Hello. Maybe this is very basic and important question for circuit analysis. Please see the attached image. KCL (Kirchhoff's Current Law) is applied to red arrow-indicated point and I choose the convention that current flowing out from the point is positive. - side of the capacitor is actually grounded thus its voltage is 0 and red arrow point is V. I1 = V/R and I2 = C(dV/dt). Substituting these to KCL as I1 + I2 = 0 give the right equation as V/R + C(dV/dt) = 0 The solution of exponentially decaying with time constant of RC. Until this problem is solved well. Good! But what if I choose the direction of I2 in opposite way? In this case KCL becomes V/R - C(dV/dt) = 0 and solution is not right. V increases forever! Only solution to solve this problem is to accept the idea that C(dV/dt) is positive only when current is from + to - through the capacitor. But why? The current can go + to - in rounding circuit all the way. (It is counter-clock wise direction here.) Could you tell me why i = C(dV/dt) has positive polarity only when current is across the capacitor from + to negative? Is there any physical reason?