Problem No 3 Determine or estimate the net heat flux density $P$ between two parallel plates at distance $L$ from each other, which are at temperatures $T_1$ and $T_2$, respectively.  The space between the plates is filled with a monoatomic gas of molar density $n$ and of molar mass $M$. You may use the following approximations: 1. The gas density is so low that the mean free path $\lambda \gg L$; 2. $T_1 \gg T_2$. 3. When gas molecules bounce from the plates, they obtain the temperature of the respective plates (for instance, this will happen if they are absorbed/bound for a short time by the molecules of the plate, and then released back into the space between the plates). 4. You may neglect the black body radiation. 5. "Estimate" means that the numeric prefactor of your expression does not need to be accurate.