Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley PowerPoint ® Lectures for University Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman Lectures by James Pazun Chapter 19 The First Law of Thermodynamics
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Introduction A steam locomotive is driven by a steam engine. The hotter the flame, the more energy may be withdrawn to turn the wheels. Applying the same principles, air conditioners and heat pumps provide examples to those among us who are too young to have ever traveled on a train like the one in the picture to the right.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Symbols, signs, and definitions for heat and work Hot coffee is poured into a room- temperature mug and over time, they reach thermal equilibrium. What is the sign of Q for the coffee? Sign of Q for mug? What is the sign of work for the coffee mug, if it slides along a table with friction?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Work as the change of pressure and volume Figures 19.4 (below left) and 19.5 (below right) will help expand our idea of work from “a force through a distance.”
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Work is determined by integration Work in a PV change is determined by integrating the area under the curve describing the change. Follow Example 19.1.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Does the path of the PV change matter? The start, the finish, and the shape of the curve are all significant.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Compressed air A compressed air cartridge at a starting pressure of p 1 = 50 atm and starting volume V 1 = 5 cm 3 is put into an empty, sealed balloon. It pops and causes the balloon to expand to 10 times the volume of the cartridge, V 2. Assuming the air undergoes an isothermal expansion and behaves like an ideal gas, draw the pV diagram of this process. What is the final pressure of the balloon? Two other ways different than the first method 1) of inflating the balloon to the same final volume are 2) a constant pressure p 1 = 50 atm inflates the balloon from V 1 to V 2 or 3) a constant pressure p 2 (calculated above) inflates the balloon from V 1 to V 2. Draw pV diagrams and rank the work done by the expanding air for the three cases of isothermal expansion.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Changes in a system’s internal energy The First Law of Thermodynamics is formally stated as the change in internal energy equal to heat transferred and work done.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Signs of heat and work on a system Is the work W, the heat Q, and the change in internal energy U, positive (+), negative (-) or zero (0) for the following situations? Does the temperature increase (+), decrease (-), or stay the same (0) WQ U T 1.You hit a nail with a hammer 2.You hold a nail over a Bunsen burner 3.You compress the air in a bicycle pump by pushing down on the handle very rapidly 4.You turn on a flame under a cylinder of gas, and the gas undergoes an isothermal expansion 5.A flame turns liquid water into steam 6.High pressure steam spins a turbine 7.Steam contacts a cold surface and condenses
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Condensing steam 1671 cm 3 of steam condenses to form 1 gram of water (1 cm 3 ) when held at a constant pressure of 1 atm (1.013 x 10 5 Pa). The heat of vaporization at this pressure is Lv = x 10 6 J/kg. 1.Draw the pV diagram for this process 2.What is the work done by the water when it condenses? 3.What is its increase in internal energy?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Cyclic processes and isolated systems Consider Problem-Solving Strategy Follow Example 19.2.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Study of thermodynamic processes The cyclic process shown proceeds counterclockwise from a in the pV diagram to b and back and the total work is W = -500J. Why is the work negative? Find the change in internal energy and the heat added during this process
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Note the definitions on page 657 Adiabatic Isochoric Isobaric Isothermal
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley The processes on a PV diagram Notice the subtle differences for each curve in Figure
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Measuring heat capacities Heat capacities may be measured at constant volume in a fairly complex process using a bomb calorimeter. Heat capacities may be measured at constant pressure using equipment as simple as a coffee cup.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Relating heat capacities at constant volume and pressure
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Heat capacities tabulated for selected gasses
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Adiabatic changes In an adiabatic process, no heat is transferred from system and surroundings.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Cyclic process A cyclic thermodynamic process occurs as shown, where path c-b is isothermal. Predict the Q, W and U for each process: What changes if c-b is adiabatic? QW UU a-c c-b b-a Whole cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Examples of adiabatic changes Follow Example 19.7 and Figure below. Follow Example 19.8.