Hi, I never really understood this fundamental concept of integration of rings to find the electric field for say a flat disk. In my book to find the electric field on a point charge on the axis of a disk a distance z away they just integrate all the rings of charge to the end of the disk. If it is a flat disk with no depth, then why can't we just use the area of the disk ∏R2dR. If we are given the charge per unit area σ then for our dQ why can't the dQ just be σ∏R2dR instead of the sum of all the rings 2∏RdRσ?
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