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John Popelish electronics forum Guru
Joined: 29 Apr 2005
Posts: 1601

Posted: Mon Jun 05, 2006 11:18 pm Post subject:
Re: Impedance Low/High



Rich Grise wrote:
Quote:  On Sun, 04 Jun 2006 14:38:03 0400, John Popelish wrote:
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?

Probably somewhere between Vietnam and Virginia. 

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Alan B electronics forum Guru Wannabe
Joined: 05 May 2006
Posts: 137

Posted: Mon Jun 05, 2006 11:41 pm Post subject:
Re: Impedance Low/High



On Sat, 03 Jun 2006 15:01:38 0700, in message
<vb14829k098mhjf00umrrkv0p08uopud83@4ax.com>, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> scribed:
Quote:  DC resistance is resistance, not impedance. AC has impedance. If you have
a coil, it will have a DC resistance and an AC impedance which will
normally be much differant.
The same with a capacitor, it will have a DC resistance (normally very high
for low values) and an AC impedance.
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. 

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Alan B electronics forum Guru Wannabe
Joined: 05 May 2006
Posts: 137

Posted: Mon Jun 05, 2006 11:43 pm Post subject:
Re: Impedance Low/High



On Sat, 03 Jun 2006 21:12:19 GMT, in message
<TWmgg.1016$o4.381@newsread2.news.pas.earthlink.net>, "Ralph Mowery"
<rmowery28146@earthlink.net> scribed:
Quote:  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. 

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ehsjr electronics forum Guru
Joined: 03 May 2005
Posts: 863

Posted: Tue Jun 06, 2006 4:51 am Post subject:
Re: Impedance Low/High



Dorian McIntire wrote:
Quote:  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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