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Calculating the resistance from R = pl/A

  1. Jun 5, 2011 #1
    1. The problem statement, all variables and given/known data

    A laboratory lead consists of 16 strands of fine copper wire twisted together. Each strand is 30 cm long with a diameter of 0.15 mm. Calculate the potential difference across the lead when it is carrying a current of 2.0 A.
    (The resistivity of copper = 1.7 × 10^-8 Ohm*m)

    2. Relevant equations
    What I would do is R = pl/A and then multiply by 2
    BUT I can't find R

    3. The attempt at a solution

    (the answer is. apparantly, 0.036 V but I cant see how)

    I said R = [(1.7*10^-8)*16*0.3)/(pi*0.15/20000)^2)
  2. jcsd
  3. Jun 5, 2011 #2


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    Homework Helper

    Putting strands of wire in parallel decreases the total resistance, because the electrons now have many more paths to follow. 16 strands would decrease the resistance by a factor of 16.

    Also, why are you dividing 0.15 by 20,000? Why is pi being squared? The area of a circle of pi*r^2, not (pi*r)^2.
  4. Jun 5, 2011 #3
    er, sorry abou the pi^2 bad use of brackets

    it was meant to be pi * (0.15/2000)^2

    again sorry,

    Question: it is still ONE strand of wire, just twisted, are you sure total R decreases
  5. Jun 5, 2011 #4
    seems you are correct - I now get the correct answer
  6. Jun 5, 2011 #5
  7. Jun 5, 2011 #6
    oh, don't worry I get it
    first example 16 seperate strands
    second example one strand with 8 branches
    its ok
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