The current, I, is a coarse, average quantity that tells what is happening in an entire wire. If we want to describe in more detail how the charges move through a conductor, we use the concept of the current density. Since it is a vector quantity, it can indicate the direction of the current flow. This formula says that if free electrons are drifting to the right, then the resulting current flows to the left because the electrons are negative. It is difficult to keep this straight, so when we work with circuits, we usually just think of current as moving positive charges, even though we know that the electrons carry the current. The current density is an area density, I.e., is the current per unit area.