1. Not finding help here? Sign up for a free 30min 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!

Find the magnetic field of an infinite uniform surface

  1. Oct 14, 2015 #1
    1. The problem statement, all variables and given/known data
    234234-png.90190.png
    This example is from 3rd edition of Griffiths' textbook. Ex. 5.8 on page 226
    I understand that by reversing the direction of the current, sign of B is switched. but i can't get it that highlighted part. and why B doesn't have z-component???

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Oct 14, 2015 #2
    Think about what would happen if we rotated the coordinate system. Originally the current was in the +x direction, but we rotate our original coordinate system -90 degrees around the z-axis so that now the flow of K is in the +y direction. The z-component of B wouldn't change, correct?

    Now if we changed the direction of the flow of current in the plane, it is analogous to the rotation above. The z-component of B can't change by simply changing where we define x and y. Therefore there is no z-component of B.
     
  4. Oct 14, 2015 #3


    2342323.PNG

    But the current flow in the infinite plane. if i draw a lot of loops as seen in the picture,
    i think B is canceled out and entire B can not be existed , what is wrong with my thought??
     
  5. Oct 16, 2015 #4
    The magnetic field beneath the plane of current travels in the opposite direction of that above the plane (use the right hand rule... above the plane, B is in the -y direction, and below the plane, B is in the +y direction). Thus when using Biot-Savart Law, the integral around the loop is the sum of the field above the plane times the length l plus the field below the plane times l (the vertical edges have no contribution). Notice that both above the plane and below the plane, the magnetic field B is in the same direction as the path ds. Therefore the fields will not cancel; they will add.
     
  6. Oct 16, 2015 #5


    thank you. i got it !
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted