# Few electromagnetics questions

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.

berkeman
Mentor
You need to show some of your own work on these in order for us to help you. What have you done on -1- so far?

quasar987
Homework Helper
Gold Member
You can do the first part of a), right?

The second part sounds more difficult. You need to know how the conductor below react (i.e. how charges will arrange themselves on the surface). But this is a classic problem used to illustrate the mthod of images. If you've seen the method of images, you know how to do this.

When I have used the Method of Images it has only ever been with 1 point charge, how does it work with a dipole?

quasar987