Calculating the magnetic field

[fluidistic]

Homework Statement

A current whose density is \vec J circulate through a conductor lattice infinitely long but with a depth a. The current circulate in a direction which is parallel to the direction in which the lattice is infinitely long.
1)Calculate the magnetic field \vec B in a point P situated at a distance d from the center of the lattice.

Homework Equations

Ampère's law I guess.

The Attempt at a Solution

I had a hard time understanding the situation, but I finally think I got it.
However here come my problems. \oint \vec B d \vec l = \mu _0 I = \mu _0 \int _S \vec J d \vec S, but I don't know what surface I should take as S.
I've found \vec B = \mu _0 \vec J a. Which is obviously wrong since I think I should get something like \frac{\text{constants}}{r}.
Any help is greatly appreciated!

 

[ApexOfDE]

imo, in this case, I = J.A = Ja^2

From Ampere's law:

<br /> \oint \vec B d \vec l = B . 2\pi d<br /> <br /> ==&gt; B = Ja^2 / 2\pi d<br />

p/s: I am not sure that i understand your problem completely :(

 

[fluidistic]

imo, in this case, I = J.A = Ja^2

From Ampere's law:

<br /> \oint \vec B d \vec l = B . 2\pi d<br /> <br /> ==&gt; B = Ja^2 / 2\pi d<br />

p/s: I am not sure that i understand your problem completely :(

Thanks for the help. I think the problem is like this : An infinite conductor plane with a depth of a. The current flows in a direction. I have to calculate the magnetic field inside the lattice (from a depth of -\frac{a}{2} if I consider the origin as being just over the lattice) and outside it.
So I don't think that \oint d \vec l =2\pi d. I'm unsure though.