Question

A point charge is used to determine the electric field around a charged particle. Why is it necessary that the point charge does not affect the position of the charge being tested?

The amount of charge will increase if the charged particle moves closer to the point charge, so any change in position changes the field.
The amount of charge will decrease if the charged particle moves closer to the point charge, so any change in position changes the field.
Electric fields are inversely proportional to the square of the distance, so any change in position changes the field.
Electric fields are directly proportional to distance, so any change in position changes the field.

Answer 1

Electric fields follow Coulomb's law, E ∝ 1/r^2, so any change in the separation r changes the field. Therefore the point (test) charge must not disturb the position of the source charge — otherwise the field being measured would be altered. (So choose: "Electric fields are inversely proportional to the square of the distance.")