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Physics
Classical Physics
Electromagnetism
Why can't excess charges leave the surface of a conductor?
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[QUOTE="jaumzaum, post: 5931905, member: 339620"] I think what you are having some trouble to understand is that a charged metal can still be in equilibrium. Think about a metal bar. If one supposedly charge it with some negative charge, those new electrons (that will swap over time with the internal electrons) will be orbiting mainly the external (surface) of the bar. This is because this state of equilibrium has minimum energy. Each atom has an electron cloud that shapes a probability density function of the electron be in that point at that time. When all the atoms join to become the bar, all of these clouds interact with one another and merge, and for the case of metals the energy that is necessary for an electron to pass from one metal atom to another is surpassed by this new rearrangement of clouds, making the electrons of the valence shell “free to move between the atoms” (of course there are some places with higher probability density as usual). When you charge a metal, the new electrons change mainly the external clouds. This results in an EQUILIBRIUM with higher energy, but it’s still an equilibrium. It’s easier to take this electrons out than it was to take the electrons out of the neutral bar. But the energy of the arrangement is still negative, that means the electrons prefer to stay at the bar than to stay flying to the infinity. Now you can understand the work function. For the electrons to escape the bar/ be absorbed by another molecule out of the bar (e.g. air molecules) you have some things to consider: Is the new equilibrium more energily viable than the first? If so all of the electrons tend to go to the new equilibrium if given some incentive (an energy to overcome the still negative energy of the charged bar). But in almost all cases the bar will not be electronically isolated so that a fotoelectric experiment will not be consider as an equilibrium. This way if one gives enough energy for the surface electrons to pass that negative energy of the bar, they will leave and never go back. That energy is the work function, it has to overcome the external electrical clouds. [/QUOTE]
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Electromagnetism
Why can't excess charges leave the surface of a conductor?
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