1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Magnetic field

  1. Aug 10, 2009 #1
    1. The problem statement, all variables and given/known data

    the current density is given by J(p) = (I/pi) * p^2 * e^(-p^2) in z direction
    The question is to first show that the cureent flowing through the wire is 'I' and then to find then to find the B field.

    2. Relevant equations

    stokes theorem.
    integral of B.dl = I

    I = J.dS

    3. The attempt at a solution

    i can find the magnetic field. however i am stuck on the first part that requires me to proof the total current is I.
    I set up the problem in cylindrical coordinates and tired the double integration between 0 to 2pi and o to a(arbitsry distance) however this does not give the correct result. plese point me in the right direction
    Last edited: Aug 10, 2009
  2. jcsd
  3. Aug 10, 2009 #2


    User Avatar
    Homework Helper

    Hi iontail,

    Can you verify your equation? You have:

    J(p) = (I/pi) * p^2 * e^(-x^2)

    Is that supposed to be p^2 in the exponential instead of x^2? Also, can you show your work for the integration?
  4. Aug 10, 2009 #3
    sorry about that it is supposed to b p^2, a typo. I tried integrating by parts on the ,p, terms and using the cylindrical coordinates formula for for dS. I get e^-p(p+3) as result
  5. Aug 10, 2009 #4
    i updated the question as well
  6. Aug 10, 2009 #5


    User Avatar
    Homework Helper

    I don't believe the integral should be cut off at an arbitrary limit (like you are doing with the quantity a); instead the radial varible p should be integrated from 0 to infinity. If you are still getting the wrong answer, please post the integration steps you are taking.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook