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Homework Help: Pretty easy ENGE question

  1. Sep 17, 2007 #1
    Pretty easy Circuit question...

    So, im in my first year ENGE Class second week in, and im given this diagram

    1. The problem statement, all variables and given/known data
    Basically Im given this Diagram (attachment)

    And told to find the total Resistivity if using AWG 18

    So, i got

    P=800W
    E=100V
    R=E^2/P = 15.125 Ohms

    2. Relevant equations

    So for the Resistivity R = p l/a.

    3. The attempt at a solution

    But im completly unsure how to get p l & a.

    This isn't even a assignment question it was a example done in class on the board, but my instructor just randomly says everything and doesnt explain anything.

    So when alls said and done I have

    R = 4.13
    I = 110/23.5= 4.7 Amps

    But absolutly no clue as to how to get it :(
     

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    Last edited: Sep 17, 2007
  2. jcsd
  3. Sep 18, 2007 #2

    Astronuc

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  4. Sep 18, 2007 #3
    So,
    p = resistivity of copper = 10.37 CM
    l = length of wire = 200m / 0.3048 = 656.17 ft
    a = area of 18 AWG = 1624 CM

    R = 10.37 * 656.17 / 1624 = 4.18

    So then to get I i got

    I = E / R
    I = E / R1 + R2 + R3
    I = 110V / 4.18 + 4.18 + 15.125
    I = 110/23.485
    I = 4.68

    Is there any way i could have done this without having to convert the meters to feet?
     
  5. Sep 18, 2007 #4

    Astronuc

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  6. Sep 18, 2007 #5
    cm is a unit of length
    CM is circular mil

    you said they should all be in my text. Everything in my text is based off CM, i dont know why but it is >_<
     
  7. Sep 18, 2007 #6
    A circular mil(or CM) is diameter is mills squared

    OR

    1 mil = 0.001 in.

    That's what my book says....

    Your resistivity(or rowe) should be in Ohms * CM / ft.
     
  8. Sep 18, 2007 #7
    yah that sounds right, i just dont know how to make symbols on here so well... i didnt @_@
     
  9. Sep 19, 2007 #8

    Astronuc

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    Ah, my mistake. I should have recognized it, but its a unit I now use seldom. 1624 cmil is correct.

    If resistivity is given in SI/metric units, one does not have to use feet. The problem in the US is that most engineering is still taught with English/British units, and there are sometimes mixed units.

    I use both at work, but prefer metric.
     
  10. Sep 19, 2007 #9
    I agree.......prefer metric to work problems with.....but still understand english better, when someone yells out a distance or figure or something....
     
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