question_answer

In defining electric field due to a point charge, the test charge has to be vanishingly small. How this condition can be justified, when we know that charge less than that on an electron or a proton is not possible?

Answer:

                Because of charge quantisation, the test charge \[{{q}_{0}}\] cannot go below e. However, in macroscopic situations, the source charge is much larger than the charge on an electron or proton, so the limit \[{{q}_{0}}\to 0\]test charge is justified.