blackbody radiation is electromagnetic radiation kept in a closed cavity in eguilibrium with the walls at some temperature T.one finds that , if the volumeof the cavity is increased for fixed temperature T . more radiation is generated but the (energy /volume ) e(T) remains constant. thus , the internal energy has the forom U=Ve( T) ,WHERE e(T) DEPENDS onlly ..... one finds that radiation exert pressure p=( 1/3) e(T) on the walls the temperature depenence on the internal energy (U) pressure (p) , and the entropy