A test charge is taken to be vanishingly small. Why?

The correct answer is Option (2) → A larger test charge would disturb the source charges and change the field being measured.

Option (1) A larger charge is not available. — Incorrect. Charges of any practical size can be produced. The size of the test charge is a choice made for the measurement.

Option (2) A larger test charge would disturb the source charges and change the field being measured. — Correct. The test charge itself exerts a force on the source charges and tends to move them. This would change the field being measured. Making the test charge negligibly small keeps the sources in place, while the ratio \( F/q \) stays finite and defines \( \vec{E} \).

Option (3) There is no hard and fast rule; it can just as well be taken larger. — Incorrect. A larger test charge pushes or pulls on the source charges and shifts them. So the field it reports is no longer the original field.

Option (4) It is positive. — Incorrect. Taking the test charge positive only fixes the direction of \( \vec{E} \). Its sign has nothing to do with how small it must be.

NCERT (2026-27):
Electric Field – “… The charge Q, which is producing the electric field, is called a source charge and the charge q, which tests the effect of a source charge, is called a test charge. Note that the source charge Q must remain at its original location. However, if a charge q is brought at any point around Q, Q itself is bound to experience an electrical force due to q and will tend to move. A way out of this difficulty is to make q negligibly small.”