In order to gain an understanding how moving charges relate to electric current, consider the following case. A copper wire has a cross section of 1 mm. During a time interval of 10 seconds, all electrons move 1 mm towards one end of the wire, resulting in a speed of 0.1 mm/s. What is the value of the electric current?

First, let’s calculate the volume of a small cylinder of length 1mm and a radius of 0.5 mm:

^{2}.(1 mm) = 0.785 mm

^{3}

Since there are 8.49 x 10^{19} copper atoms in a cubic millimeter, the total number of atoms in this cylinder is:

^{3}).(8.49 x 10

^{19}atoms/mm

^{3}) = 6.67 x 10

^{19}atoms

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:

^{19}).(-1.6 x 10

^{-19}C) = -10.67 C

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.