This is probably a very stupid question but I couldn't find an answer on google.
Consider a neutral material - say, some block of metal. There is exactly as many electrons as protons and the electrons are bound to the nuclei with the electromagnetic force.
Now, add some electrons to it. It becomes negatively charged, until something with a lower charge contacts it.
The electromagnetic force makes negative charges away from other negative charges.
So why doesn't the object directly repel its overload of electrons? What prevents them to escape into the vacuum? Is there another interaction that I don't know which is occuring?