- #1

goodphy

- 216

- 8

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.

I

Substituting these to KCL as I

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 I

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?

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.

I

_{1}= V/R and I_{2}= C(dV/dt).Substituting these to KCL as I

_{1}+ I_{2}= 0 give the right equation asV/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 I

_{2}in opposite way? In this case KCL becomesV/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?