I have a question about the electric field of a uniformly charged disk with radius a. I'll move point by point till i reach the part i really can't get. First of all, the surface area is made of infinite number of RINGS! So, we basically integrate the charge of the rings from r="0" to r="a". Since we considered a ring to be made of infinite points, we said that a point carries a charge dq of the whole Q. dq= Lambda ds. By analogy, the Q of one ring in the surface is a fraction of the big Q of the surface area which is equal dq= sigma * dA. (I assumed this part by analogy and not by understanding the concept) .. Anyway, it is given that dA itself is 2pi r dr. How can we get this?? I mean, mathematically A= pi a^2.. So dA= 2pi a da. This is from the mathematical side. From the physical side, i really can't get the point the rings with thickness dr. I mean from where did this dr come. I was told that if you get a regtangle with a width dr, and wrap it, you will get a ring with some thickness.. I really can't visualize this. This is my whole problem which is how to get dA which I originally can't know how did we reach to dA itself. Please clarify and i would be thankful to whoever gives me a hand to the full argument.