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Impedance Low/High

J

John Fields

Jan 1, 1970
0
Setting the imaginary term to zero leaves only resistance which is what I
stated can only be observed in DC circuits.

---
That's right, but it's wrong in that in a series RCL circuit
connected to a DC source, setting the imaginary term for
capacitance, Xc, to zero will result in charge flowing through the
circuit limited only by the resistance of the resistor, the
resistance of the inductor, and the source voltage, according to
Ohm's law:

E
I = ---
R
---

This is merely a semantic argument

....

Then quantify it and bring it to a satisfactory conclusion.
 
J

John Fields

Jan 1, 1970
0
John,

Looks like your second post and my third post almost crossed in the system
and say essentially the same thing expressed two different ways.
 
J

John Fields

Jan 1, 1970
0
Setting the imaginary term to zero leaves only resistance which is what I
stated can only be observed in DC circuits.
 
J

John Larkin

Jan 1, 1970
0
Setting the imaginary term to zero leaves only resistance which is what I
stated can only be observed in DC circuits.

Why only in DC circuits? Resistance can be measured in AC circuits, at
any frequency, as the real component of the impedance. A vector
network analyzer, an ac-only device, can accurately measure a 50-ohm
termination, separately reporting the real and any reactive
components.

John
 
J

John Popelish

Jan 1, 1970
0
Dorian said:
Setting the imaginary term to zero leaves only resistance which is what I
stated can only be observed in DC circuits. This is merely a semantic
argument so I'll leave you with the Wikipedia definition for impedance (not
my first choice but readily available):


"Electrical impedance, (sometimes Electric impedance) or simply impedance,
is a measure of opposition to a sinusoidal electric current. The concept of
electrical impedance generalizes Ohm's law to AC circuit analysis. Unlike
electrical resistance, the impedance of an electric circuit can be a complex
number. Oliver Heaviside coined the term impedance in July of 1886"

Resistance is not a DC effect. It is an instantaneous effect. Ohm's
law is an instantaneous law, not a DC one. Infinite duration is not
required for it to be a valid description of resistance.

10 ohms resistance means that the instantaneous ratio of the
instantaneous voltage across the resistance is 10 times the
instantaneous current through it, regardless of the duration or speed
of variation or either voltage or current.

Besides DC is a sinusoidal frequency. It just happens to be a
frequency of zero.

Impedance generalized the (frequency independent) concept of
resistance to include reactance, which is frequency dependent and does
not have a fixed instantaneous ratio of voltage to current throughout
an AC cycle. This requires replacing the instantaneous ratio of
voltage to current with a two dimensional value, related to frequency.

One way to describe impedance is to call the magnitude of the
impedance the ratio of the RMS single frequency voltage across the
pair of nodes divided by the RMS single frequency current through the
nodes and add a term that describes the phase shift between voltage
and current. This is the polar form of impedance. Note the two
dimensions required (magnitude and phase). There is also a Cartesian
form that keeps track of the two dimensions as a real and imaginary
part, with the imaginary part being the frequency dependent part.
 
R

Robin

Jan 1, 1970
0
Uriah said:
I seem to be real slow in grasping these simple concepts. Another
question on Impedance.
What is low impedance? and what does it mean? Can you have low
impedance with high voltage or does it only apply to current? Is it
just a relative thing. When dealing with digital circuits does
low/high impedance imply certain things? What things?
thanks again


<quote>...does low/high impedance imply certain things? What
things?<end of quote>

Yes e.g. if a *source* is "low impedance" then that implies that it is
dangerous, it has plenty of drive power e.g. a fully charged car
battery. If you short it out then it will be capable of delivering
plenty of current.

The opposite, a "high impedance" source, implies less danger e.g. a
nearly flat car battery. If you short it out, the current is nearly
zero.


Cheers
Robin
 
K

Kevin Aylward

Jan 1, 1970
0
Dorian said:
Setting the imaginary term to zero leaves only resistance which is
what I stated can only be observed in DC circuits.

Not true at all. What do you propose is the current in ac circuit for an
rms ac voltage of 1V with a pure resistance of 1 ohm, compared to a
resistance of 2 ohms?

If resistance can not be observed, just what to you think a measurement
is? To wit, ther differance in current in the above example.
This is merely a
semantic argument

No ot isnt. Its a matter of facts.
so I'll leave you with the Wikipedia definition for
impedance (not my first choice but readily available):

Fortunately, I know way more that Wikipedia on this matter.

You need to understand that Wikipedia are not experts on what they
state, unlike myself. Its meant for non-specialists, i.e. mostly for
laymen like you.

Dont try and teach your granpa to suck eggs sonny.
"Electrical impedance, (sometimes Electric impedance) or simply
impedance, is a measure of opposition to a sinusoidal electric
current.

This is inaccurate. Impedance is a measure of opposition to current,
either DC or AC.
The concept of electrical impedance generalizes Ohm's law to
AC circuit analysis.
Yes.

Unlike electrical resistance, the impedance of
an electric circuit can be a complex number. Oliver Heaviside coined
the term impedance in July of 1886"

Yes.

Kevin Aylward B.Sc.
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"
 
R

Rich Grise

Jan 1, 1970
0
Resistance is not a DC effect. It is an instantaneous effect. Ohm's
law is an instantaneous law, not a DC one. Infinite duration is not
required for it to be a valid description of resistance.

10 ohms resistance means that the instantaneous ratio of the
instantaneous voltage across the resistance is 10 times the
instantaneous current through it, regardless of the duration or speed
of variation or either voltage or current.

Besides DC is a sinusoidal frequency. It just happens to be a
frequency of zero.

Impedance generalized the (frequency independent) concept of
resistance to include reactance, which is frequency dependent and does
not have a fixed instantaneous ratio of voltage to current throughout
an AC cycle. This requires replacing the instantaneous ratio of
voltage to current with a two dimensional value, related to frequency.

One way to describe impedance is to call the magnitude of the
impedance the ratio of the RMS single frequency voltage across the
pair of nodes divided by the RMS single frequency current through the
nodes and add a term that describes the phase shift between voltage
and current. This is the polar form of impedance. Note the two
dimensions required (magnitude and phase). There is also a Cartesian
form that keeps track of the two dimensions as a real and imaginary
part, with the imaginary part being the frequency dependent part.

Where the heck were you when they were trying to teach me this stuff? ;-)

Thanks!
Rich
 
R

Rich the Philosophizer

Jan 1, 1970
0
Silly me, I keep sending them money!


Well, when your cup runneth over, I guess it's your call where it gets
spilled. ;-)

Cheers!
Rich
 
J

John Popelish

Jan 1, 1970
0
Rich said:
Where the heck were you when they were trying to teach me this stuff? ;-)

Probably somewhere between Vietnam and Virginia.
 
A

Alan B

Jan 1, 1970
0
Gosh, that's not what they taught me in engineering school. They
claimed that impedance is the vector sum of resistance and reactance.

Thanks for straightening me out. I'll ask for my money back.

Me too! 'Cept that the piece of paper they gave me has come in real handy.
Reckon I'll wait a while on that refund demand. :)
 
A

Alan B

Jan 1, 1970
0
DC resistance is resistance, not impedance. AC has impedance.

You've crossed terms between "reactance" and "impedance." As others have
mentioned, impedance is the vector sum of resistance and reactance.
 
E

ehsjr

Jan 1, 1970
0
Dorian said:
Setting the imaginary term to zero leaves only resistance which is what I
stated can only be observed in DC circuits.

Tell that to my toaster.

Ed
 
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