Solving by finite difference method

[a_hoseni110]

hi;
I have 3 hyperbolic electrodes ,one as a ring and 2 others as endcap
electrodes which have potential v and 0 respectively.(quadrupole ion trap)
I want to solve potential inside the trap by finite difference method.

I don't know how general equations for unshaped materials will change , If I truncate my shape .
I don't know how to determine the magnitude of nodes which are on truncated line or exactly before that?

please help me
thanks a lot

 

[mfb]

Where do you truncate your shape?
If the field is not too far away from uniform, you can assume a uniform field (in direction of the border) as boundary condition.