First Law of Thermodynamics 3a Students know heat flow and work are two forms of energy transfer between systems. 3b Students know that heat flow into an engine at hgh temperature and the heat flow out a lower temperature in proporequal to the work output of the engine. Also the first law of thermodynamics is arestatement of the law of conservation of energy. 3c Students know the internal energy of an object includes the energy of random motion of the objects atoms and molecules often refered to as thermal energy. The greater the temperature of the atoms of the object, the greater the energy of motion of the atoms and molecules that make up the object.

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. Question: What conclusion did 19th century physicist come two about the relationship between work add internal energy? Rumsfeld found that Cannons heated up when bored Joule found that temperature of water increased and was proportional to distance mass fell Temperature at bottom of water fall was greater than temperature at top of water fall due to potential energy being converted to heat Penny heats up do to work hammer does on penny WORK can become HEAT

The First Law of Thermodynamics 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 work work Internal Energy Question: Write the 1st law of thermodynamics in words and as an equation. When are the variables Q, W and U posivte? When is this law true?What conservation law is the 1st law of thermodynamic s a restatement of? System is collection of particles ie molecules, atoms, objects. In true system none of the particles enter or leave system. Energy in the form of heat or work can be transferred in and out of the system Work is considered positive if system expands against its surroundings (ie does work on surroundings) and negative if system contracts (ie has work done on it by surroundings) Q is positive for heat added to system and negative for heat flowing out of system Law stating that internal energy will remain constant as long as no work done or heat change occurs the internal energy remains constant Can change internal energy by work or by stealing it (heat). heat heat

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

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 -

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 Questions: What is a heat engine? How is the work it does related to the heat inputed and expelled from the engine? Give some examples of heat engines.

Case 3: The Atmosphere As air increases elevation Pressure decreases Volume increases Temperature decreases (-10 C / 1000 m) Relative humidity increases Question: As an air mass increased altitude, what happens to the pressure, volume, temperature, and relative humidity of the air?

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. Question: Draw a picture of an air mass moving up a mountain and indicate its temperature and volume. Explain why the Santa Ana winds that occur during the spring in Southern CA are warm. Under what conditions does a temperature inversion form in the inland empire?

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

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 Question: A cylinder contains a gas which supports a piston. A) If the piston is held steady and the cylinder is heated, what happens to the internal energy and temperature of the gas? If the gas causes the piston to move upward, what happens to the internal energy and temperature of the gas? If the piston is compressed and the the gas decreases in volume what happpens to the temperature and internal energy of the gas?Equation for constant volume case becomes Q=Change of U Full equation used for piston system and thus temperature change of gas not as great. Discussing gases because they have greatest change in internal energy with temperature compared to solids and liquids Remind students that temperature is measurement of internal energy of an object Hewitt Video Disk Chapter 9