Magnetic Field due to a current carrying wire

[irivce]

Homework Statement

A 2.0 cm length of wire centered on the origin caries a 20 A current directed in the positive y direction. Determine the magnetic field at the point x = 5.0 m on the x-axis.

Homework Equations

B = uI/2piR

The Attempt at a Solution

Using the equation above I just plugged in 5m for R and 20A for I and I got 8e-7 T. I was looked up current in a straight wire and found a calculator in hyperphysics that gave me 8e-7 as well, but the answer that was given to me is 1.6nT in the negative z direction.

 

[TSny]

The formula B = $\mu_0$I/(2$\pi$R) is only for a straight wire that is much longer than the distance from the wire to the field point. You have the opposite case where the wire is much shorter than the distance from the wire to the field point.