Initially charged capacitor disharged through a resistor

[Mike88]

Homework Statement

A capacitor with initial charge q0 is discharged through a resistor.
(a) In terms of the time constant τ, how long is required for the capacitor to lose the first one-third of its charge? answer x tau
(b) How long is required for the capacitor to lose the first two-thirds of its charge?
answer x tau

Homework Equations

i = -(q₀/RC)e^(-t/RC)

The Attempt at a Solution

I tried solving for time t and got -RC*ln(q/q₀)= t
I'm lost i don't know what to do

 

[Doc Al]

I tried solving for time t and got -RC*ln(q/q₀)= t

Looks good to me (except for the minus sign). So what's the time when q = q0/3?

 

[Mike88]

Looks good to me (except for the minus sign). So what's the time when q = q0/3?

i then get t = RC*ln(1/3) where RC = tau

is this correct?

 

[Doc Al]

i then get t = RC*ln(1/3) where RC = tau

is this correct?

Yes. But put that minus sign back in. (I didn't see that you had q0 on top! )

 

[Mike88]

Yes. But put that minus sign back in. (I didn't see that you had q0 on top! )

thanks for the help i figured out everything.