In the definition of (4) the charge ql must remain stationary. This requires that the test charge q2 be negligibly small so that its force on ql does not cause ql to move. In the presence of nearby materials, the test charge q2 could also induce or cause redistribution of the charges in the material. To avoid these effects in our definition of the electric field, we make the test charge infinitely small so its effects on nearby materials and charges are also negligibly small. Then (4) will also be a valid definition of the electric field when we consider the effects of materials. To correctly map the electric field, the test charge must not alter the charge distribution from what it is in the absence of the test charge.