The exact formula for the electric field at the axis of a charged disk is given by
E = k*2II*R^2*a/{ x*sqrt(R^2 + x^2)}.
Here k =9*10^9 Nm^2/C^2, II stands for Pi (3.14), R = 10 cm = 0.1 m, a is used for alpha and = 3*10^10 C/m^2 and x = 1 cm = 0.01 m.
