Just doing an exercise and I am stuck on the following questions. Any help would be appreciated: 1) A vertical dipole, with charge +40C at 10km high, and -40C at 6km high, what is the E field at the ground directly below (ground assumed perfect conductor). And at what distance on the ground will the E field be 0? 2) A coaxial cable is constructed using 2 dielectrics. Dielec 1 occupies 1/6th of the total area between the conductors who's radii are 1 and 7mm. Relatice Perm. of E1 = 5, E2 = 3. Calculate capacitance per unit length. 3) A small dipole centered at the origin, has components (0,0,p). By differentiating the expression for potential at (x,y,z) find the electric field components of x, y and z for (x^2 + y^2 + z^2) >> d^2 where p = dq, q is the charge and a is the seperation Think I have the others sorted, just these left. Thanks, David.