  Starting from the Boltzmann equation in the relaxation time approximation and
employing a Chapman-Enskog like expansion for the distribution function close
to equilibrium, we derive second-order evolution equations for the shear stress
tensor and the dissipative charge current for a system of massless quarks and
gluons. The transport coefficients are obtained exactly using quantum
statistics for the phase space distribution functions at non-zero chemical
potential. We show that, within the relaxation time approximation, the
second-order evolution equations for the shear stress tensor and the
dissipative charge current can be decoupled. We find that, for large values of
the ratio of chemical potential to temperature, the charge conductivity is
small compared to the coefficient of shear viscosity. Moreover, we show that in
the relaxation-time approximation, the limiting behaviour of the ratio of heat
conductivity to shear viscosity is qualitatively similar to that obtained for a
strongly coupled conformal plasma.
