This document discusses a formula for calculating inductance in applied electricity. The formula is L= N2(μ0)(μr)al, where L is inductance, N is the number of turns in the coil, μ0 is the permeability of free space, μr is the relative permeability, a is the length of the coil, and l is the cross-sectional area of the coil. The document notes that the cross-sectional area l can be calculated using the formula πr2.