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PHY 113 C General Physics I 11 AM – 12:15 P M MWF Olin 101 Plan for Lecture 24:

PHY 113 C General Physics I 11 AM – 12:15 P M MWF Olin 101 Plan for Lecture 24: Review: Chapters 17-18, 14, 19-22 Sound; Doppler effect & standing waves Physics of fluids; pressure, buoyant force, Bernoulli’s equation Temperature & heat & ideal gas law First law of thermodynamics

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PHY 113 C General Physics I 11 AM – 12:15 P M MWF Olin 101 Plan for Lecture 24:

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  1. PHY 113 C General Physics I 11 AM – 12:15 PM MWF Olin 101 Plan for Lecture 24: Review: Chapters 17-18, 14, 19-22 Sound; Doppler effect & standing waves Physics of fluids; pressure, buoyant force, Bernoulli’s equation Temperature & heat & ideal gas law First law of thermodynamics Cycles and their efficiency PHY 113 C Fall 2013 -- Lecture 24

  2. PHY 113 C Fall 2013 -- Lecture 24

  3. Comment about Exam 3: • Part I – take home portion (1 problem): available at end of class today -- 11/21/2013; must be turned in before part II • Part II – in-class portion (3 problems) --Tuesday 11/26/2013 • Some special arrangements for early exams have been arranged by prior agreement • Of course, all sections of the exam are to be taken under the guidelines of the honor code PHY 113 C Fall 2013 -- Lecture 24

  4. iclicker question • How are you doing on preparing your equation sheet for Exam 3? • It is completed • It is almost completed • I am in a panic because there are too many equations this time PHY 113 C Fall 2013 -- Lecture 24

  5. Webassign – Assignment #21 The work done by an engine equals one-fourth the energy it absorbs from a reservoir. (a) What is its thermal efficiency? (b) What fraction of the energy absorbed is expelled to the cold reservoir? PHY 113 C Fall 2013 -- Lecture 24

  6. Webassign – Assignment #21 What is the coefficient of performance of a refrigerator that operates with Carnot efficiency between temperatures -3.00°C and +27.0°C? PHY 113 C Fall 2013 -- Lecture 24

  7. Webassign – Assignment #21 A gasoline engine has a compression ratio of 6.00 and uses a gas for which γ = 1.40. (a) What is the efficiency of the engine if it operates in an idealized Otto cycle? (b) If the actual efficiency is 16.0%, what fraction of the fuel is wasted as a result of friction and energy losses by heat that could by avoided in a reversible engine? (Assume complete combustion of the air-fuel mixture.) fraction lost= ideal-actual=0.51-0.16=0.35 PHY 113 C Fall 2013 -- Lecture 24

  8. Webassign – Assignment #21 An idealized diesel engine operates in a cycle known as the air-standard diesel cycle shown in the figure below. Fuel is sprayed into the cylinder at the point of maximum compression, B. Combustion occurs during the expansion B → C, which is modeled as an isobaric process. Show that the efficiency of an engine operating in this idealized diesel cycle is given by the following expression. PHY 113 C Fall 2013 -- Lecture 24

  9. Comment on adiabatic process (Q=0) -- PHY 113 C Fall 2013 -- Lecture 24

  10. Comment on adiabatic process (Q=0) -- continued PHY 113 C Fall 2013 -- Lecture 24

  11. Comment on adiabatic process (Q=0) – continued • Suppose you were asked to calculate the final pressure for an expansion process where Vi/Vf=1/10 when Pi=1 atm. and when g=1.3? PHY 113 C Fall 2013 -- Lecture 24

  12. Review of main ideas from Chapters: 17-18 – Sound waves 14 -- Physics of fluids 19-22 – Temperature, heat, thermodynamics PHY 113 C Fall 2013 -- Lecture 24

  13. Physics of sound waves • Sound waves are described by the wave equation time position Change of average air density or pressure PHY 113 C Fall 2013 -- Lecture 24

  14. Standing waves. Two sinusoidal waves, same amplitude, same f, but opposite directions Standing wave: PHY 113 C Fall 2013 -- Lecture 24

  15. Standing waves between reflecting walls PHY 113 C Fall 2013 -- Lecture 24

  16. Doppler effect PHY 113 C Fall 2013 -- Lecture 24

  17. toward away Relative velocity of source toward observer PHY 113 C Fall 2013 -- Lecture 24

  18. Typical question concerning Doppler effect: A driver travels northbound on a highway at a speed of 30.0 m/s. A police car, traveling southbound at a speed of 34.0 m/s, approaches with its siren producing sound at a frequency of 2500 Hz. (a) What frequency does the driver observe as the police car approaches?(b) What frequency does the driver detect after the police car passes him? PHY 113 C Fall 2013 -- Lecture 24

  19. The physics of fluids. • Fluids include liquids (usually “incompressible) and gases (highly “compressible”). • Fluids obey Newton’s equations of motion, but because they move within their containers, the application of Newton’s laws to fluids introduces some new forms. • Pressure: P=force/area 1 (N/m2) = 1 Pascal • Density: r =mass/volume 1 kg/m3 = 0.001 gm/ml PHY 113 C Fall 2013 -- Lecture 24

  20. General relationship between P and r: Buoyant force for fluid acting on a solid: FB=rfluidVdisplacedg A Dy mg PHY 113 C Fall 2013 -- Lecture 24

  21. Bernoulli’s equation: PHY 113 C Fall 2013 -- Lecture 24

  22. Bernoulli’s equation: PHY 113 C Fall 2013 -- Lecture 24

  23. Webassign questions on fluids (Assignment #17) A hypodermic syringe contains a medicine with the density of water (see figure below). The barrel of the syringe has a cross-sectional area A = 2.40  10-5 m2, and the needle has a cross-sectional area a = 1.00  10-8 m2. In the absence of a force on the plunger, the pressure everywhere is 1.00 atm. A force  of magnitude 2.65 N acts on the plunger, making medicine squirt horizontally from the needle. Determine the speed of the medicine as it leaves the needle's tip.  PHY 113 C Fall 2013 -- Lecture 24

  24. Notion of temperature: Effects of temperature on materials – continued -- ideal gas “law” (thanks to Robert Boyle (1627-1691), Jacques Charles (1746-1823), and Gay-Lussac (1778-1850) 8.314 J/(mol K) temperature in K volume in m3 # of moles pressure in Pascals 1 mole corresponds to 6.022 x 1023 molecules PHY 113 C Fall 2013 -- Lecture 24

  25. Notion of heat Heat can be used to change temperature: Heat capacity: C = amount of heat which must be added to the “system” to raise its temperature by 1K (or 1o C). Q = C DT Heat capacity per mass: C=mc Heat capacity per mole (for ideal gas): C=nCv C=nCp PHY 113 C Fall 2013 -- Lecture 24

  26. Some typical specific heats PHY 113 C Fall 2013 -- Lecture 24

  27. Heat and changes in phase of materials Example: A plot of temperature versus Q added to 1g = 0.001 kg of ice (initially at T=-30oC) PHY 113 C Fall 2013 -- Lecture 24

  28. Typical question concerning heat: Suppose you have a well-insulated cup of hot coffee (m=0.3kg, T=100oC) to which you add 0.3 kg of ice (at 0oC). When your cup comes to equilibrium, what will be the temperature of the coffee? PHY 113 C Fall 2013 -- Lecture 24

  29. Important equations for macroscopic and microscopic descriptions of thermodynamic properties of matter PHY 113 C Fall 2013 -- Lecture 24

  30. Question from previous exam: PHY 113 C Fall 2013 -- Lecture 24

  31. FB mg T PHY 113 C Fall 2013 -- Lecture 24

  32. Question from previous exam: PHY 113 C Fall 2013 -- Lecture 24

  33. PHY 113 C Fall 2013 -- Lecture 24

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