Electromagnetic radiation at temperature t in an enclosure of volume vhas pressure p and energy density u given by и —а т* р 3 1sat" and where a is a constant. (a) write down the ftr and use it to show that as = 4avt2; дт (i) as 4 =-al 3 (ii) ду hint: write e = vu and hence find an expression for de in terms of du and dv; note that uis a function only of t 5 marks] (b) use the results of part (a) to show that the entropy s of the radiation is given by: (i) 4 s = (ii the helmholtz free energy of the radiation is given by f = -pv.