Negligible ionic Space Charge

When the charge on the center wire is relatively low, the ionic space charge density is assumed to be negligible, and Laplace s equation is... 

Suppose that some ionic species i is strongly rate-limiting and that the rapid flow of an electronic species produces a constant voltage VB across the oxide, independent of oxide thickness L. Furthermore, assume the electric field in the oxide to be due primarily to surface charge, such that space-charge effects are negligible. Then the electric field, E(x), in the oxide can be written as a constant E0... 

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