Field Electron Emission: How Much Charge Can You Remove From a Sphere?

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Smacal1072
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If you have a sphere of metal electrically isolated by a vacuum, and continuously add negative charge to it, the metal will eventually discharge due to field electron emission. This depends primarily on the work function.

Take the same situation as above, but continuously remove negative charge from the sphere. I know that the positive charge will accumulate on the outside of the metal sphere - how much negative charge can you remove before the skin of the sphere starts shedding?
 
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Probably a lot more, the positive charge are the ions of the metal atoms that are being held in the lattice. The electrons are the charges that are mobile and easily stripped off. To expell the ions you would probably have to compare the resulting ionic (Coulomb) forces with the Van der Waals/London forces that hold the metal lattice together.