This question relates to Thermodynamics. In the book Thermodynamics: An Engineer

Question

This question relates to Thermodynamics. In the book Thermodynamics: An Engineering Approach has a table at the back as shown in the image below.

Could you please help me with understanding the difference between Internal Energy (u) and Enthalpy (u). I would like to know under what circumstanses or type of question would I need to use the values for internal energy and under what circumstances or types of questions would I need to use the values for enthalpy?

I know that internal energy is an energy occuring at micrscopic scale, without considering external properties such as exerting pressure and volume of a matter. Enthalpy is a thermodynamic function in a system considering both internal and external properties of a matter. However, I am confused about which type of question require the values for internal energy and which tyes of questions require the values for enthalpy. Could you please clarify this and possibly give me examples for both?

915 APPENDIX 1 TABLE A4 Saturated water-Temperature table (Concluded) Specific volume m3/kg Internal energy Enthalpy Sat. 872.86 1723.5 2596.4 874.87 1920.0 2794.8 2.3776 4.0154 6.3930 895.38 1702.9 2598.3 897.61 1899.7 2797.3 2.4245 3.9318 6.3563 2105.9 0.001181 0.094680 918.02 1681.9 2599.9 920.50 1878.8 2799.3 2.4712 3.8489 6.3200 2319.6 0.001190 0.086094 940.79 1660.5 2601.3 943.55 1857.4 2801.0 2.5176 3.7664 6.2840 2549.7 0.001199 0.078405 963.70 1638.6 2602.3 966.76 1835.4 2802.2 2.5639 3.6844 6.2483 2797.1 0.001209 0.071505 986.76 1616.1 2602.9 990.14 1812.8 2802.9 2.6100 3.6028 6.2128 3062.6 0.001219 0.065300 1010.0 1593.2 2603.2 1013.7 1789.5 2803.2 2.6560 3.5216 6.1775 3347.0 0.001229 0.059707 1033.4 1569.8 2603. 1037.5 1765.5 2803.0 2.7018 3.4405 6.1424 3651.2 0.001240 0.054656 1056.9 1545.7 2602.7 1061.5 1740.8 2802.2 2.7476 3.3596 6.1072 3976.2 0.001252 0.050085 1080.7 1521.1 2601.8 1085.7 1715.3 2801.0 2.7933 3.2788 6.0721 255 4322.9 0.001263 0.045941 1104.7 1495.8 2600.5 1110.1 1689.0 2799. 2.8390 3.1979 6.0369 260 4692.3 0.001276 0.042175 1128.8 1469.9 2598.7 1134.8 1661.8 2796.6 2.8847 3.1169 6.0017 5085.3 0.001289 0.038748 1153.3 1443.2 2596.5 1159.8 1633.7 2793.5 2.9304 3.0358 5.9662 5503.0 0.001303 0.035622 1177.9 1415.7 2593.7 1185.1 1604.6 2789.7 2.9762 2.9542 5.9305 946.4 0.001317 0.032767 1202.9 1387.4 2590.3 1210.7 1574.5 2785.2 3.0221 2.8723 5.8944 6416.6 0.001333 0.030153 1228.2 1358.2 2586.4 1236.7 1543.2 2779.9 3.0681 2.7898 5.8579 6914.6 0.001349 0.027756 1253.7 1328.1 2581.8 1263.1 1510.72773.73.1144 2.7066 5.8210 7441.8 0.001366 0.025554 1279.7 1296.9 2576.5 1289.8 1476.9 2766.7 3.1608 2.6225 5.7834 7999.0 0.001384 0.023528 1306.0 1264.5 2570.5 1317.1 1441.6 2758.7 3.2076 2.5374 5.7450 8587.9 0.001404 0.021659 1332.7 1230.9 2563.6 1344.8 1404.8 2749.6 3.2548 2.4511 5.7059 9209.4 0.001425 0.019932 1360.0 1195.9 2555.8 1373.1 1366.3 2739.4 3.3024 2.3633 5.6657 9865.0 0.001447 0.018333 1387.7 1159.3 2547.1 1402.0 1325.9 2727.9 3.3506 2.2737 5.6243 315 10,556 0.001472 0.016849 1416.1 1121.1 2537.2 1431.6 1283.4 2715.0 3.3994 2.1821 5.5816 320 11,284 0.001499 0.015470 1445.1 1080.9 2526.0 1462.0 1238.5 2700.6 3.4491 2.0881 5.5372 325 12,051 0.001528 0.014183 1475.01038.5 2513.4 1493.4 1191.0 2684.3 3.4998 1.9911 5.4908 330 12,858 0.001560 0.012979 1505.7 993.5 2499.2 1525.8 1140.3 2666.0 3.5516 1.8906 5.4422 335 13,707 0.001597 0.011848 1537.5945.5 2483.0 1559.4 1086.0 2645.4 3.6050 1.7857 5.3907 340 14,601 0.001638 0.010783 1570.7 893.8 2464.5 1594.6 1027.4 2622.0 3.6602 1.6756 5.3358 345 15,54 0.001685 0.009772 1605.5 837.7 2443.2 1631.7 963.4 2595.13.7179 1.5585 5.2765 350 16,529 0.001741 0.008806 1642.4 775.9 2418.3 1671.2 892.7 2563.9 3.7788 1.4326 5.2114 355 17,570 0.001808 0.007872 1682.2 706.4 2388.6 1714.0 812.9 2526.9 3.8442 1.2942 5.1384 360 18,666 0.001895 0.006950 1726.2 625.7 2351.9 1761.5 7201 2481.6 3.9165 1.1373 5.0537 365 19,822 0.002015 0.006009 1777.2 526.4 2303.6 1817.2 605.5 2422.7 4.0004 0.9489 4.9493 370 21,044 0.002217 0.004953 1844.5 385.6 2230.1 1891.2 443.1 2334.3 4.1119 0.6890 4.8009

Explanation / Answer

Basically Internal energy means the energy which hold by the fluid when it is at rest or the energy because of
temperature.
Apart from this we use internal energy in those problems where constant volume condition persists.
For. eg. If there is a gas entrapped in piston and cylinder arrangement and we are heating the gas without moving the piston then the internal energy of the gas will increase.
Enthalpy is defined as = U+PV
where PV denotes the flow work that means we are going to use enthalpy when fluid is flowing.
For e.g in turbine , compressor etc.
We use the enthalpy becasue it include the energy both becasue of temperature as well becasue of flow.
For turbine generally we written
h1 = h2 + W
where h is enthalpy and W is work done.
subscripts 1 denotes the inlet and 2 for outlet.

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