A post$-$tensioned prestressed concrete beam of rectangular section, $250$ mm x $450$ mm in size, is simply supported over a span of $6$ m$.$ The beam is prestressed by a parabolic cable of area $600$ mm$?$ carrying an effective force of $500$ kN$,$ at an eccentricity of $150$ mm at the centre of span, and concentric at supports. The permanent load on the beam $($inclusive of self$-$weight$)$ is $35$ kN$/$m and the total load $($inclusive of non$-$permanent live load$)$ is $45$ kN$/$m$.$ Determine the difference in the instantaneous curvatures undertotal and permanent loads. Take Es $=210$ GPa and E$,=35$ GPa. f$,=4$ MPa