not sure if this question's been asked before, though I couldn't see one in my brief search on here... Well, the problem I am trying to solve is that I want to determine the electric field strength between two parallel rectangular-shaped plates with different voltages on each plate. For example, rectangular plate 1 has a voltage of +180V and rectangular plate 2 (parallel to plate 1) has a voltage of -5V. What equation(s) could we use to determine the field strength at any point between these two plates? The distance of separation of the plates is finite, say 8.6mm.
If you are not worried about fringing effects near the edges of the plates (which you shouldn't, if the plate dimensions are much larger than the gap distance), then finding the field is simple: V/d, where V is the voltage between the plates and d is the gap distance between the plates.
So, in the example you gave, there would be a field of 185V/(0.0086m) = about 21512 V/m, between the two plates. The voltage 1mm from the -5V plate would be -5 + (0.001m)(21512 V/M) = 21.512V-5V = 17.512V