Charging and Discharging RC circuit

1. Apr 10, 2010

lha08

1. The problem statement, all variables and given/known data
I'm confused by the interpretation of the charging and discharging formulas:
Charging: Q= Qo (1-e^-t/RC)
Discharging: Q=Qoe^(-t/RC)
Like for Qo, like apparently its the max charge, so does that mean at the very end of charging? Initially, i thought that Qo meant that the initial charge (obviously it can't be zero...) but then for the Qo in discharging, what does that one mean? Like apparently it's the initial charge, do they mean the max charge of the capacitor at the end of charging?

And also, for the current formula for charging I(t)=Ie^-t/RC, does 'I' refer to the initial current that is maximum at the beginning of this process? And for the discharging one, i thought that there is no current at the beginning, but then in my book it says that I is the initial current in the circuit..are they referring to the charging current?
I know I'm really confused but any help would be much appreciated...

2. Relevant equations

3. The attempt at a solution

2. Apr 11, 2010

ehild

Plug in t=0, what do you get for Q? What is the limit of Q if t tends to infinity?
When you start to charge a capacitor, the charge is usually zero on it at the beginning. And you can discharge a capacitor if it has some charge.

As for the current, plug in t=0, what do you get? Then find the limit of I when t tends to infinity.

ehild

Last edited: Apr 11, 2010
3. Apr 11, 2010

Phrak

Dude. Qo is the ultimate charge you can put on the capacitor after you charge it forever.
So you put a really big t in your equation Q= Qo (1-e^-t/RC) and e^-t/RC becomes like zero because you can try it. Put bigger and bigger t/RC in your calculator but make it negative. e^-t/RC starts going to zero with t way bigger than RC. So Q=Qo when it's totally charged.