# Finding the Energy in Capacitor?

Discussion in 'Homework Help' started by Digit0001, Apr 3, 2011.

1. ### Digit0001 Thread Starter Member

Mar 28, 2010
89
0
Hi

I am having trouble with the following question:
View attachment tut4qns.doc

For the first question i got the answers for a) however the energy for the 20 and 40 micro ohms capacitors are incorrect.

This is what i did:

sub 0.5 into the equations that i got for a) and i get
v1 = 840.150v
v2 = 208.030v

w_12 = 0.5 * (12 * 10^-9) * v1^2
=4.23J

w_20 = 0.5 * (20 * 10^-9) * v2^2
=0.424J

w_40 = 0.5 * (40 * 10^-9) * v2^2
=0.866J

Question 2 how would i start it?

P.S

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
A hint for (2) is to equate the equations of the energy of the capacitor with the energy of the inductor to find a relationship between the voltage across the capacitor and the current though the inductor. Also, at steady state the voltage across the inductor is zero and the current through the capacitor is zero.

3. ### Digit0001 Thread Starter Member

Mar 28, 2010
89
0
what about my first question what have a done wrong?

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
You've got the wrong value for V2. It should be 178V.