  We invoke the concepts of magnetic boundary layer and magnetic Rayleigh
number and use the magnetic energy dissipation rates in the bulk and the
boundary layers to derive some scaling laws expressing how Nusselt number
depends on magnetic Rayleigh number, Prandtl number and magnetic Prandtl number
for the simple case of turbulent magnetohydrodynamic Rayleigh-Benard convection
in the presence of uniform vertical magnetic field.
