First, let’s calculate the volume of a small cylinder of length 1mm and a radius of 0.5 mm:
Since there are 8.49 x 1019 copper atoms in a cubic millimeter, the total number of atoms in this cylinder is:
Assume now that every atom has one free electron that can be displaced. Given a charge of -1.6 x 10-19 C per electron, the total charge to be moved is:
Now, it takes 10 seconds for this 1 mm long cylinder full of free electrons to move 1 mm in one direction, so it corresponds to a current of:
Notice the sign of this current. Since the moving charges are negative (the electrons), the electric current is also negative.