What is the value of t when q(t) is equal to 1.204 seconds in a circuit?

[Sajjad]

The charge flowing through a circuit is

q(t)=[3e^(-t) - 5e^(-2t)]--------(1)

find the value of t and then current i.
as i=dq/dt.
i am doing it like this

i=-3e^-t + 10e^-2t.....taking derivative

let e^-t=u...for eaze

i=10[u^2-3u/10]

let 1=0 then

10[u^2-3u/10]=0

10[u^2 - 2(u)(3/20) + (3/20)^2 - (3/20)^2]=0 ...using formula

10[u-3/20]^2 - 10[3/20]^2=0

10[u-3/20]^2=10[3/20]^2

10[u-3/20]^2=9/40

[u-3/20]^2=9/400

u-3/20=sqrt[9/400]

u=sqrt[9/400] + 3/20

e^-t = 3/20 + 3/20...as u=e^-t

Taking log on both side

ln[e^-t]= ln[3/10]

-t= -1.204
-----------------|
t= 1.204 seconds |------ am i doing ok till here?
-----------------|
by putting this in equation 1 we will get the vale for charge,q.

 

[Hootenanny]

Like I said in the physics forum, when do you want to find t? If you let q'(t) = 0, then you are finding the maximum / minimum charge.