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!

Ampere's Law

  1. Jun 6, 2007 #1
    1. The problem statement, all variables and given/known data

    What is the magnitude of the magnetic field at point P if a=R and b=2R (see attached image)

    2. Relevant equations
    Ampere's Law where integral of B ds over a closed surface = μ0I where μ0= 4πx10-7

    3. The attempt at a solution

    I used the formula and found after integrating (ds becomes 2*Pi*2R the circumference of the circle) that for the outer circle (radius 2R) B= μ0I/4*Pi*R

    I then thought that the current through the inner circle would be in the same ratio as that of the areas of the two circles.
    So inner current/outer current = Pi*(2R)^2/Pi*R^2
    so inner current = 4I

    i then subsituted this into my integral for the inner circle using Ampere's Law and found B for the inner circle to be equal to μ0*4I/2*Pi*R which simplifies to μ0*2I/Pi*R

    the next step is what i'm having problems with as i'm not 100% certain as to what to do with these two values of B i have found. current flows in the same direction in both so i thought id have to add one B to the other but none of these answers show up in the five multiple choice options so i must have gone wrong somewhere. have i perhaps missed a crucial step along the way or was my working out in the first place off the mark? any advice would be great

    cheers :smile:

    Attached Files:

  2. jcsd
  3. Jun 6, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I can't quite follow the path that you are integrating over. Rather that attempt to use Ampere's law, why not use the Biot_Savart law to calculate the magnetic field at the center of a current loop. Unless you happen to the know the formula already.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Ampere's Law
  1. Ampere's law (Replies: 2)

  2. Ampere's Law (Replies: 1)

  3. Ampere's Law (Replies: 1)