# 2nd Kirchhoff's law for all passive elements

1. Jun 7, 2014

### Jhenrique

How would be the 2nd Kirchhoff's law for this circuit here:

I'm asking because I don't know how is the behavior of the inductor, capacitor and current source wrt the 2nd Kirchhoff's law...

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2. Jun 8, 2014

### CWatters

Kirchhoff's 2nd law states that the sum of the voltages around a loop is zero. It also applies to inductors and capacitors so..

VI + VR + VL + VC = 0

The voltage (particularly on Inductors and capacitors) is time dependant but the above still applies at any instant in time.

3. Jun 8, 2014

### Jhenrique

And if I add a voltage source in series in this circuit, the equation will be so:

VI + VR + VL + VC = VV

Yeah!?

4. Jun 8, 2014

Yeah!

5. Jun 9, 2014

### CWatters

Yes. Although it might be better to write it as..

VI + VR + VL + VC - VV = 0

Did you realise you may have implied a different orientation for the additional voltage source VV?

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Last edited: Jun 9, 2014
6. Jun 9, 2014

### CWatters

or if you prefer

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7. Jun 9, 2014

### Jhenrique

Interesting and confused!

8. Jun 9, 2014

### Jhenrique

But I don't think so... I place in the left side of the equation the potential difference that favors the circulation of the current and I place in the right side the potential differences opposes the circulation of the current. I can't think in another way more intuitive...

9. Jun 9, 2014

### CWatters

It's not always obvious if a "voltage" opposes or increases the current. Best practice is to mark your diagram in a consistent way, then go around each loop summing all the voltages. Then solve any simultaneous equations (eg for circuits with multiple loops). You may well find some unknown voltages turn out -ve when you were expecting them to be +ve.