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Homework Help: Efield in Composite WirePls help

  1. Feb 12, 2008 #1
    Efield in Composite Wire..Pls help!!

    1. The problem statement, all variables and given/known data
    A 2.0m length of wire is made by welding the end of a 1.2m long silver wire to the end of an 0.8m long copper wire. Each piece of wire is 0.60mm in diameter. The wire is at room temperature. A potential difference of 5.0V is maintained between the ends of the 2.0m composite wire. What is the magnitude of E in the copper?



    2. Relevant equations
    E='roe'J , 'roe' is resistivity
    J=I/A, A is cross sectional area
    I= 45amperes
    A= 2.827*10^-7 sqm

    Resistivity of copper= 1.72*10^-8
    Resistivity of silver= 1.47*10^-8

    3. The attempt at a solution
    I tried to calculate J first by substituting in values of I and A using the formula J=I/A and multiplying by 'roe'_copper but i still cant get the right answer!! I would like to ask the concept behind the E of a composite wire too..
     
  2. jcsd
  3. Feb 12, 2008 #2

    Kurdt

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    First, if you could post what you've done it might be easier to spot a mistake. Secondly what value were you using for I?
     
  4. Feb 12, 2008 #3
    Mmm.. i found out that I=45amperes and A=2.827*10^-7 sqm and found out that J=1.59*10^8. To find E in copper section i multiplied this J by 'roe'_copper= 1.72*10^-8 to get E=2.74V/m. But the answer is wrong. thanks!
     
  5. Feb 13, 2008 #4

    Kurdt

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    When I work out the current I get something different. I take it you found current by:

    [tex] I = \frac{VA}{\rho l}[/tex]
     
    Last edited: Feb 13, 2008
  6. Feb 13, 2008 #5
    Yup i found it by using this formula whereby i added the resistance of each component using their respective length and 'roe' since they are in series before dividing 5V with the total resistance to get 45amperes.
    i obtained J by I/A, where I=45amperes and A= 2.827*10^-7 sqm to get J=1.59*10^8 A/m^2.
    I cant get the idea of finding E using E= 'roe'_copper*J to find E in copper section though.
     
  7. Feb 13, 2008 #6

    Kurdt

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    It could just be that you are using the wrong resistivity for silver. It is listed as 1.59x10-8 elsewhere.
     
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