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First Law of Thermodynamics

First Law of Thermodynamics. Some Experiments. Joule’s falling weight and paddle wheel Joule’s Waterfall Experiment Count Rumsfeld’s Cannons Hammering a Penny Conclusion: Both work and heat are energy transfers that can change the internal energy of a system.

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First Law of Thermodynamics

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  1. First Law of Thermodynamics

  2. Some Experiments • Joule’s falling weight and paddle wheel • Joule’s Waterfall Experiment • Count Rumsfeld’s Cannons • Hammering a Penny • Conclusion: Both work and heat are energy transfers that can change the internal energy of a system.

  3. The First Law of Thermodynamics work work • Change in Internal Energy = Heat added to system- Work done by system (DU=Q - W) • Q>0, when heat added, W>0 when work done by system (volume expands, W=PDV) • Restatement of Conservation of Energy Internal Energy heat heat

  4. Check Question • Heat flows into the gas of a piston-cylinder system. The gas expands and the cylinder moves upward. How does the temperature change compared to when the piston is held in place? • A) larger temperature change • B) lower temperature change • C) No temperature change

  5. Thermodynamic Processes • Isovolumetric (volume remains constant) • Heat flow (Q) into or out of material • Q, T and DU change, W =0 • Adiabatic (No heat added or subtracted) • Volume of gas increases or decreases • Temperature increases or decreases • W and DU change, Q=0 • Isothermic (temperature constant) • Volume changes, work either + or – • Q changes, either + or -

  6. Case 2: Heat Engine • Engine in which some heat is converted to work as the energy moves from a high to low temperature source. • Examples: Steam Engines, Internal Combustion Engines, Diesel Engines • Work done =Qinput-Qoutput

  7. Case 3: The Atmosphere • As air increases elevation • Pressure decreases • Volume increases • Temperature decreases (-10 C / 1000 m) • Relative humidity increases

  8. Local Climatic Results • Mojave is rain shadow desert • Air cools as moves up mountains releasing moisture on west side of mountain. • Little rain released on east side of slopes as air moves down • Santa Ana Winds • Air warms as moves from mountains into valleys • Temperature Inversion • Hot air traps cooler air below it as sea breeze blows cool air under hot.

  9. Thermodynamic Processes • Heat flow into or out of system • Q=DU, work = 0 (Isovolumetric change) • Heat flow in increases DU and temperature • Heat flow out decreases DU and temperature • Work done by or on system • W=DU, Q=0 (adiabatic change) • Work done by system results in expansion and decrease in internal energy and temperature • Work done on system results in compression and increase in internal energy and temperature

  10. Case 1: Cylinder and Piston • Changing internal energy w/ heat • Heat flows in increases DUand T • Heat flowing out decreases DU and T • Changing internal energy w/ work • Piston pushed down=>Work done on gas compresses it resulting in increasing DU and T • Piston Lifted up=>Work done by gas results in expansion and DU and T decreasing • Adiabatic change means no heat

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