• Support PF! Buy your school textbooks, materials and every day products Here!

Capacitor equivalent

  • Thread starter WeiLoong
  • Start date
  • #1
35
0

Homework Statement


https://scontent-kul1-1.xx.fbcdn.net/v/t34.0-12/13228153_10206233880597531_197570509_n.png?oh=32466b5e1ab50b8c8a0f49be9d044411&oe=573C076F
Find the equivalent capacitance between A and B

Homework Equations


Capacitor

The Attempt at a Solution



https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-xat1/v/t34.0-12/13227986_10206233888797736_1816077667_n.png?oh=b89640bb9bb064cd8dfb82adec3cf894&oe=573BF525&__gda__=1463494744_04b6bbc1efbb4f70cbc46240f62cdcf4
I assume 1 and 2 is in parallel
There for 1/C = 1/1+1/1+1/3
C=8/3
Seems like this idea is not so convincing, Anyone has better idea on how to solve this question?
 

Answers and Replies

  • #2
cnh1995
Homework Helper
Gold Member
3,306
1,074
Parallel components are connected between same two points. Which capacitors are in parallel then?
Also, your formula for equivalent capacitance for parallel capacitors is incorrect.
 
  • #3
ehild
Homework Helper
15,427
1,827
@WeiLoong
I assume 1 and 2 is in parallel
There for 1/C = 1/1+1/1+1/3
C=8/3
Seems like this idea is not so convincing, Anyone has better idea on how to solve this question?

Which capacitors are 1 and 2? C1 and C2 are really parallel, but what about the others? Somehow you presented a correct result for the equivalent capacitance, but how? Where is that 1/3 from?
The red line means the same node A in the circuit.

upload_2016-5-16_11-55-59.png
 
  • #4
963
213
I assume 1 and 2 is in parallel
There for 1/C = 1/1+1/1+1/3
C=8/3
there is some error in addition.
moreover your circuit given above does not show the relation.
check it.
 
  • #5
35
0
@WeiLoong



Which capacitors are 1 and 2? C1 and C2 are really parallel, but what about the others? Somehow you presented a correct result for the equivalent capacitance, but how? Where is that 1/3 from?
The red line means the same node A in the circuit.

View attachment 100804
c1 and c2 is parallel meanwhile c3 and c4 are series
c3 and c4 will give me 2/3F
2/3F is parallel to C1 and C2
hence C=1+1+2/3=8/3 is that correct?
 
  • #6
ehild
Homework Helper
15,427
1,827
c1 and c2 is parallel meanwhile c3 and c4 are series
No, C3 and C4 are not in series, as their common point is connected to B. How is the equivalent of C1 and C2 connected to C3?

upload_2016-5-16_13-2-50.png
 
  • #7
35
0
so that mean C1 C2 C3 are parrallel?
C123= 3F?
 
  • #8
cnh1995
Homework Helper
Gold Member
3,306
1,074
so that mean C1 C2 C3 are parrallel?
C123= 3F?
Parallel components are connected between same two points.
 
  • #9
35
0
https://scontent-kul1-1.xx.fbcdn.net/v/t34.0-12/13231083_10206234376729934_1801165661_n.png?oh=6c602ace0e3669a2fc756dec529a82d2&oe=573C5142
Like this? Then 2F and 1 F on top are in series
 
  • #10
cnh1995
Homework Helper
Gold Member
3,306
1,074
https://scontent-kul1-1.xx.fbcdn.net/v/t34.0-12/13231083_10206234376729934_1801165661_n.png?oh=6c602ace0e3669a2fc756dec529a82d2&oe=573C5142
Like this? Then 2F and 1 F on top are in series
Right.
 
  • #11
gneill
Mentor
20,792
2,770
so that mean C1 C2 C3 are parrallel?
C123= 3F?
Just because components can be drawn so that their symbols are parallel does not mean that they are connected in parallel. Parallel and series connectivity is a matter of topology, not drawing aesthetics. On a schematic diagram wires and components can be moved or turned or twisted or routed or placed all over the page, but so long as the connections remain the same, the topology of the circuit remains the same.

You need to get a firm grasp on what parallel and series connections mean and be able to recognize the necessary and sufficient conditions for both.

Here is a quick test that will let you recognize if two components are in parallel: If you can trace a complete, closed path through the two components following existing wiring and without passing through any other component along the way, then those two components are parallel.

Your C1 and C2 are such a pair:
upload_2016-5-16_8-21-57.png

Note the closed path in green that passes only through C1 and C2 and no other components along the way.
 
  • #12
ehild
Homework Helper
15,427
1,827
https://scontent-kul1-1.xx.fbcdn.net/v/t34.0-12/13231083_10206234376729934_1801165661_n.png?oh=6c602ace0e3669a2fc756dec529a82d2&oe=573C5142
Like this? Then 2F and 1 F on top are in series
Yes.
 

Related Threads on Capacitor equivalent

  • Last Post
Replies
1
Views
6K
  • Last Post
Replies
4
Views
2K
Replies
1
Views
2K
  • Last Post
Replies
10
Views
2K
  • Last Post
Replies
8
Views
450
Replies
5
Views
3K
Replies
7
Views
4K
Replies
2
Views
1K
Replies
3
Views
4K
Replies
1
Views
2K
Top