Why isn't the real part of z just the coefficient R?

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Discussion Overview

The discussion revolves around the interpretation of the real part of the complex impedance \( z \) in the context of the equation \( \frac{1}{z} = \frac{1}{R} + \frac{1}{i\omega L} \). Participants are exploring the mathematical rearrangements and implications of complex numbers in electrical engineering, particularly focusing on the relationship between real and imaginary components.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asserts that if \( z = R + i\omega L \), then the real part of \( z \) should be \( R \), questioning the validity of their rearrangement.
  • Another participant challenges this by stating that the rearrangement is incorrect, providing a counterexample to illustrate that \( \frac{1}{a} \neq \frac{1}{b} + \frac{1}{c} \) leading to \( a \neq b + c \).
  • A participant seeks clarification on whether \( \frac{1}{R + i\omega L} = \frac{1}{R} + \frac{1}{i\omega L} \) is indeed correct.
  • One participant expresses uncertainty about their previous reasoning and indicates a willingness to re-evaluate their approach.
  • Another participant attempts a new expression for \( z \) but realizes complications arise with the terms involved, particularly with the imaginary unit in the denominator.
  • A suggestion is made to multiply by the complex conjugate to simplify the expression, detailing the standard procedure for handling complex fractions.
  • A participant provides a partial expression for \( z \) and notes the need to realize the denominator correctly.
  • Another participant encourages a simpler computation of \( \frac{1}{a + ib} \) as a preliminary step before returning to the original problem.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct interpretation of the real part of \( z \) and the validity of the mathematical rearrangements. Multiple competing views and approaches remain present in the discussion.

Contextual Notes

The discussion includes unresolved mathematical steps and assumptions about the rearrangement of complex fractions. Participants are navigating through the complexities of complex impedance without a definitive resolution.

whatisreality
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In the equation 1/z = 1/R +1/(iwL), why isn't the real part just R?

Unless my rearrangement is wrong, z=R+iwl, so Re(z) should definitely be R!
 
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Yes, your rearrangement is wrong. [itex]\frac{1}{a}= \frac{1}{b}+ \frac{1}{c}[/itex] does NOT give "a= b+ c". As a check, suppose b= c= 2. The 1/a= 1/2+ 1/2= 1 so a= 1 But b+ c= 4, not 1.

From [tex]\frac{1}{z}= \frac{1}{R}+ \frac{1}{iWL}[/tex], multiply on both sides by all denominators, RiWLz, to get RiWL= iWLz+ Rz= (R+ iWL)z. Solve that for z to find the real and imaginary part.
 
So you're saying 1/(R+iwL)=1/R+1/(iwL) ?
 
Yes. I did actually think that was weird, not sure why I didn't check it. Let me try that rearrangement again...
 
z = -R2ω2L2/(iωL+R) is my second attempt!Edit: maybe not. L squared too. And that makes life more difficult! What do I do about i on the denominator? Multiply by the conjugate?
 
Last edited:
Pretty standard: multiply both numerator and denominator by the complex conjugate of the denominator.
[tex]\frac{a+ bi}{c+ di}= \frac{a+ bi}{c+ di}\frac{c- di}{c- di}[/tex][tex]= \frac{ac+ bd+ (bc- ad)i}{c^2+ d^2}=[/tex][tex]\frac{ac+ bd}{c^2+ d^2}+ \frac{bc- ad}{c^2+ d^2} i[/tex]

Here you want to multiply numerator and denominator by [tex]R- i\omega L[/tex].
 
z = RiwL/(R+iwL)
Realise the denominator:
(iR2wL+Rw2L2)/(R2+w2L2)
 
You can do better than that (your first line is OK though).
Try this as a warm up : forget the exercise, just compute [tex]\frac{1}{a+ib}[/tex]. Then go back to z.
 

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