At 11:23 03/13/99 -0600, Eric Oliver wrote:
>I have
>been struggling with truly understanding the concept of impedance.

i'll give it a try. when you try to describe the behaviour of a given
circuit, you use the voltages at all the connection points (also called
"nodes") and the currents between them (in the "branches" -- do i use the
proper english terms here?)., so for any "branch," ie. for any connection
between two nodes, you have a difference in potential (a voltage
difference) between them and a current flowing between them, like this:

... (node1, v1)  -----/\/\/\----- (node2, v2)
                    R (->I)

so, if all that happens with DC only, we say

R = (v2-v1)/I = v/I

that's the "concept of resistance;" it relates voltage and current. and it
does it in a way that leaves both of them "in phase," which means that when
you look at both with a scope, they have the same form and the peaks at the
same time.

now lets put some changes with time in there and go to AC. usually, in all
"real world" situations, the peaks of current and voltage are not exactly
at the same time (although the difference may be so small that it's hard to
measure). this is caused by the effect that the resistance is not a pure
resistance, but that it is an "impedance." which means nothing more than
that it has a capacitive and/or inductive component. the capacitive
component makes the current partly dependent of the =change= of the voltage
(differential), and the inductive component makes the voltage partly
dependent on the =change= of the current (again the differential).

C = I / (dv/dt)
L = v / (dI/dt)

so, an impedance is a generalized resistance, capacitance, inductance. the
three of them are basically three special forms of impedance, the "pure"
forms. and every impedance can be described in combining these three
elements, in the appropriate serial or parallel circiut.

for example, a wire. for most purposes, you can just assume it's a mere
(ideal) conductor. sometimes, with high currents, you have to look at it as
a resistor and take the dissipated power into accout. at other times, with
high frequencies, its inductive characteristics may be important, or its
capacity (with, say, another wire). so, it often depends on what you're
looking at which parts of a "real" impedance you're using in your
calculations and which ones you find negligible. but basically, =every=
resistance, capacitance, inductance, conductor... =everything= is an
"impedance" and has all three characteristics to varying degrees.


i hope this gave you an idea -- i'm sure there are some more "enlightened"
ones out there to correct or clarify or give better examples :)

ge