Consider a test charge in the vicinity of many source charges: q-i, where i ranges from 1 to N. The principle of superposition of Coulomb forces implies that the force on Q is the vector sum of the forces due to each q. This expression can be rearranged to express the force as Q times a vector quantity, called the electric field. If the test charge were different, the electric field would not change. Thus, it helps formulate the effect of source charges independent of the test charge. By definition, it follows the principle of superposition. At any point, the electric fields of the source charges are vectorially added to give the net electric field. The direction of the electric field is chosen to be the direction of the force that a positive test charge would experience. Hence, a negative test charge would experience a force opposite to the field’s direction. Thus, the electric field of a positive point charge is directed away from it, and a negative point charge is directed towards it.