1. The problem statement, all variables and given/known data What is the magnetic field a distance z above a rotating disk (ang.velocity w) with surface charge density inversely proportional to the square of the distance from the centre of the disk. 2. Relevant equations K = v*surface charge density B = (constant)*integral of (K x rhat / r^2 )da 3. The attempt at a solution I just wanna know if i'm setting up this question right (basically, if i'm setting up the part in the integral correct, which is why i ommited the constant) charge desntiy = c/s^2 where c is a constant (since it is inversely proportional to square of distance from centre of disk) so K = v*charge density = ws*c/s^2 = wc/s so K cross rhat = Ksin(alpha) where alpha is the angle between k and r but in this case, the angle will always be 90 degrees since k and r are in perpendicular planes so i need to inegrate (K/r^2)da but all the horizontal components will cancel so i need to take only the vertical components, so i multiply by cos theta where theta is the angle r makes with the disk so i have : integral of (Kcostheta/r^2) da costheta = s/(s^2+z^2)^0.5 K = wc/s r^2 = s^2 + z^2 so i get ingeral of wc/s * s/(s^2+z^2)^3/2 * da da = sdsdtheta so I have to integrate (wcs/(s^2+z^2)^3/2)dsdtheta from s=0 to s=R and theta= 0 to theta=2pi is this correct?