# Capacitor

Discussion in 'Homework Help' started by jstrike21, Oct 13, 2009.

1. ### jstrike21 Thread Starter Member

Sep 24, 2009
104
0
The voltage across a 0.18 micro Farad capacitor is:
v=40v for t <= 0
v= Ate^(-5000t)+Be^(-5000t) for t>=0

The current in the capacitor at t=0+ is 152 milliamperes. Assume passive sign convention.

1. Find initial energy stored in capacitor.
For this i used w=.5cv^2=.5(.00000018)(40^2)= 144microJules

2. Find the coefficient A in v/s.
I don't know how to get this one

3. Find the coefficient B.
I used the equation for t>=0 and put 40 in for v and 0 in for t and solved and got B=40v

4. Find the current in the capacitor when t=2.76 milliseconds.
I believe i need to figure out number 2 before I can do this one.

Can I get some reassurance on 1 and 3 and some help on 2 and 4

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
You should be able to make use of the differential relationship

Ic(t)=CdVc(t)/dt

Where Ic(t) is the capacitor current with respect to time

and

Vc(t) is the stated capacitor voltage relationship wrt time.

So differentiate the voltage relationship and make use of the stated initial conditions at t=0+ by plugging them into the derived current function. Given you have B you should then find A.

3. ### jstrike21 Thread Starter Member

Sep 24, 2009
104
0
I get 844,444 for A and -1.976*10^-6 for I on number 4. Do these seem right?

4. ### Thav Member

Oct 13, 2009
82
0
I think you should solve 3 and get B before you do 2. It looks like if you take the point t=0- you know Vc and t so should be able to find B. Try that and then use tnk's advice for solving 2.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
You were getting there but must have missed something. Always helps if you post your working as you go. I've attached my solution - hopefully it's correct. You may want to delay looking at the pdf and retry your solution.

I get A=1.0444x10^5 and i(2.76ms)=-0.281uA

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6. ### jstrike21 Thread Starter Member

Sep 24, 2009
104
0
thanks for the help, i just made a simple differentiation error