Electrons flow through a wire from the source to the receiver, but they cannot be retained in the wire once the source and receiver are disconnected. In copper, there are approximately 1.38*10^22 conduction electrons per gram, regardless of external conditions. Unlike water in a hose, which flows rapidly when a tap is opened, electricity behaves differently; the wire is already filled with electrons. When voltage is applied, electrons begin to move, but their actual speed is quite slow, typically only a few centimeters per second. The effectiveness of electricity lies in the vast number of electrons rather than their speed.