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Homework Help: Electric field strength of wire problem

  1. Aug 4, 2008 #1
    The electric field strength 5 cm from a very long charged wire is 2000N/C . What is the field strength 10 cm from wire.
    A)1x10^-4 N/C
    B)1x10^3 N/C
    C)9.9x10^7 N/C
    D) None of these

    I have
    E=kq/r^2 and infinite line charge E=(2k[tex]\lambda[/tex])/R

    2000 * .05 m^2 /K =q
    q=5.56 E-8
    k5.56E-8/.^21 m = 500 N/C so my answer would be D

    But its wrong so where am I off? Thanks for the help
    Last edited: Aug 4, 2008
  2. jcsd
  3. Aug 4, 2008 #2


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    The field from a line charge and the field from a point charge have completely different forms. Don't mix them. Why don't you solve for lambda instead of q?
  4. Aug 4, 2008 #3


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    You have an infinite line of charge, so use that formula. The formula you are using is for point charges, not lines of charge.

    (Edit: Dick beat me to it.:smile:)
  5. Aug 4, 2008 #4
    okay so
    E = k2lambda/r
    (2000*.05)/2k =5.56x10^-9 = lambda
    plug this in
    5.56x10^-9 * k * 2 / .1 = E = 9.8x10^4 still not any of the answers
  6. Aug 4, 2008 #5


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    Put units on numbers, ok? I get 999.41 V/m. The difference between that and one of your numbers is pure round off. BTW, you didn't even have to solve for lambda. You are trying to compare 2*k*lambda/(0.05m)=2000 V/m with 2*k*lambda/(0.1m). Everything else is the same, but the denominator doubled. What's the exact answer?
  7. Aug 4, 2008 #6
    Oh okay I see it. sorry I converted everything to meters. So then my answer would be the same as yours 999.99 which would be 1 x 10^3 . Thank you guys so much. I was having trouble with this because the logic just didnt make any sense. I gather I could've conceptually answered the problem but I wanted to know the math behind it. Thanks again.
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