The Laws of Thermodynamics - General Physics | PHYSICS 103, Exams of Physics

Download The Laws of Thermodynamics - General Physics | PHYSICS 103 and more Exams Physics in PDF only on Docsity! 4/13/09 Physics 103, Spring 2009, U.Wisconsin 1 Chapter 12 The Laws of Thermodynamics Today: The First Law of Thermodynamics And its applications 4/13/09 Physics 103, Spring 2009, U.Wisconsin 2   State of a thermal system  Description of the system in terms of state variables » Pressure (P) » Volume (V) » Temperature (T) (Internal Energy U) (not work, heat, etc.)  A macroscopic state of an isolated system can be specified only if the system in internal thermal equilibrium 4/13/09 Physics 103, Spring 2009, U.Wisconsin 5   When the gas is compressed   ΔV is negative   The work done on the gas is positive (to the system, just a convention)   When the gas is allowed to expand   ΔV is positive   The work done on the gas is negative (the system does work, negative sign)   When the volume remains constant   No work is done on the gas 4/13/09 Physics 103, Spring 2009, U.Wisconsin 6 If the pressure remains constant during the expansion or compression (an isobaric process)  Simple formula: W = - P (Vf - Vi) If the pressure changes, the average pressure may be used to estimate the work done 4/13/09 Physics 103, Spring 2009, U.Wisconsin 7   Used when the pressure and volume are known at each step of the process   The work done on a gas that takes it from some initial state to some final state is the area under the curve on the PV diagram   This is true whether or not the pressure stays constant 4/13/09 Physics 103, Spring 2009, U.Wisconsin 10   Isobaric   Pressure stays constant   Horizontal line in PV diagram   Isovolumetric   Volume stays constant   Vertical line on the PV diagram   Isothermal   Temperature stays the same (PV = constant)   Hyperbola in PV diagram   Adiabatic   No heat is exchanged with the surroundings 4/13/09 Physics 103, Spring 2009, U.Wisconsin 11   Quantities of interest   Q - Heat » Positive if energy is transferred to the system   W - Work » Positive if done on the system   U - Internal energy » Positive if the temperature increases   The relationship among U, W, and Q can be expressed as  ΔU = Uf – Ui = Q + W (The 1st Law)   This means that the change in internal energy of a system is equal to the sum of the energy transferred across the system boundary by heat and the energy transferred by work (just another form of energy conservation) 4/13/09 Physics 103, Spring 2009, U.Wisconsin 12 46% 21% 33% 0% 10% 20% 30% 40% 50% Shown in the picture below are the pressure versus volume graphs for two thermal processes, in each case moving a system from state A to state B along the straight line shown. In which case is the change in internal energy of the system the biggest? 1. Case 1 2. Case 2 3. Same A B 4 2 3 9 V(m3) Case 1 A B 4 2 3 9 V(m3) P(atm) Case 2 P(atm) correct T increase higher in case 1 than in case 2 Therefore, U increases more in case 1 than in case 2 P1AV1A=2x3=6, P1BV1B=4x9=36, 6 to 36 P2AV2A=4x3=12, P2BV2B=2x9=18, 12 to 18 4/13/09 Physics 103, Spring 2009, U.Wisconsin 15   Energy transferred by heat is zero   The work done is equal to the change in the internal energy of the system Q = 0, thus ΔU = W. (more on p. 305)   One way to accomplish a process with no heat exchange is => for an thermally isolated system (no thermal contact) => or to have the process happen very quickly   In an adiabatic expansion, the work done is negative and the internal energy decreases 4/13/09 Physics 103, Spring 2009, U.Wisconsin 16   No change in volume, therefore no work is done (p.306) ΔU = Q   The energy added to the system goes into increasing the internal energy of the system  Temperature will increase   Sometimes called “Isochoric”  Pressure remains constant, therefore the work is simply W = - P ΔV , and thus ΔU = Q - P ΔV. (p.303 for ideal gas. A summary table on p.308) 4/13/09 Physics 103, Spring 2009, U.Wisconsin 17   The First Law is a general equation of Conservation of Energy   Internal energy, heat transfer, work are inter-related. There is no practical, macroscopic, distinction between the results of energy transfer by heat and by work