effective capacitance

It's two capacitors in series. Imagine a thin conducting surface between E1 and E2. For equal size plates on both ends, and assuming no fringing effects around the edges, C1 is proportional to E1/t1 and C2 is proportional to E2/t2, where E1 and E2 are the permittivities (or relative dielectric constants) and t1 and t2 are the plate spacings or thincknesses. Since the net capacitance is C1*C2/(C1+C2), the net capacitance is proportional to E1E2/(E1*t2+E2*t1). That can be rewritten as 1/(t2/E2 + t1/E1). In other words, you can average the

Sanity check: if you have equal thicknesses and E1=999*E2, averaging the permittivities yields 500*E2. But clearly the capacitance cannot be more than that of just the E2 section alone.

Cheers, Tom

If you do the ascii-art thing please use fixed-point font, and no tabs.

Your three-plate structure, with the middle plate floating, should pretty much have the same capacitance as the left-to-middle capacitor in series with the middle-to-right capacitor. You'll have some direct coupling from left to right through fringe effects, but by and large you should be able to estimate the capacitance.

I assume you don't really have a conductor in the middle, but are just trying to break the problem down in approved engineering fashion. I expect the answer will be substantially the same with or without that conductor.

Thanks. My worry was just that: the legitimacy of breaking down the problem by assuming that conductor in the center. From the responses I've seen here looks like it Ok. Thanks guys.

Thanks. My worry was just that: the legitimacy of breaking down the problem by assuming that conductor in the center. From the responses I've seen here looks like it Ok. Thanks guys.