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Resistance, cross-sectional area of long coated wire

  1. Feb 27, 2017 #1
    1. The problem statement, all variables and given/known data
    upload_2017-2-27_16-26-36.png
    upload_2017-2-27_16-27-11.png

    2. Relevant equations


    3. The attempt at a solution
    I attempted all of the parts:
    upload_2017-2-27_16-31-15.png
    upload_2017-2-27_16-31-49.png


    I think I did the right things for a-b-c-d, but I am pretty unsure about e & f. Can someone verify if my logic is right? Thanks.
     
  2. jcsd
  3. Feb 27, 2017 #2

    haruspex

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    You have d wrong. The formula you quoted is not quite right.
     
  4. Feb 27, 2017 #3
    Ah I see it should be 1/Req. If I change that, will the following steps be right assuming I use that value ?
     
  5. Feb 27, 2017 #4

    haruspex

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    Your method in e) is right.
    In f, you seem to have assumed all of I is in the inner wire when calculating the current density there.
     
  6. Feb 27, 2017 #5

    haruspex

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    If you read posts 2 and 3, you'll see that d) was wrong, and this led to the wrong expression in e.
     
  7. Feb 27, 2017 #6
    How do I calculate it without assuming this? By the way, this isn't homework, I am just working on problems to prepare for an exam tomorrow.
     
  8. Feb 27, 2017 #7

    haruspex

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    One way would be to suppose the potential applied to the wire is V and compute what current would flow in each component.
     
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