What Percent of Final Charge Does a Capacitor Reach After 4.5 Time Constants?

[itryphysics]

Homework Statement

Consider an RC circuit in which a capacitor is being charged by a battery connected in a circuit. Initially the capacitor is uncharged. Then the switch is closed.
After a time equal to 4.5 time constants, what percent of the final charge is present on the capacitor?

Homework Equations

Q = Q_o [1 - e^ (-t/RC)]

The Attempt at a Solution

I picked a value of 100 for Q_o
so
Q= 100 [1 - e^ (-t/4.5)]
my approach was going to be to find Q and then see wht percent that is of the initial charge Q_o
but I don't know wht to put in for the time "t"
please help me out

 

[LowlyPion]

I think you would take t = 4.5RC

 

[itryphysics]

so for the expression e^(-t/RC) I do e^(-4.5) ?

 

[Delphi51]

4.5 time constants means a time of 4.5*R*C.
R*C comes out in units of time.

 

[LowlyPion]

so for the expression e^(-t/RC) I do e^(-4.5) ?

Yes that should do it.

 

[itryphysics]

Awsome! Thank you so much!