# Voltage Across Capacitors

kd2amc

## Homework Statement

I was going through this PDF on my own to review for my AP Physics C E&M exam: https://apcentral.collegeboard.org/...course=ap-physics-c-electricity-and-magnetism

I was also watching this video (which discusses the solutions):

The part I am stuck on is part 2(a) (it is hard for me to copy and paste the contents of the PDF into here).

(The solution to part 2(a) is at about 16 minutes into the video).

Q=CV

## The Attempt at a Solution

Why is voltage across C0 equal to V0? I understand how this be the case if the resistor R1 was not there (since voltages are the same for components connected in parallel); however, since the resistor is connected to the capacitor in series, I thought that the voltage V0 would be split across C0 and R1 (which means C0 would have a voltage less than V0). Why is this not the case?

Thank you so much for any help!

Homework Helper
Gold Member
I thought that the voltage V0 would be split across C0 and R1 (which means C0 would have a voltage less than V0).
Yes, that will be the case when the capacitor is 'charging' i.e. the charge on the capacitor will be increasing with time. You need to find the equation for charge as a function of time.

kd2amc
Yes, that will be the case when the capacitor is 'charging' i.e. the charge on the capacitor will be increasing with time. You need to find the equation for charge as a function of time.

Thank you for your help. According to my notes, the charge as a function of time is Q = CV(1-e^-t/RC). I'm not sure how to use this equation to conclude that the voltage across C0 = V0, though. Doesn't this equation actually imply that Q = 0 at time t=0?

Homework Helper
Gold Member
Doesn't this equation actually imply that Q = 0 at time t=0?
Right.
that the voltage across C0 = V0
You get this condition at t=∞(mathematically) and that represents the steady state. In steady state, the current through the capacitor is zero, meaning that it is fully charged.

Practically, you don't need infinite time to reach steady state. It is assumed that the steady state is reached at t=5RC (or 6RC)(where RC is the time constant) since the capacitor will have more than 99% of its steady state voltage by that time.

• berkeman
kd2amc
Oh! I see now! Thank you!

• berkeman