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Homework Help: Inverse relationship between radius and resistance

  1. Jul 10, 2012 #1
    1. The problem statement, all variables and given/known data

    We did an experiment using resistance putty and a multimeter. We changed the radius of the putty but kept the length the same. We recorded the resistances at different radius'. I know that physics theory says that resistance is inversely proportional to cross-sectional area of wire. For example, if the radius doubles, the cross sectional area increases four times and the resistance decreases by four times.

    2. Relevant equations

    radius (m) resistance (Ω)
    0.0015 601
    0.003 270
    0.0075 72.4
    0.0115 50
    0.015 43

    3. The attempt at a solution

    However my results don't follow the theory at all. the resistance decreases by closer to 2 or 3 times when the radius doubles.

    I'm kind of stumped by this. Thanks for any help or pointers in the right direction :)
  2. jcsd
  3. Jul 10, 2012 #2


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    Staff Emeritus
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    Gold Member

    Every experimental result has an error. What is the uncertainty in your measurement of the radius? How precisely were you able to control the length? What do you think the error in this was? Are these errors able to account for the discrepancy?

    P.S. the plural of radius is radii
  4. Jul 10, 2012 #3
    I did a curve fit to the data (using a graphics package), and the resistance decreased with the radius to the ~ 1.2 power. This is much lower sensitivity than to the 2.0 power. So Navras' question still stands: What is the reason for the lower sensitivity?
  5. Jul 10, 2012 #4


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    How should I know? I was just trying to get him to take experimental error into account, and to determine whether or not it could account for the discrepancy. If it can't, then I don't know what to say other than that our assumptions about the properties of the system must not have been in line with the actual properties.
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