Notices
Results 1 to 15 of 15

Thread: Percent of a Composite from Resistivity values

  1. #1 Percent of a Composite from Resistivity values 
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    I am working on a resistor lab. I found the resistivity of 5 resistors all made of the same material, a silver-nickel composite. I would like to find the percent silver/nickel of the wire.

    Average Resistivity: 4.56e-7
    Resistivity of Silver: 1.59e-8
    Resistivity of Nickel: 6.99e-8

    Here is my equation:
    Ω_silver (percent)+ Ω_nickel (100-percent)= Ω_composite

    With everything substituted:
    [1.59〖10〗^(-8) ][%]+ [6.99〖10〗^(-8) ][100-%]= 4.56〖10〗^(-7)

    But this gives me % = 121.

    Now I realize that these resistivity values are at 20c and my lab was warmer and that the wire most likely has some other materials in it, but shouldn't I be able to get a reasonable answer?

    What am I doing incorrectly?

    P.S. Sorry for the simplicity of this question in comparison to all the other questions here. This was the first place that showed up on Google. Thanks for your help.


    Reply With Quote  
     

  2.  
     

  3. #2  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    I'd have thought the resistivity of the composite would be somewhere between the two metals it is made of. Your data shows it is higher than either one, so naturally you won't get a reasonable answer.


    Reply With Quote  
     

  4. #3  
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    Quote Originally Posted by Harold14370
    I'd have thought the resistivity of the composite would be somewhere between the two metals it is made of. Your data shows it is higher than either one, so naturally you won't get a reasonable answer.
    Right, that's the problem I was having; it doesn't make any sense. I've double check my data and calculations, even checking with others, so my average is correct with the data I had. What do you think is the most reasonable explanation?

    1)Other metals in the wire of the resistor
    2)Temperature Variation
    3)Human reading error (Amperes/Volts)

    It's probably a combination of all three and more. It just seems like my answer is too off. I tried the experiment with a copper resistor and got 1.71e-8 which is really close to 1.72e-8 (resistivity of copper). So I can't imagine the temperature has much to do with it (it's only a few degrees) and I'm pretty sure that the data was collected properly. So that really only leaves impurities in the wire. Does this seem accurate? Again it just seems like my answer is too far off? Ideas?

    Thanks for your help.
    Reply With Quote  
     

  5. #4  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    Resistivity and resistance is a molecular structure kind of thing.

    Perhaps this alloy formed from the two metals has a significantly different molecular arrangement that alters the resistivity to a higher value than either pure metal
    everything is mathematical.
    Reply With Quote  
     

  6. #5  
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    Quote Originally Posted by organic god
    Resistivity and resistance is a molecular structure kind of thing.

    Perhaps this alloy formed from the two metals has a significantly different molecular arrangement that alters the resistivity to a higher value than either pure metal
    Sounds plausible. I don't know anything about this; can anyone confirm this theory?

    Edit: I see that Harold14370 is the Mod of the Electrical/Electronics category. Perhaps, I ought to ask someone there about the material in resistors. Thoughts?
    Reply With Quote  
     

  7. #6  
    Suspended
    Join Date
    Apr 2008
    Posts
    2,176
    Nichrome wire is like that (80 percent nickle, 20 percent chrome) 108.0 10^6, ohms * cubic centimeters.

    Yet nickle 99.4 pure is 9.5 10^6, ohms * cubic centimeters.
    And Chromium is 13 10^6, ohms * cubic centimeters.

    I show silver as 1.59 10^6, ohms * cubic centimeters.

    I had some copper wire, with a nickle and maybe chromium electroplating, it was heater supply wire, that showed no ohms on a 500 foot roll, with one ohm meter I used. That is pretty odd I thought.

    Sincerely,


    William McCormick
    Reply With Quote  
     

  8. #7  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    How consistent was the resistance of each of the 5 wires you tested?

    One possibility is that there is an oxide film on the resistors so that you are not making good contact with your ohmmeter probes.

    Another possibility is the thermocouple effect. There is a voltage at the junction of dissimilar metals which is a function of temperature. The copper leads of your test equipment will form a thermocouple with the silver/nickel material at each connection. This shouldn't matter if the two junctions are at the same temperature, because the voltages will cancel out. But if the resitor wire is long, there could be a temperature difference from one end to the other. That's probably not the problem - just throwing that out there for you to think about.
    Reply With Quote  
     

  9. #8  
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    Quote Originally Posted by William McCormick
    I show silver as 1.59 10^6, ohms * cubic centimeters.
    Just FYI, I'm in ohms*meters.

    If Silver and nickel act in the same manner, then I suppose my math doesn't work. Can anyone specifically state that Silver/Nickel acts this way. Does anyone have a descent Table of Resistivity with composites?
    Quote Originally Posted by Harold14370
    How consistent was the resistance of each of the 5 wires you tested?
    I'm assuming you mean resistivity; the resistance was all over the place because of different lengths.
    4.50〖10〗^(-7)
    4.59〖10〗^(-7)
    4.51〖10〗^(-7)
    4.65〖10〗^(-7)
    4.54〖10〗^(-7)

    Quote Originally Posted by Harold14370
    Another possibility is the thermocouple effect. There is a voltage at the junction of dissimilar metals which is a function of temperature. The copper leads of your test equipment will form a thermocouple with the silver/nickel material at each connection. This shouldn't matter if the two junctions are at the same temperature, because the voltages will cancel out. But if the resitor wire is long, there could be a temperature difference from one end to the other. That's probably not the problem - just throwing that out there for you to think about.
    Well, the lengths of the Silver/Nickel aren't too long.
    .80 m
    1.20 m
    1.60 m
    2.00 m
    2.00 m

    The Copper was 20.00 m.

    Obviously, I was using some other wire in the circuit to connect all the equipment together. I don't know what it was made of, but it was well insulated and rather thick. The only thing is that this can't be the problem because the copper was right. Does this sound valid? Just thought I would mention it.

    Quote Originally Posted by Harold14370
    One possibility is that there is an oxide film on the resistors so that you are not making good contact with your ohmmeter probes.
    That would be possible as I'm not quite sure how long these resistor have been around.

    Thank you all for your input.
    Reply With Quote  
     

  10. #9  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    There is no alloy of nickel and silver listed on the Wikipedia list of alloys.
    http://en.wikipedia.org/wiki/List_of_alloys
    There is something called nickel silver which is actually nickel and copper and does not contain silver. Could that be what you are dealing with?

    The copper wire would not have a thermocouple effect if your test leads are copper and so are the resistors. There would be no volatage at a copper-copper junction. I doubt that's it anyway since the resistivity you measured is pretty consistant and the thermocouple effect would be somewhat random depending on the temperature differentials.
    Reply With Quote  
     

  11. #10  
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    Quote Originally Posted by Harold14370
    There is no alloy of nickel and silver listed on the Wikipedia list of alloys.
    http://en.wikipedia.org/wiki/List_of_alloys
    There is something called nickel silver which is actually nickel and copper and does not contain silver. Could that be what you are dealing with?

    The copper wire would not have a thermocouple effect if your test leads are copper and so are the resistors. There would be no volatage at a copper-copper junction. I doubt that's it anyway since the resistivity you measured is pretty consistant and the thermocouple effect would be somewhat random depending on the temperature differentials.
    Even if it is nickel silver (which I don't think it is, but I don't have the resistors with me), I still have the problem of the "out of bounds" average. Would the molecular structure effect that?

    I found a table here, but I don't know how it applies.
    Reply With Quote  
     

  12. #11  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    Here is a table of AgNi alloys with resistivity values.
    http://www.alibaba.com/product-gs/51630546/AgNi.html
    It looks like the resistivity is supposed to be between that of silver and nickel, as one would expect. So this doesn't help explain your results.
    Reply With Quote  
     

  13. #12  
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    Quote Originally Posted by Harold14370
    Here is a table of AgNi alloys with resistivity values.
    http://www.alibaba.com/product-gs/51630546/AgNi.html
    It looks like the resistivity is supposed to be between that of silver and nickel, as one would expect. So this doesn't help explain your results.
    Hmmm... I'll take a look at the resistor tomorrow to see if I overlooked something and post the results.
    Reply With Quote  
     

  14. #13 Wire 
    Forum Ph.D.
    Join Date
    Jan 2009
    Posts
    951
    The resistance of your wire is also a function of length and diameter, you are not taking that into account! Bresides its an alloy not a composite.
    Reply With Quote  
     

  15. #14 Re: Wire 
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    Quote Originally Posted by fizzlooney
    The resistance of your wire is also a function of length and diameter, you are not taking that into account!
    Pardon? Please explain. I have to use the length and diameter (albeit area) to find the resistivity from the resistance.

    Quote Originally Posted by fizzlooney
    Bresides its an alloy not a composite.
    How does this affect the results?
    Reply With Quote  
     

  16. #15 Answer 
    Forum Freshman
    Join Date
    Feb 2009
    Posts
    8
    So it turns out William McCormick was right. When silver and nickel are combined you get a resistivity that isn't within the resistivity of the two metals. Silver and Nickel are not very commonly merged,and therefore, finding a table for the metal is near impossible.

    Thank you all for your input. I appreciate it.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •