An annulus (a disk with a hole) made of paper has an outer radius R and an inner radius R/2, see figure below. An amount Q of electric charge is...

An annulus (a disk with a hole) made of paper has an outer radius R and an inner radius R/2, see figure below. An amount Q of electric charge is uniformly distributed over the paper. a) Find the potential as a function of distance on the axis of the annulus. b) Find the electric field on the axis of the annulus.