# Thévenin resistance when branch is 0V (RC circuit)

#### mabtjie

Joined Oct 16, 2011
1 Prior to t=0, switch is in position A (for long time).
Afterward, switch is in position B.

We're using $$v(t)=v(\infty)+[v(0)-v(\infty)]e^{\frac{-t}{\tau}}$$ where $$\tau=RC$$.

My problem is in the calculation of the thévenin resistance from the perspective of the capacitor.

My answer: $$R=R_\mathrm{th}=(15+10)\parallel 100=20\,\mathrm{k\Omega}$$
Homework answer: $$R=100\,\mathrm{k\Omega}$$.

I was told that it has to do with the entire bottom of the circuit being at 0 V relative to the 100 V, so the left hand loop of the circuit is disabled. I'm not really satisfied with that answer though. I was hoping that someone could shed some light on this?

#### JoeJester

Joined Apr 26, 2005
4,390
The switch disables the left hand side when in the "b" position. Follow the current flow.

#### crutschow

Joined Mar 14, 2008
32,926
The Thevenin equivalent from the capacitor's point-of-view is whatever the capacitor sees when going from one capacitor terminal until it reaches the other capacitor terminal. As Joe said, follow the current.