the same sort of charge distribution as if the confetti were conducting, and it will feel a net force from the rod.

You'll notice that none of this involves transfer of charge between the confetti and any other object. If the confetti is resting on aluminium foil, I'd think that negative charge would flow from foil to confetti, and that the rod would continue to attract the confetti, which now has a net negative charge.

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  • $\begingroup$ Hi, I asked my teacher. But she said it's supposed to be weaker attraction between the confetti and the plate because aluminium foil is a conductor. So when the plate comes near and the flow of the charge in the confetti starts to change, the aluminum foil will try to neutralize that, which in effect prevents the attraction with the plate. $\endgroup$ – Ke Ke Sep 14 '18 at 11:48
  • $\begingroup$ I couldn't understand what she meant at first, but now I think about it, it makes sense. Since aluminium foil (= a.f.) is a conductor, where e's can freely move around from atom to atom, when the positively charged plate is brought near the neutral confetti, e's in the confetti will be attracted to the plate, leaving the surface facing the a.f. relatively more positive. $\endgroup$ – Ke Ke Sep 14 '18 at 12:14
  • $\begingroup$ As soon as this happens, e's in the a.f. will transfer to the confetti, making it net negative and the a.f. net positive charge. Since the a.f. and the confetti are opposite charges, the attraction between them will prevent the attraction between the confetti and the plate. $\endgroup$ – Ke Ke Sep 14 '18 at 12:14
  • $\begingroup$ Thank you for this explanation. I'm sorry that I haven't been able to reply earlier. I understand the reasoning, but a counter-argument is that the confetti, now with a net negative charge, will be attracted all the more strongly to the rod/plate (as well as being attracted to the foil). I have a suspicion that your teacher is right, and that the resultant pull on the confetti, though still towards the rod/plate, will be weaker when it's resting on the foil, but I don't think that the case is yet proved! Well done for your persistence. $\endgroup$ – Philip Wood Sep 23 '18 at 16:57
  • $\begingroup$ I've just realised the likely significance of your teacher using a charged plate, rather than a charged rod, to attract the confetti. Whereas dipoles (equal and opposite charges) would be attracted to the rod as explained in my answer, they should not be attracted to the plate if it's uniformly charged and the confetti is close to it, because in that case the electric field due to the plate is $uniform$. However, with the foil in place, the confetti will have a net negative charge (as you said) and will then experience a force even in a uniform field! $\endgroup$ – Philip Wood Sep 25 '18 at 12:40

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