  Using direct numerical simulations of three-dimensional hydromagnetic
turbulence, either with helical or non-helical forcing, we show that the ratio
of kinetic-to-magnetic energy dissipation always increases with the magnetic
Prandtl number, i.e., the ratio of kinematic viscosity to magnetic diffusivity.
This dependence can always be approximated by a power law, but the exponent is
not the same in all cases. For non-helical turbulence, the exponent is around
1/3, while for helical turbulence it is between 0.6 and 2/3. In the
statistically steady state, the rate of the energy conversion from kinetic into
magnetic by the dynamo must be equal to the Joule dissipation rate. We
emphasize that for both small-scale and large-scale dynamos, the efficiency of
energy conversion depends sensitively on the magnetic Prandtl number, and thus
on the microphysical dissipation process. To understand this behavior, we also
study shell models of turbulence and one-dimensional passive and active scalar
models. We conclude that the magnetic Prandtl number dependence is
qualitatively best reproduced in the one-dimensional model as a result of
dissipation via localized Alfven kinks.
