# Finding the voltage of a circuit

## Homework Statement

Capacitor 3 in Figure (a) is a variable capacitor (its capacitance C3 can be varied). Figure (b) gives the electric potential V1 across capacitor 1 versus C3. The horizontal scale is set by C3s = 20 μF. Electric potential V1 approaches an asymptote of 10 V as C3 approaches infinity. What are (a) the electric potential V across the battery, (b)C1, and (c)C2?

http://edugen.wileyplus.com/edugen/courses/crs7165/art/qb/qu/c25/fig25_44_wiley.gif

## Homework Equations

C=q/V=(permitivity constant)A/d

## The Attempt at a Solution

I've been stuck on this problem for 1 hour. I just can't figure it out. I can't figure out the voltage of the battery, which is supposed to be 10V.

What I've tried to do is look at when C3=infinity which means that the distance between the capacitor is 0, so its just a wire. Then I get a parallel circuit with C1, and C2, where the voltage going through C1 is 10V. This makes a series circuit with C1, C2. So I know that the voltage of the battery = 10V + the voltage through C2. I can't figure out how to get 10V for the battery

TSny
Homework Helper
Gold Member
What I've tried to do is look at when C3=infinity which means that the distance between the capacitor is 0, so its just a wire. Then I get a parallel circuit with C1, and C2, where the voltage going through C1 is 10V. This makes a series circuit with C1, C2. So I know that the voltage of the battery = 10V + the voltage through C2. I can't figure out how to get 10V for the battery

I'm not following where you state that C3 = ##\infty## makes a parallel circuit and then in the next sentence say it's a series circuit.

Can you derive a general expression for the voltage V1 in terms of the battery voltage, V, and C1, C2, and C3? You can then see what happens as C3 goes to infinity.

NascentOxygen
Staff Emeritus