▁relative ▁to ▁the ▁above - ment ioned ▁boundary ▁conditions ▁and ▁ ▁is ▁a ▁capac itive ▁coefficient ▁of ▁the ▁h th ▁electro de ▁given ▁by ▁▁ ▁. ▁▁ ▁is ▁the ▁voltage ▁difference ▁between ▁the ▁h th ▁electro de ▁and ▁an ▁electro de ▁held ▁to ▁a ▁constant ▁voltage ▁( DC ), ▁for ▁instance , ▁directly ▁connected ▁to ▁ground ▁or ▁through ▁a ▁DC ▁voltage ▁source . ▁The ▁above ▁equations ▁hold ▁true ▁for ▁the ▁above ▁Dir ich let ▁conditions ▁for ▁ ▁and ▁for ▁any ▁other ▁choice ▁of ▁boundary ▁conditions ▁on ▁. ▁ ▁A ▁second ▁case ▁can ▁be ▁that ▁in ▁which ▁ ▁also ▁on ▁ ▁so ▁that ▁such ▁equations ▁reduce ▁to ▁▁ ▁, ▁▁ ▁. ▁ ▁As ▁a ▁third ▁case , ▁explo iting ▁also ▁to ▁the ▁arbitr ar iness ▁of ▁ ▁, ▁we ▁can ▁choose ▁a ▁Ne umann ▁boundary ▁condition ▁of ▁ ▁tang ent ▁to ▁ ▁in ▁any ▁point . ▁Then ▁the ▁equations ▁become ▁▁ ▁, ▁▁ ▁. ▁ ▁In ▁particular , ▁this ▁case ▁is ▁useful ▁when ▁the ▁device ▁is ▁a ▁right ▁par alle le pip ed , ▁being ▁ ▁and ▁ ▁the ▁later al ▁surface ▁and ▁the ▁bases , ▁respectively . ▁ ▁As ▁a ▁fourth ▁application ▁let ▁us ▁assume ▁ ▁in ▁the ▁whole ▁the ▁volume ▁, ▁i . e ., ▁ ▁in ▁it , ▁so ▁that ▁from ▁the ▁first ▁equation ▁of ▁Section ▁ 1 ▁we ▁have ▁▁ ▁, ▁ ▁which ▁recover ▁the ▁Kirch hoff ▁law ▁with ▁the ▁inclusion ▁the ▁disp lacement ▁current ▁across ▁the ▁surface ▁ ▁that ▁is ▁not ▁covered ▁by ▁electro des . ▁ ▁En closed