Determine the heat exchange per m² of area a, between the surface specified below if surface 1 is at 1000 k while surface 2 is at 400 k. all surfaces are black. (i) large parallel planes (ii) long coaxial cylinders of 0.5 m and 1 m dia with inner cylinder hotter. (iii) concentric spheres of 1 m and 2 m dia with inner surface hotter. (iv) parallel planes of 2 m x 2 m size placed symmetrically 1 m apart.