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?


  • \$\begingroup\$ Could you tell us what time he says that so we can get a better context? It is a 53 minute video after all. \$\endgroup\$
    – RICK
    Jun 29, 2014 at 23:45
  • \$\begingroup\$ 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. \$\endgroup\$
    – M S
    Jun 30, 2014 at 16:48

1 Answer 1


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

  • \$\begingroup\$ Looks suspiciously much like Kirchhoff's Voltage Law ;o) \$\endgroup\$
    – jippie
    Jun 29, 2014 at 20:23
  • \$\begingroup\$ @jippie describe first, let the dude take it on board then give it a name LOL \$\endgroup\$
    – Andy aka
    Jun 29, 2014 at 20:27
  • \$\begingroup\$ @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. \$\endgroup\$
    – RICK
    Jun 29, 2014 at 23:45

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