“There are a number of ways to state the Second Law," he said.
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2.Saibal Mitra, a professor of physics at Missouri State University, finds the Second Law to be the most interesting of the four laws of thermodynamics. a) What is the rate of heat input into the engine per second? b) What is the heat output from the engine in 10 seconds?Ĭycles Isothermal expansion (ΔU = 0) 1.
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Hot Temperature Reservoir TH (heat source) QHĬold Temperature Reservoir TC or TL (heat sink) QC or QLĮngine uses the change in internal energy due to the temperature difference to produce work Work output from the engine W = QH – QL Week 11 ThermodynamicsĮngines Efficiency of the engine The amount of work compared to the input energy (heat) e = engine efficiency e= W W = work QH QH = input heat (energy) ExampleĪ motorcycle engine has an efficiency of 22% and produces a maximum power rating of 50 hp (1hp = 746 watts). Heat engines: car engines, power plants Heat engines reversed: refrigerators, air conditioners Work (W) Entropy in a natural process always increases.Ģnd Law of Thermodynamics 2nd Law of Thermodynamics An engine can only use a portion of the available heat to perform work. All real world processes are irreversible. The process cannot return to the original state by running the process in reverse. The process can return to the original state by running the process in reverse. Very fast process can be approximated as adiabatic. Q = ncΔT amount of heat transferred in process Molar specific heat at constant pressure cp = 5/2 R Isochoric Process Volume is held constant P W = 0 so Δ U = Q Molar specific heat at constant volume cv = 3/2 R Adiabatic Process The heat transfer (Q) is zero, the system is insulated. New Terms for Thermodynamics Isothermal Process Temperature is held constant. PV PV Find temperature by using PV = nRT or 1 1 = 2 2 T1 T2 Week 11 Thermodynamics Temperature increases as the process goes away from the origin of the graph. The area between the process line and the x axis The area contained inside a closed cycle. Volume (m3) Gas is expanding when a process has a + x direction (-Work) Gas is contracting when a process has a – x direction (+Work) PV Diagrams (cont’d) B Pressure (N / m2 or Pa) A Multiple process (usually 3 or 4) for the gas to return to the original state. One curve is represented as gas goes from one state to another state.
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Pressure is on y-axis and Volume is on x-axis Graphical method of representing the 1st Law (ΔU = Q + W) If two objects are in thermal equilibrium with a third object they are in thermal equilibrium with one another. What is the change in internal energy? Q = + 2,500 J W = - 1,800 J In your car’s engine the gas ignites adding 2500 J of heat to the cylinder at the same time 1800 J of work is being done by the gas on the piston in the cylinder. Solving for F and substituting yields W = - P ΔV Week 11 Thermodynamics Q is + for heat added to the system W is + for work done on the system ΔU = internal energy ΔU = Q + W Q = heat W = work The change in internal energy of a system is related to the heat and work done on and by the closed system.