# Approximating magnetic field as the field of magnetic dipole

1. Nov 21, 2015

### phys-student

1. The problem statement, all variables and given/known data
A square wire loop of size 2a by 2a lies in the x- y plane with its center at the origin and sides parallel to the x and y axes. A counterclockwise current I runs around the loop.
(a) Find the magnetic field on the z axis. [Answer: Bz = 2μ0Ia2/[(a2 + z2)(2a2 + z2)0.5]]
(b) Show that for z/a >> 1 the field becomes that of a magnetic dipole, and find the magnetic moment.

2. Relevant equations
Field of a magnetic dipole:
B=(μ0/4πr3)*[3(m⋅r')r'-m]

where r is the distance to the field point, m is the magnetic moment, and r' is a unit vector pointing towards the field point

3. The attempt at a solution
I have already done part a and got the correct expression for the magnetic field. For part b, I said that since z/a >> 1 the term (a2+z2) could be approximated as z2 and the term (2a2+z2) could be approximated by z2. This gives me z3 in the denominator which is good since it matches the equation for the field of a magnetic dipole but I can't get the rest of the terms to match. Can someone help me? I think I've either made the wrong assumption or I'm not properly evaluating the term in the square brackets in the equation for field of a magnetic dipole.

2. Nov 21, 2015

### nrqed

For a dipole located at the origin with its axis along z, what would be the vector $\vec{m}$? For a point on the z axis, what would be the unit vector $\vec{r}'$ ?