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I know that when there are excess positive charges in a conductor, for example, a metal sphere, the positive charges will spread out over its surface. However, I am confused about how this excess charge spreads out over the surface, if protons cannot move and only electrons can move.
Can someone please inform me on how the excess positive charge spreads out over the surfaces of conductors?

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up vote 7 down vote accepted

Physically what is happening is this:

  • When you touch the positively charged source to the conductor (the metal sphere), electrons leave the conductor through the point of contact.
  • This leaves the point of contact on the conductor with a large deficit of electrons, and thus the point has a positive charge density.
  • The positive charge density produces an electric field in the conductor, which immediately pulls on remaining electrons in the conductor.
  • The electrons remaining spread out until they have eliminated all of the electric fields in the conductor (if there were remaining fields, the electrons would continue to rearrange).
  • The electrons will now be 'more spread out' than the protons; the difference between the new electron surface density and the original tells you the distribution of 'excess positive charge' on the surface.

I hope this helps, let me know if you have an application in mind for this; I often times find it helpful in thinking about problems to temporarily ignore the fact that in practice there is only one charge carrier (the electron) and just think about excess positive charge as positively charged particles spreading out.

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Thank you so much for this insightful response! It really helped a lot. – Jake Perentosh Feb 26 '13 at 4:39

For a conductor to display positive charge it means that some of the electrons have been removed from it. The positive charge displayed homogeneously on the conductor's surface is the result of charge balance, between the positive stationary charges of the surface atoms and the negative charge of the electrons.

Because it is a conductor,the electrons are freely moving in a conduction band. and the charge difference will be uniformly distributed .

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This question seems to be from chapter 15 of the third edition of Matter & Interactions by Chabay and Sherwood (Wiley, 2012). You're correct that protons don't move. The whole story is told by how mobile electrons distribute themselves. If there are fewer electrons than protons, the conductor has a net positive charge, but there are still mobile electrons, and they still redistribute themselves.

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The electrons are redistributed over the surface, and this leads to redistribution of positive charge (for example, electrons move to the places where initially the lack of electrons was the greatest).

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protected by Qmechanic Dec 7 '15 at 19:00

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