Consider steady heat transfer between two large parallel plates at constant temperatures of T$1=300$K and T$2=200$KthatareL$=1$cmapart,asshowninFigure$1.$Assumingthe surfaces to be black $($emissivity $=1),$determine the rate of heat transfer between the plates per unit surface area assuming the gap between the plates is $($a$)$filled with atmospheric air, $($b$)$evacuated$,($c$)$filled with urethane insulation, and $($d$)$filled with superinsulation that has an apparent thermal conductivity of $0\mathrm{.}00002$W$/$m $$C$.$

Conductivities: kair $=0\mathrm{.}0219$W$/$m$/$K$,$kurathane $=0\mathrm{.}026$W$/$m$/$K$,$ksuperinsulation $=210-5$W$/$m$/$K$.$