# question about capacitors in parallel

Discussion in 'Homework Help' started by becca02, Feb 5, 2009.

1. ### becca02 Thread Starter Member

Jan 28, 2009
10
0
if i have two capacitors in parallel with two switches, and a voltage source, what would be the voltages on the two capacitors if at first switch1 is closed and switch2 is open for t<0 and then switch1 opened and switch2 closed at t=0?

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2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
What do you think about the solution?

3. ### becca02 Thread Starter Member

Jan 28, 2009
10
0
there arent any numbers ... the question just ask to find the voltage on each capacitor in terms of Vs

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Ok I am going to give a push.

Assume the capacitors are initially discharged (0V across them).
If you close the S1 with S2 open what will happen?
The if you open S1 and close S2 what do you think will happen?

5. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
So your answer will have to be given in terms of Vs, C1 and C2.

Have you tried to do the calculations?

6. ### becca02 Thread Starter Member

Jan 28, 2009
10
0
okay, so capacitors in parallel are combined. so my guess would be that the switches are doing the same thing, whether S1 is open and S2 is closed, or vice-versa. so then the vc1 = Vs-vc2 and vc2 = Vs-vc1 ?

Last edited: Feb 5, 2009
7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
When S1 is closed and S2 open capacitor C1 will charge up to the supply voltage. When S1 opens and S2 closes charge will flow from C1 to C2 until the voltage on both capacitors is equal.

The formula to find that voltage is:

V=[C1*V1+C2*V2]/(C1+C2)

where V1 and V2 are the initial voltage across C1 and C2 respectively