I Would like to come up with the capacitance between an infinite sheet and a finite area rectangle as a function of distance. I looked into my old E&M book, it did not consider edge effects I need to include those. Here is the situation. I just spent the day working with a capacitive sensor. One plate is a rectangle ~(1X5 cm) the other is a 8" Si Wafer. The sensor measures the distance to the wafer with error less then 100 [itex] \mu m [/itex] . To calibrate this sensor the software collects 5000 data points as the sensor is moved from contact to 8mm away, it is then fit to a 4th order polynomial. All very good, but through the day I noticed that the software was rounding and numbers, well not really rounding them, my thought was that they must be using single precision variables. I still not understand why or how 2 would be come 1.999999 ??? The kicker came when they used Matlab to come up with the "5th" order polynomial. The coefficient of the 3rd order was on the order of 10-14 the fourth was order 10-18 the fifth seemed to be set at 0. I do not under stand how software which cannot handle integers can meaningfully compute that polynomial. Soooo.... I want to know what theory says.. Thanks for any help you can provide.