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Homework Help: Material Thickness

  1. Dec 16, 2009 #1
    1. A conductive sheet of material has a CSA of 10mm x 20mm. Reisistivity of 103 (10 to the 3, 10,000) ohms M. Calculate thickness of the material required to give resistance of 5k ohms



    2. Ro = RA/l , where l would be thickness, l = RA/Ro



    3. Thickness = 5,000 x 0.2 / 10,000 = o.1m therefore 100m

    Comments appreciated
     
  2. jcsd
  3. Dec 16, 2009 #2

    Redbelly98

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    There are some problems with your calculation:
    103 is not 10,000.

    The area is not 0.2 (is that supposed to be in m2?). Try converting the mm values to meters first. Remember, 1 mm is 10-3 m.​
     
  4. Dec 18, 2009 #3
    Thanks:

    Thickness = 5,000 x 0.0002/ 1,000 = 1 x 10-3m therefore 1mm

    or do I have to sq the .ooo2 giving:

    =5,000 x 4x10-8 / 1,000 = 2 x 10-7
     
  5. Dec 18, 2009 #4

    Redbelly98

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    One of those is correct! :smile: Here are 2 questions to help you decide which:

    1. How did you get the 0.0002 value for the area, and therefore what are the units associated with this value?

    2. Are these units (from my previous question) correct units for an area, or would you need to square the units to to get an area?

    Note: correct units for area can be expressed as m2, m·m, mm2, or mm·mm.
     
  6. Dec 19, 2009 #5
    Hi, thanks for the reply, think I am having a christmas moment.
    The area should be 10mm x 20mm = 200mm in m 200 x 10-3 therefore 0.2m
    Thickness = 5,000 x 0.2/ 1,000 = 1mm

    How does this seem
     
  7. Dec 19, 2009 #6

    Redbelly98

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    I'll stop your post here and make a comment about units.

    Just as you multiplied 10 x 20 to get 200, we also need to multiply mm x mm, (since mm appears twice in the values being multiplied) and that gives us mm2. So the units will be mm2, not simply mm as you wrote.

    Since mm2 is a unit of area, we have A=200 mm2 for the area. Do not square this value to do the calculation; that would result in units of mm4 which is not a unit of area.

    Of course, if you seem to be working in MKS units, we do need to convert from mm2 to m2
     
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