# Three capacitors in a loop and dependence?

I was watching a lecture on State Variables on YouTube here

The professor mentioned that "if three capacitors form a loop, the three cannot be independently specified. Only two can be done as the third becomes dependent on the other two". I was wondering how the two capacitors and can independent and the third dependent?

Thanks!

• Could you tell us what time he says that so we can get a better context? It is a 53 minute video after all.
– RICK
Jun 29, 2014 at 23:45
• It's around the 40 minute mark. When I get home from work, I can give you the exact time the professor talks about it.
– M S
Jun 30, 2014 at 16:48

Three caps in series across a "10V" power supply. If one cap (C1) has 2V across it and another (C2) has 5 volts across it, the third HAS to have 3 volts across it because: -

2 + 5 + 3 =10

If the power supply was 0V it makes no difference: -

C1 has (say) 2V across it, C2 has 5V across it and the third must have -7V across it because

2 + 5 - 7 = 0

• Looks suspiciously much like Kirchhoff's Voltage Law ;o) Jun 29, 2014 at 20:23
• @jippie describe first, let the dude take it on board then give it a name LOL Jun 29, 2014 at 20:27
• @Andyaka I think the author is referring to the capacitance of, not that voltage across each. That's how I remember it from school at any rate.
– RICK
Jun 29, 2014 at 23:45