A linear quadrupole (Fig. 11.8) consists of charge +2Q at the origin and two charges -Q at (-d, 0) and (+d, 0). (i) Write down the magnitude of the electric field at P on the x-axis where x > d . (ii) If x >> d show that the field varies inversely as the fourth power of distance from the origin. (iii) If Q = 2 nC and d = 0.01 mm, calculate the field at x = 20 cm.