815.6 A new concept: entropy Example 1: the change of entropy in the process of adiabatic expansion of ideal gas. System Stopcock closed °。° Vacuum (a)Initial state i () Final state Solution:0=W+AU=0 W=0 AU=0 815.6 A new concept: entropy This process is not a reversible process, how to calculate the entropy Because the entropy is a state variable, we can use the reversible isothermal process to calculate the entropy. AS=nc;,In2+nRIn′ =nRIn nRIn 2 ∠S>0 1818 Example 1: the change of entropy in the process of adiabatic expansion of ideal gas. Solution: Q = W + ∆U = 0 W = 0 ∆U = 0 §15.6 A new concept: entropy This process is not a reversible process, how to calculate the entropy? Because the entropy is a state variable, we can use the reversible isothermal process to calculate the entropy. ln ln 2 ln ln nR V V nR V V nR T T S nc i f i f i f V = = ∆ = + ∆S > 0 §15.6 A new concept: entropy