Filament torsional stiffness
= Filament torsional stiffness
{title2=$C=G_sJ_p$}
For an isotropic elastic rod with shear modulus $G_s$, polar second moment of area $J_p$, and twist rate $\Omega$, the torque is $C\Omega$ and the energy per length is $C\Omega^2/2$. For a circular section of radius $r$, $J_p=\pi r^4/2$. This material twist stiffness is distinct from the energy of bending filament centerlines into a helix.