A uniform disc of radius r and mass m is charged uniformly with the charge q. This disc is placed flat on a rough horizontal surface having coefficient of friction $\mu .$ A uniform magnetic field is present in a circular region (a > r) but varying as $k{t}^3$ as shown in figure. Find the time in second after which the disc begins to rotate. (Given r = 1m, m = 18 kg, q = 1C, $\mu =0.1,$ K = 4, g = 10 $m/s^2$)