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So I had a discussion with my research advisor about resistance vs. impedance

  1. Mar 1, 2007 #1
    He insists they are synonymous. I said I didn't agree. I told him that impedance is specifically the ratio of voltage to current in the s-domain or frequency domain. Impedance carries much more information that resistance alone.

    Is it technically correct to think of impedance as being the same thing as resistance?

    Did I just end up making a fool of myself?
     
  2. jcsd
  3. Mar 1, 2007 #2

    berkeman

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    Staff: Mentor

    Impedance is in general a complex quantity. It is represented by a 2-D vector in the Real-Imaginary plane. The vertical axis is the imaginary (reactive) component of the impedance, and the horizontal axis is the real (resistive) component of the impedance.

    The input impedance of a circuit will generally be complex, and will vary with frequency. You can google or wiki "vector impedance meter" to see some of the instruments that are commonly used for making those measurements. We use the venerable HP 4194 here in our lab (actually several of them).
     
  4. Mar 1, 2007 #3
    It's a matter of choice, when you're talking about matched circuit, then impedance is the same thing as resistance since circuit is matched and complex values are canceled out , elsewhere resistance is a part of impedance as berkeman pointed out. So you might say that they are synonymous. But still, you're not making a fool out of yourself and you're correct when saying that impedance is carrying much more info than pure resistance .

    Btw, I think of resistance as Z = R + j0 ;)
     
  5. Mar 1, 2007 #4
    yeah you say it is 'purely resistive'
     
  6. Mar 1, 2007 #5

    rbj

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    they are the same species of animal dimensionally, so they are commensurate meaning that they can be meaningfully added, subtracted, and, in some sense, compared. normally we think of the term "impedance" as a complex quantity where the real part is "resistance" and the imaginary part is "reactance". since you can only compare (< or >) real quantities, you can't compare a complex impedance to a real resistance unless you know for some reason that the imaginary part is zero, or you are comparing some function of the impedance, such as its magnitude or its real part, to the resistance.

    many times specs on parts are expressed as "impedance" when the quantity is mostly or entirely real. the characteristic impedance of a transmission line or of free space would be examples.
     
  7. Mar 1, 2007 #6
    right, I understand impedance is a complex number and I understand its interpretation. I was just looking for someone to back up my argument.

    Thanks.
     
  8. Mar 1, 2007 #7
    yeah, that's exactly where this topic came up. We were talking about the matching network used to match the RF power supply to the load.

    Sometimes I feel like I bring up insignificant technicalities and I fear my professors find that annoying.
     
  9. Mar 1, 2007 #8
    yeah, I mentioned to him that the impedance carries a real resistive component and an imaginary reactive component.

    meh, I shouldn't have brought it up in the first place. When he said "impedance is resistance, correct?" I should have just agreed. :tongue:
     
  10. Mar 2, 2007 #9

    Ouabache

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    Concepts can be confusing enough to someone new to EE.

    However since you already were comfortable with the fact that impedance contains more than resistance (i.e. can also have inductive & capacitive reactance), a good stategy, would be just to let it go by. (as you are now thinking in retrospect)

    If this was one of your profs, then this person 'holds the cards', in issuing grades. It's not a bad idea to stay on their good side. Beyond school, I also find it a useful, in building or maintaining rapport with people; not to press a point for the sake of being right.:rolleyes:
     
  11. Mar 2, 2007 #10
    yeah, I pretty much just agreed with his point and left it alone. I didn't drag it on too long. I don't think I annoyed him about it.
     
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