**1. The problem statement, all variables and given/known data**

Find Bz(0,0,z1), the z component of the magnetic field at the point P located at x=y=0,z=z1 from the current I flowing over a short distance dl = |dl|*j located at the point rc = x1*i.

**2. Relevant equations**

Biot-Savart law:

(μo/4∏)*[I*(dL x r)/(|r|^2)]

**3. The attempt at a solution**

I concluded that the radius would be √(( x1 )2 + (z1 )2) and submitted:

(μo/4∏)*[I*dL/√(( x1 )2 + (z1 )2)]

which was wrong. Then I remember that the equation called for r^2 not r, so I submitted:

(μo/4∏)*[I*dL/(( x1 )2 + (z1 )2)]

which was also wrong. I thought that the cross product would be even but just in case I tried:

(μo/4∏)*[I*-dL/(( x1 )2 + (z1 )2)]

which was still wrong.

Am I getting the radius wrong? I’m not quite sure what’s going on.

http://ift.tt/1fmlnUw