Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S16 / 2 / 2 / Solution

Choose $N_D=(4\pi /3)n_e{\lambda }_D^3$. The stipulated estimate then givesThus density cancels and conductivity grows as $T^{3/2}$ at fixed ion charge. Physically faster electrons are less effectively deflected by Coulomb encounters. Numerical factors depend on the Debye-volume convention, and a realistic collision calculation introduces a Coulomb logarithm and evolving ionization; the cancellation is approximate rather than exact for every real plasma.