where δ denotes a functional deriva

where δ denotes a functional derivative. In the second equality in Eq. (4.29), we used the fact that n is a functional of μ−V , by virtue of Eq. (4.28). In a strong magnetic field, this excess density moves along equipotentials with the guiding-center-drift velocity E/B (E ≡ ∂V/e∂r being the electric field). The component vdrift of the drift velocity in the y-direction (along the conductor) is