I am getting stuck trying to find the total electric charge on the annulus. The Question reads: "A thin disk with a circular hole at its center, called an annulus, has inner radius R_1 and outer radius R_2. The disk has a uniform positive surface charge density, sigma, on its surface. Determine the total electric charge on the annulus." I realize that I need to intergrate dE_mag(r) from R_2 to R_1, but I'm not sure how to do it using the correct variables .