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 force on the loop

  1. Apr 13, 2016 #1
    1. The problem statement, all variables and given/known data

    Rigid contour of an isosceles triangle and a long line cable lie in a plane and are located in vacuum . Determine the intensity of the magnetic force F acting on the loop. Current I = 100 A.
    free upload image
    Correct answer:
    F = 386 mN.
    I am getting 2 mN.
    Notice that I found dS by splitting while triangle in two triangles.
    2. Relevant equations

    3. The attempt at a solution
    upload gambar
  2. jcsd
  3. Apr 13, 2016 #2


    User Avatar
    Gold Member

  4. Apr 13, 2016 #3

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    How is the derivative in the last line not zero?
    Does the magnetic flux through the loop change with time?

    In the problem description, the wire loop is not carrying a current.
    ds is a line element and dS is a surface element... not always clear which you intend.
    ie. The area element between x and x+dx, inside the triangle, is dS=2y(x)dx : y=(x-a)/2
    So dS=(x-a)dx ... that what you mean?
    Then: ##\vec B = \frac{\mu_0 I}{2\pi x}\hat k## and ##d\Phi = \vec B\cdot d\vec S##
    ... means the flux density $$\Phi = \frac{\mu_0 I}{2\pi}\int_a^{2a}\frac{x-a}{x}\text{d}x$$ ... which is where you ended up.

    What about ##\vec F= I\oint (d\vec s\times \vec B)## ?
  5. Apr 14, 2016 #4
    upload image free
    You mean something like this , still I don't understand how there can be force on the loop if there is no current I flowing through loop.

    image sharing

    By the way I think I messed something up because a didn't cancle out in F.Guess because I didnt do the loop integral the right way
    Last edited: Apr 14, 2016
  6. Apr 14, 2016 #5
  7. Apr 16, 2016 #6
    Please guys help me I am trying to figure out the whole day how my solution is incorrect, in the solution there is mi zero * I^2 / (2*pi) [1/2 - ln2] and I am having 1 - ln2 in the brackers , where did he get that one half from.
    img upload
    free photo upload
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted