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Thread: Difference between impedance and resistance?

  1. #1 Difference between impedance and resistance? 
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    Hi all,

    I am trying to measure the conductivity of a polymer electrolyte using EIS (Electrochemical Impedance Spectroscopy). I've been getting really high impedance values on my instrument and so I tried to test a 50x50x1.5mm block of aluminum so that I could compare my values with the known. I've been getting impedance values as high as 0.7Ohm*m for the aluminum block, which has known resistance in the nano Ohms range.

    As far as I understand it, impedance is just resistance but in an AC circuit. At least that is what I used to think, but now I am not so sure.

    What is the difference between impedance and resistance? Is it possible to use math to convert between the two?

    Thanks

    -bdc69


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    Quote Originally Posted by bdc69 View Post
    Hi all,

    I am trying to measure the conductivity of a polymer electrolyte using EIS (Electrochemical Impedance Spectroscopy). I've been getting really high impedance values on my instrument and so I tried to test a 50x50x1.5mm block of aluminum so that I could compare my values with the known. I've been getting impedance values as high as 0.7Ohm*m for the aluminum block, which has known resistance in the nano Ohms range.

    As far as I understand it, impedance is just resistance but in an AC circuit. At least that is what I used to think, but now I am not so sure.

    What is the difference between impedance and resistance? Is it possible to use math to convert between the two?

    Thanks

    -bdc69
    Impedance is a generalized resistance. Resistance is purely real (meaning that voltage and current are perfectly in phase). Impedance can be complex in general, allowing for arbitrary phase between voltage and current. So there's no conversion possible between the two in general; one subsumes the other.

    The wrong readings you are getting suggest any number of problems, such as the method of connection to the sample, a miscalibrated instrument, or something that is plain broken. Without knowing more details, it isn't possible to say anything more definite. Perhaps you could post details about the model of instrument, how you've connected to the sample, etc.


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    Impedance is the combination of reactance and resistance.

    Reactance comes in two forms; capacitive reactance and inductive reactance.

    Reactance is frequency dependent while resistance is not.

    Capacitive reactance is found by



    where f is the frequency.

    Inductive reactance is

    X_L= 2 \pi f L

    Everything is going to have some reactance, so if you run an AC current through it, the impedance will be greater than the resistance.

    for an inductance, the impedance is found by:



    for a capacitance it is



    To combine all three you use



    Note that if you can get XL and Xc to be equal, this reduces to just R.

    By picking the right values for L and C you get something that has its lowest impedance at a set frequency, which is the idea behind a band-pass filter.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
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    To put it in simple terms for any who might not understand electronic theory; impedance is a measure of resistance in a circuit or component where the current does not change in phase with a change in voltage, so we are talking about alternating current. As has been stated, capacitance and inductance will cause the current to vary out of phase with the applied (alternating) voltage. Your problem with measuring a metal block will almost certainly be the quality of connection between the probes and the block. 0.7 Ohms sounds like a reasonable value taking into account the resistance of the probe wires and there may be grease and contamination etc on the probe tips - if you connect the meter probes together you will probably get a similar reading - I bet you won't get less than 0.2 Ohms. Aluminium also has an oxide layer on its surface which might cause extra resistance. To measure such low resistances, accurately you will need specialist equipment, but that is not my field.
    Last edited by One beer; January 20th, 2014 at 10:03 AM.
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    This helps. Testing has been conducted on the PARSTAT 4000 with an electrode and counter electrode in which the sample is clamped between. I've lightly sanded the surfaces of the block in an effort to remove an oxide layer but this does not make much of a difference.

    Now another question; The aluminum block is 1.59mm x 40.42mm x 39.13mm. If the known value of resistance for this aluminum alloy is 3.700E-08 Ohm*m, than what value should I expect to get for my aluminum block? Would I make conversions using the thickness of the block or the surface area?
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    Quote Originally Posted by bdc69 View Post
    This helps. Testing has been conducted on the PARSTAT 4000 with an electrode and counter electrode in which the sample is clamped between. I've lightly sanded the surfaces of the block in an effort to remove an oxide layer but this does not make much of a difference.

    Now another question; The aluminum block is 1.59mm x 40.42mm x 39.13mm. If the known value of resistance for this aluminum alloy is 3.700E-08 Ohm*m, than what value should I expect to get for my aluminum block? Would I make conversions using the thickness of the block or the surface area?
    The resistance of a uniform block of material is rho*length/area, where rho is the resistivity (your 3.7e-8 ohm-m number), length is the distance between the electrodes, and area is the cross-sectional area of the faces. You didn't identify which dimension goes with which feature, so you'll just have to crunch the numbers yourself. No matter what, you are going to get very tiny numbers. So tiny, in fact, that it isn't clear that you will be able to measure the resistance of the aluminum; you may very well be characterising everything but.
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