= Force per unit length
{title2=$\mathbf f=d\mathbf F/d\ell$}
A <force per unit length> describes a distributed force on a filament and has units $\mathrm{N\,m^{-1}}$. For a charged filament with <linear charge density> $\lambda$ and longitudinal current $I$, the external <Lorentz force> is $\mathbf f=\lambda\mathbf E+I\widehat{\mathbf t}\times\mathbf B$. Length, density and force must all refer to the same measurement frame. An infinite uniform filament generally has a finite force density but no finite total force.
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