Your English isn't too bad at all -- and I guarantee you it is MUCH, MUCH better than my facility with YOUR native tongue (no matter what it is)!

You are correct that, for sinusoidal steady state conditions that the V-I "phasors" exhibit the same type of linear relationship that we are used to seeing with resistors in DC circuits. The V-I "characteristic" does not. This is very likely an English-subtle-point issue.

If you plot voltage versus current for a capacitor in sinusoidal steady state, you get an ellipse, not a straight line.

And, just to be clear (again, very possibly an English-subtle-point item), the validity of superposition is not only the main criterion for establishing if a system is linear, it is THE criterion -- as in most sources DEFINE a linear system as being one in which superposition holds. It is a necessary and sufficient condition. A fine-point distinction that can sometimes be very valuable.

 

If you plot voltage versus current for a capacitor in sinusoidal steady state, you get an ellipse, not a straight line.

Yes - again you are right. I should have restricted my example to the magnitudes of the sinusoidal signals.
Again I see - when math meets electronics we must be very, very careful by describing properties of parts. This is true also because, in fact, in the field of electronics no formula is 100% correct. We always neglect third or fourth order influences and are using equality signs - knowing that this an approximation only. But that`s good and proven engineering practice.

 

You are doing fine.

One of the strong points about humans is that we have a very strong ability to take very poorly defined concepts when we communicate with each other and, far more often than not, infer the correct intended meaning. But, in engineering, that same ability (which we use and rely on all the time) can get in our way because it tends to leave us sloppy and often unaware of the lack of precision in our communication. Again, more often than not the correct concept gets across. But not always. So the more we try to be clear and precise, even though none of us ever will be truly so all the time (and it would not be fun to try to communicate with someone that truly was), the more likely we will be to spot when a miscommunication is happening because of this and to figure out how to correct it.

And what you say about higher order effects is very true. A very good rule to live by is, "If it's good enough, then STOP! It's GOOD ENOUGH!"

I've designed ICs where the fab house supplied transistor models that were, in fact, subcircuits consisting of over 300 components. Talk about taking higher-order effects into account! But that was the level of complexity that was needed to obtain simulation results that were "good enough" for the range of designs that those transistor models were used in. But in many, even most, of the circuits on a chip an extremely simple, first-order model was more than sufficient for the needed accuracy. This was indicated by the fact that most of the transistors on the chip were initially sized by hand using that simple model and never had to be tweaked. But some circuits (and not always obvious ones) had to be changed considerably because it turned out the transistors operated in a realm where high-order effects dominated the performance.

One of our biggest problems was long simulation times because of these hugely complicated models. For most of the circuits, they were TOO good! We paid a huge penalty in simulation time for no meaningful benefit. In theory, we could have stripped them down and made several progressively more complex models and then used whichever level model was needed for each transistors. That would probably have cut our simulation times down an order of magnitude and we talked about doing it, but decided that the risks outweighed the rewards because of the chance of using too simple a model in the wrong place by accident.

 

Quote WBahn:So the more we try to be clear and precise, even though none of us ever will be truly so all the time (and it would not be fun to try to communicate with someone that truly was), the more likely we will be to spot when a miscommunication is happening because of this and to figure out how to correct it.

Yes, and we have a similar situation with terms which describe the properties of parts, circuits and systems (in reality !):
Are there any circuits/systems that are "linear"? No.
Are there sinusoidal signals ? No.
Are there squarewave signals ? No.
.......
And each (good) engineer should know what he means using these terms.

 

Quote WBahn:So the more we try to be clear and precise, even though none of us ever will be truly so all the time (and it would not be fun to try to communicate with someone that truly was), the more likely we will be to spot when a miscommunication is happening because of this and to figure out how to correct it.

Yes, and we have a similar situation with terms which describe the properties of parts, circuits and systems (in reality !):
Are there any circuits/systems that are "linear"? No.
Are there sinusoidal signals ? No.
Are there squarewave signals ? No.
.......
And each (good) engineer should know what he means using these terms.

Careful - I got ticked off for nit picking by WBahn.

 

Oh, I've put this one on the "shrug pile".

 

i think you guys just gave me more home work lol