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Question regarding Electrical conduction(Resistance)

  1. Jun 19, 2005 #1
    (a) Find the resistance between two opposite edge faces of a piece of square carbon film of thickness 5.0 x 10^-7 n,if the resistivity of carbon is 4.0 x 10^-5 ohm meter.

    (b) A carbon film of the same thickness is formed on an insulator rod of diameter 3.0m.What is the length of the rod so that the carbon film on its curve surface has a resistance of 100 ohm ?

    Question (a) and (b) is continuous and I have no problem with question (a).My problem is question (b) and I can't really understand with the question and I can't imagine out the figure.

    I want to ask whether there is any relationship of answers in question (a) in solving the qiestion (b) and I really need someone to explain to me the phrase 'curve surface' .

    Thanks for anybody that spend some time on this question and I really need explaination for this question.
  2. jcsd
  3. Jun 19, 2005 #2
    Think of a cylinder, the curved surface is well, the surface that isn't the ends.

    Could you not just use the formula, [tex]R = \frac{\rho l}{A}[/tex] and solve for l? R is the resistance, [itex]\rho[/itex] is the resistivity and A is the cross sectional area.
  4. Jun 22, 2005 #3
    Solution and some suggestion to solve the question

    I have try to solve this question for a long time and I really can't find the correct answer.I have my doubt,solution to the question in the attachment that followed.Thanks for anybody that spend some time on this qustion.

    Attached Files:

  5. Jun 22, 2005 #4


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    Homework Helper

    Judging by your attached document, the correct diameter of the rod in part b) is 3.0 mm, rather than 3.0m. Using 3.0 mm the given answer is realized by following exactly the same approach I suggested in your previous problem. Find the resistance from one end to the other of a thin walled cylinder by finding the cross-sectional area of the cylinder walls, and the length of the cylinder. Here you know the resistance and must solve for the length.
  6. Jun 23, 2005 #5
    Thanks for your help OlderDan and I really appreciate it because I have really understand with the question and thanks for your explaination.
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