Two spheres of the same material have radii 1 m and 4 m and temperatures 4000 K and 2000 K respectively. The energy radiated per second by the first sphere is

The energy radiated per second E, by a body of surface area A, at temperature T K is given byE=kσAT4  (Stefan's law)where   k = relative emissivity           E1 = energy radiated per second byfirst sphere =kσ4π(1)2(4000)4=kσπ1024×1012 joule Similarly,E2=kσ4π(4)2(2000)4=kσπ1024×1012 jojle ∴ E1=E2