# Equivalent capacitor in the circuit

1. Jul 21, 2012

### nik jain

Find the equivalent capacitance of the combinations shown in the fig.
(refer to the file attached)

Is there any easier method to solve instead of this method by assuming a constant potiential difference is applied across the circuit and a total charge Q flown in it.

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2. Jul 21, 2012

### sirisha kotik

this can be dne by charge distribution . and applying kirchoff's loop law.

3. Jul 21, 2012

### CWatters

4. Jul 21, 2012

### Saitama

5. Jul 22, 2012

### Staff: Mentor

No easier. Let current in top 2F cap be I1, and in top 4F cap be I2.
And current down through vertical 4F cap is I1 - I2.
etc

Solve for I´s in terms of applied voltage V and ω.

Does the textbook give the answer?

6. Jul 22, 2012

### sirisha kotik

the ans is 20/7 F i think...is tht correct?

7. Jul 22, 2012

### Saitama

So, the star delta transformation isn't applicable here?

8. Jul 22, 2012

### sirisha kotik

tht is nt a wheatstone bridge hw cn u apply the star delta thn?

9. Jul 22, 2012

### nik jain

Yes the ans. is 20/7 F and I also get it by using krichoff's law

10. Jul 22, 2012

### nik jain

I also want to know that can we use star to delta formation here as in this case applying of
krichoff's law is easy , but what if all the value of capacitance is different ?

11. Jul 22, 2012

### Staff: Mentor

The formulae given in the wikipedia article are general formulae, so their transform is applicable where the three branches are all different. On this page http://en.wikipedia.org/wiki/Y-Δ_transform under the heading Equations for the transformation from Y-load to Δ-load 3-phase circuit you are shown how to relate the impedance of each arm of Δ to that of the Y.

https://www.physicsforums.com/images/icons/icon2.gif [Broken] In their formula, instead of resistances, you will use impedances, remembering that the impedance of a capacitor C = (ωC)⁻¹

I tried it on your capacitor network, transforming the upside-down Y shape of the vertical capacitor and the two lower ones into a delta. This changes the network to an uncomplicated arrangement of capacitors in parallel, and in series. I got the same answer, 20/7 F

Last edited by a moderator: May 6, 2017