Impedance Low/High

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

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.

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.

...does low/high impedance imply certain things? What things?

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

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.

No ot isnt. Its a matter of facts.

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.

This is inaccurate. Impedance is a measure of opposition to current, either DC or AC.

Yes.

Yes.

Kevin Aylward B.Sc. snipped-for-privacy@anasoft.co.uk

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

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

Thanks! Rich

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

Cheers! Rich

Probably somewhere between Vietnam and Virginia.

On Sat, 03 Jun 2006 15:01:38 -0700, in message , John Larkin scribed:

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. :-)

On Sat, 03 Jun 2006 21:12:19 GMT, in message , "Ralph Mowery" scribed:

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

Tell that to my toaster.

Ed