Dipole levitation force above a superconducting plane
= Dipole levitation force above a superconducting plane
{title2=$F_z$}
For a small fixed-current loop with moment $m=I\pi R^2$ and $R\ll h$, the image field gives upward <force> $3\mu_0m^2/(32\pi h^4)$. Differentiating the external field holds the image position fixed; moving both source and image in that derivative would double the <force> incorrectly.