A remark in my textbook goes as follows: "If an excess charge is placed on an isolated conductor, that amount of charge will move entirely to the surface of the conductor. None of the excess charge will be found within the body of the conductor."
The author explains this with the help of the above picture, which is a lump of copper hanging from an insulating thread. He says that the electrical field in the Gaussian surface must be zero, but he does not really explain it well.
Can someone explain why this must necessarily be so?