kirchauf's law

steveb

Joined Jul 3, 2008
2,431
what are conditions where KVL & KCL fails?..
I think a better way to say it is, "What are the underlying assumptions for KVL and KCL, in the context of circuit theory?".

See the following lecture which is one of the better treatments I've seen on this topic. The critical answer to your question occurs in the first 10 minutes of the lecture, so you don't need to watch the whole thing, although I do recommend that, given the quality of the lecture.

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-002-circuits-and-electronics-spring-2007/video-lectures/lecture-2/

Here Prof. Agarwal talks about the lumped circuit element assumption and the underlying assumption that the rate of change of charge in an element and the rate of change of flux around an element is zero (or at least small).

I understand the above statements that the laws don't fail because KVL and KCL are the circuit formulation of fundamental laws we all accept. However, we always need to remember that Maxwell's equations are the better approximation to circuits, and circuit theory is a simplification and abstraction of that.
 

Papabravo

Joined Feb 24, 2006
12,301
I don't remember using either KVL or KCL in the context of AC circuits. I always used them exclusively in DC circuits. The presence of non-conservative electric fields is certainly a persuasive argument. It is similar the the argument about why perpetual motion is not possible. There is no reversible adiabatic process so a heat engine is impossible. This is because the amount of work available is not independent of path. The form of the integral is exactly the same.
 
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