# impedance vs resistance

2793 Views 25 Replies 16 Participants Last post by  piperunner
Hey guys. First post. Total newb. Looking for some clarification.

My understanding (audio background) is that high impedance means the excact same as high resistance.

As far as power distribution goes, what is the difference between impedance and resistance ?
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#### backstay

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Short answer, resistance is for DC, impedance is for AC. So no they aren't the same.

#### BBQ

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Resistance is in the copper wire itself and effects both AC and DC circuits.

electric resistance - a material's opposition to the flow of electric current; measured in ohms
Impedance is an electrical phenomena that is caused by AC circuits.

Electrical impedance, or simply impedance, describes a measure of opposition to alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC) there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.
http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Electrical_impedance.html

#### Aphex's Twin

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Discussion Starter · ·
Ok thanks. So far our studies have been based almost entirely on dc circuits. Weve been working with some single phase 3 wire ac stuff but impedance hasnt come up yet so I was just curious.

That response from backstay was almost as quick as a GFCI that noticed some more current in the neut. This forum rules !

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Z=R+Xl+Xc

#### bkmichael65

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Z=R+Xl+Xc
Close. How about Z=square root of [R squared + (Xl-Xc)squared]
I couldn't figure out how to do the math symbols on this keyboard

#### Semi-Ret Electrician

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Welcome aboard Aphex!

If that is a picture of your twin you shouldn't receive much resistance on ET.:laughing:

#### BBQ

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Look past the obvious. :laughing:

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#### Bootss

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#### gold

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Impeadance is the sum of resistance of the cunductor and the resistive sum of capacitive and inductive reactance. That's what the formula zog posted describes. Xsub L being Inductive reactance Xsub c being capacitive reactance. You would have to google the formulas for each I don't remember them nor do I think I ever needed them. In more laymans terms impeadance is the total resistance including what is created by an inductor and/or capacitor within the same circuit or paralell to it.

hardworkingstiff

#### Mshea

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In trade school the teachers teach DC resistance wich in AC systems is only equivalent for pure resistive circuits. AC has active fields that are constantly changing direction and magnetic features that don't exicst in DC.

A steel conduit has greater DC resistance to currrent than a copper bonding wiire inside the pipe but if it is carrying a fault the current will still split nearly 50/50. If you put the copper bond wire outside of the pipe the load wires are in then the fault current prefers the raceway more like 10/90 than 50/50.

In both cases the DC resistance was not the best predictor of current path. It is the magnetic field that increased the impedance on the external bond wire. The resistance of the wire stayed the same.
Because impedance is dynamic I am pretty sure the trade schools only use resistance because it is easier to teach and for the most part we don't solve impedance problems as electricians.

#### Big John

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It's a good habit to get into thinking of ohms law as E=I•Z instead of E=I•R. It removes a lot of confusion when troubleshooting 95% of the windings we actually see in the field.
Look past the obvious. :laughing:
That's freaking hilarious, because I really didn't notice the face. Proof that a woman with an awesome rack can get away with anything. :laughing:

#### ampman

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#### Zog

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bkmichael65 said:
Close. How about Z=square root of [R squared + (Xl-Xc)squared]
I couldn't figure out how to do the math symbols on this keyboard
Well thats not really accurate either, but I was trying to keep it simple for the new guy. Just talking about circuit impedance complex numbers are usually used where Z= R +jZ , phasors can also be used.

Your formula is a simplified version of a much more complex device impedance formula that I won't even attempt to type on this keyboard.

#### Aussielec

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Just talking about circuit impedance complex numbers are usually used where Z= R +jZ , phasors can also be used.
I've never seen that equation. Shouldn't it be Z=R+jX?

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#### Zog

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Aussielec said:
I've never seen that equation. Shouldn't it be Z=R+jX?
Yes, my bad. Fat finger error.