Last question I promise,,,, A uniformly charged disk has a radius of 2.5 cm and carries a total charge of 4*10^-12C. Find the electric field on the x-axis at a distance of 20cm away. I used the equation: Ex = (sigma/ (2*Eo)) * (1 - ( 1/ sqrt( ( R^2/x^2) + 1)) this is what i did so far Ex = (sigma/ (2*8.85*10^-12)*(1 - ( 1/ sqrt( ( 0.025m^2/0.002m^2)-1) I am not sure how to calculate sigma in this case. I am guessing that it is the total charge dived by the area. But, i think i am missing something. The answer is 0.89 N/C Thanks!