 Consider a large electrically charged sphere with a charge capital Q and a small sphere with a charge small q. The electric force between these two spheres, which are the distance r from each other, is given by Coulomb's law. Here, 1 over 4 pi epsilon 0 is a constant pre-factor with the electric field constant epsilon 0, which provides the right unit of force, the unit Newton. Now, what if you know the value of the big charge and want to know the value of the force that the big charge exerts on the small charge? However, you do not know the exact value of this little charge, or you intentionally leave that value open and only want to look at the electric force exerted by the big charge. So, you have to somehow eliminate the small charge Q from Coulomb's law. To do so, you simply divide Coulomb's law by Q on both sides. This way, on the right-hand side, the small charge drops out and instead lands on the left-hand side of the equation. The quotient on the left-hand side, f over Q, is defined as the electric field E of the source charge Q. By calling it source charge, we imply that the charge Q is the source of the electric field. The electric field E thus indicates the electric force, which would act on a small charge when placed at the distance r to the source charge.