上游充通大￥ SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lectures 7-8 Spring,2018 Prof.,Dr.Yonghua HUANG 强 A月n是n http://cc.sjtu.edu.cn/G2S/site/thermo.html 1日G
Engineering Thermodynamics I Lectures 7-8 Spring, 2018 Prof., Dr. Yonghua HUANG http://cc.sjtu.edu.cn/G2S/site/thermo.html
Closed System Energy Balances Total energy Total energy Change in the total entering the system energy of the system En-Eout=△Esystem 1st law (Qin-Cou)+(Win -Wou) AE=E2-E,=△U+△KE+APE Only two types of interactions are =m(4-4)+5m(c-c2)+mg(32-2) possible between a closed system and its surroundings Surroundings System W 上降文通大学 March 9,2018 2 SHANGHAI JIAO TONG UNIVERSITY
March 9, 2018 2 Closed System Energy Balances 2 1 2 2 2 1 2 1 2 1 KE PE 1 ( ) ( ) ( ) 2 E E E U m u u m c c mg z z Only two types of interactions are possible between a closed system and its surroundings 1 st law
Closed System Energy Balances (gn-Qo)+(Wn-Wou）=△Fsystem change in the amount net amount of energy net amount of energy of energy contained transferred in across transferred out across within a system the system boundary by the system boundary during some time heat transfer during by work during the interval the time interval time interval △E=Q-W 1st law AE=E2-E,=△U+△KE+△PE i.e.,the energy of a system (U+PE+KE)changes due to work =m(,-4)+5m(c-c)+mg(32-2) and heat transfers between the system and surroundings 上降文通大学 March 9,2018 3 SHANGHAI JIAO TONG UNIVERSITY
March 9, 2018 3 Closed System Energy Balances i.e., the energy of a system (U+PE+KE) changes due to work and heat transfers between the system and surroundings 1 st law 2 1 2 2 2 1 2 1 2 1 KE PE 1 ( ) ( ) ( ) 2 E E E U m u u m c c mg z z
Important Notes on 1st Law 1.E is always measured relative to reference point! ··Reference plane for PE 。·Reference frame for KE .Reference state for U (i.e.U=0@reference state) 2.Changes in E are important,not total values of E 3.AE depends only on beginning and end states(property) 4.Q and W depend on process path (could get to the same end state with different combinations of Q and W) 国 5.Energy balance for a particular problem will depend on the system that you select >YOU MUST ALWAYS SPECIFY YOUR SYSTEM IN ORDER TO APPLY THE 1ST LAW 6.Total heat transfer is the summation of all heat transfers across the system boundary Q-∑Q, 7.Total work is the summation of all work transfers across the system boundary W=∑W 上游充通大学 March 9,2018 4 SHANGHAI JLAO TONG UNIVERSITY
March 9, 2018 4 Important Notes on 1st Law 1. E is always measured relative to reference point! • • Reference plane for PE • • Reference frame for KE • • Reference state for U (i.e. U = 0 @ reference state) 2. Changes in E are important, not total values of E 3. ΔE depends only on beginning and end states (property) 4. Q and W depend on process path (could get to the same end state with different combinations of Q and W) 5. Energy balance for a particular problem will depend on the system that you select YOU MUST ALWAYS SPECIFY YOUR SYSTEM IN ORDER TO APPLY THE 1ST LAW 6. Total heat transfer is the summation of all heat transfers across the system boundary 7. Total work is the summation of all work transfers across the system boundary
Forms of the 1st Law for Closed Systems 1.Finite process △E=Q-W 2.Differential form dE=δQ-δW ©3.Rate basis dE =9-W dt 国4.Steady-state o-w *steady-state>state does not change with time** 上游充通大学 March 9,2018 5 HANGHAI JIAO TONG UNIVERSITY
March 9, 2018 5 Forms of the 1st Law for Closed Systems 1. Finite process ΔE = Q −W 2. Differential form dE =δQ −δW 3. Rate basis 4. Steady-state ** steady-state state does not change with time**