One side of a circular disc of radius R has an electric double layer of uniform strength mm spread over it. Prove that, along the line of symmetry which is normal to the plane of the disc (and hence passing through the centre of the circle), the electric field at a distance x from the layer is given by 2πε_0(a2+x2)3/2m_pπa2 
Fig. 1.23 Electric double layer.