forcing and forced functions, natural and stead-state responses

[PG1995]

Hi

Could you please help me with this query? Thanks.

Regards
PG

 

[steveB]

I think it is OK to say that all circuits have inductance and capacitance, if we include small parasitic effects. However, if we neglect parasitics in a circuit that is primarily non-reactive at the frequencies we are dealing with, the transient response is practically instantaneous, and practically speaking you can say that a transient response doesn't really exist since the steady state response exists alone, immediately.

One might argue that delay effects are separate from reactance from the point of view of circuit theory, and in this case the system with delay does have a transient response. However, delay effects are a manifestation of field effects, which then leads to circuit models as transmission lines, which can be viewed as distributed inductance and capacitance.

It will be interesting to see if someone can think of an exception. Essentially, you are describing a system without dynamic state variables and state equations.

 

[MrAl]

Hello there,


Strictly speaking the only thing in the universe that does not have inductance and capacitance is a node, and by node i mean a point in a circuit that has no dimension, only position. We use this concept in nodal analysis.

This is because as soon as we introduce distance, the signal has to travel from one point to another point, and once it has to travel any distance at all it will encounter natural capacitance and inductance. Inductance because it has to travel in a line through space establishing an electromagnetic field, and capacitance because the potential difference between the forward path and the return path establishes an electrostatic field.

But in theory we often used the lumped circuit principle (ie non distributed) and we also ignore everything that we dont specifically decide beforehand that we want to include in the circuit, and this usually means we ignore a lot of inductances and capacitances that are small relative to the other circuit elements. For a simple example, we often ignore even the series resistance of a battery.

 

[PG1995]

Thank you, Steve, MrAl.

@MrAl: I really liked your reply. It was simple, straightforward and to the point. I'm saying this because sometimes you include the details which make things somewhat more difficult for me to capture and I'm like Although I'm sure even those details will also help me someday when I've overcome the my limitations!

Best wishes
PG