A point charge +Q is fixed at point O in vacuum. Three points A, B and C lie on the same straight line through O at distances r, 2r and 3r respectively.
a. (i) State how the magnitude of the electric field produced by a point charge varies with distance from the charge.
b. (ii) Without performing any numerical calculation, arrange the magnitudes of the electric fields at A, B and C in decreasing order and justify your answer.
c. (iii) If the charge at O were replaced by −Q, would the order obtained in part (ii) change? Explain your reasoning.