Reference states and reference values Recall reference values for PE,KE u,h,s cannot be measured directly;p,T,v can be measured directly Changes in u,h,s computed using derived relations with p,T,v Note:only changes can be computed,not u,h,s,at a specific state But fortunately in 1st and 2nd laws only changes in these properties are needed arbitrary 国 We choose a convenient reference state and set properties to zero at that state ● For water,u=0,s=0,at 0.01C for Sat.Liq.state; h=u+pv;could have negative values relative to reference; For ammonia,propane,refrigerants,h =0 sat.liq.at-40C; u=h-pv Ch available in table but not u) Different ref.state for different substances;ref.state cancels out 上游通大学 March 14,2019 4 SHANGHAI JLAO TONG UNIVERSITYMarch 14, 2019 4 Reference states and reference values Recall reference values for PE, KE u,h,s cannot be measured directly; p,T,v can be measured directly Changes in u,h,s computed using derived relations with p, T, v Note: only changes can be computed, not u, h, s, at a specific state But fortunately in 1st and 2nd laws only changes in these properties are needed We choose a convenient reference state and set properties to zero at that state For water, u = 0, s = 0, at 0.01ºC for Sat. Liq. state; h=u+pv; could have negative values relative to reference; For ammonia, propane, refrigerants, h =0 sat. liq. at -40ºC; u=h-pv (h available in table but not u) Different ref. state for different substances; ref. state cancels out arbitrary