PHY 113 C General Physics I 11 AM
- Slides: 33
PHY 113 C General Physics I 11 AM – 12: 15 PM MWF Olin 101 1. 2. 3. 4. 5. 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 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 1
11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 2
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 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 3
iclicker question How are you doing on preparing your equation sheet for Exam 3? A. It is completed B. It is almost completed C. I am in a panic because there are too many equations this time 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 4
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? 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 5
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? 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 6
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 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 7
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. 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 8
Comment on adiabatic process (Q=0) -- 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 9
Comment on adiabatic process (Q=0) -- continued 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 10
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? 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 11
Review of main ideas from Chapters: 17 -18 – Sound waves 14 -- Physics of fluids 19 -22 – Temperature, heat, thermodynamics 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 12
Physics of sound waves Sound waves are described by the wave equation time position Change of average air density or pressure 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 13
Standing waves. Two sinusoidal waves, same amplitude, same f, but opposite directions Standing wave: 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 14
Standing waves between reflecting walls 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 15
Doppler effect 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 16
toward away Relative velocity of source toward observer 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 17
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? 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 18
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. ØPressure: P=force/area 1 (N/m 2) = 1 Pascal ØDensity: r =mass/volume 1 kg/m 3 = 0. 001 gm/ml 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 19
General relationship between P and r: Buoyant force for fluid acting on a solid: FB=rfluid. Vdisplacedg A Dy mg 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 20
Bernoulli’s equation: 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 21
Bernoulli’s equation: 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 22
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 crosssectional area A = 2. 40 10 -5 m 2, and the needle has a crosssectional area a = 1. 00 10 -8 m 2. 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. 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 23
Notion of temperature: Effects of temperature on materials – continued -ideal gas “law” (thanks to Robert Boyle (16271691), Jacques Charles (1746 -1823), and Gay. Lussac (1778 -1850) 8. 314 J/(mol K) temperature in K volume in m 3 # of moles pressure in Pascals 1 mole corresponds to 6. 022 x 1023 molecules 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 24
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 1 K (or 1 o C). Q = C DT Heat capacity per mass: C=mc Heat capacity per mole (for ideal gas): C=n. Cv C=n. Cp 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 25
Some typical specific heats Material Water (15 o. C) Ice (-10 o. C) Steam (100 o. C) Wood Aluminum Iron Gold 11/21/2013 J/(kg·o. C) 4186 2220 2010 1700 900 448 129 PHY 113 C Fall 2013 -- Lecture 24 cal/(g·o. C) 1. 00 0. 53 0. 48 0. 41 0. 22 0. 11 0. 03 26
Heat and changes in phase of materials Example: A plot of temperature versus Q added to 1 g = 0. 001 kg of ice (initially at T=-30 o. C) 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 27
Typical question concerning heat: Suppose you have a well-insulated cup of hot coffee (m=0. 3 kg, T=100 o. C) to which you add 0. 3 kg of ice (at 0 o. C). When your cup comes to equilibrium, what will be the temperature of the coffee? 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 28
Important equations for macroscopic and microscopic descriptions of thermodynamic properties of matter 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 29
Question from previous exam: 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 30
FB mg T 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 31
Question from previous exam: 11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 32
11/21/2013 PHY 113 C Fall 2013 -- Lecture 24 33
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