Question about circuits

  • #1

Homework Statement



A circuit is constructed with five capacitors and a battery as shown. The values for the capacitors are: C1 = C5 = 4.1 μF, C2 = 3 μF, C3 = 6.8 μF, and C4 = 3.4 μF. The battery voltage is V = 12 V.

Picture is here: http://s3.amazonaws.com/answer-board-image/7cae5cec-ff95-4f38-b803-29ac0b6ddae6.png

I need to find the equivalent capacitance between points a and c.

Homework Equations



I have the answer but I am a bit confused as to how it is arrived at. See below.

The Attempt at a Solution



I found the equivalent capacitance between a and b as c234=(c4)+(1/c23)

I see next that c234 is in series with c1 and c5. Why does the equation then become Cac=(1/((1/C1)+(1/C234)) and not (1/((1/C1)+(1/C5)+(1/C234))?
 

Answers and Replies

  • #2
559
8
You can neglect c5 since it doesn't affect the capacitance at a or b since the current is flowing clockwise.
 
  • #3
gneill
Mentor
20,925
2,866
Apparently they're only looking for the capacitance to the right of the points a and c. This is not clear in your problem description, so perhaps the problem is stated differently in the original?
 
  • #4
Apparently they're only looking for the capacitance to the right of the points a and c. This is not clear in your problem description, so perhaps the problem is stated differently in the original?

This was exactly the way the problem was written, so it left me a little confused as well
 
  • #5
gneill
Mentor
20,925
2,866
You can neglect c5 since it doesn't affect the capacitance at a or b since the current is flowing clockwise.

Current direction doesn't affect equivalent capacitance! Not unless there's a nonlinear circuit element like an open switch or a diode involved that would prevent "seeing" the capacitance.
 

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