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11/21/2013PHY 113 C Fall 2013 -- Lecture 241 PHY 113 C General Physics I 11 AM – 12:15 PM MWF Olin 101 Plan for Lecture 24: Review: Chapters 17-18, 14,

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Presentation on theme: "11/21/2013PHY 113 C Fall 2013 -- Lecture 241 PHY 113 C General Physics I 11 AM – 12:15 PM MWF Olin 101 Plan for Lecture 24: Review: Chapters 17-18, 14,"— Presentation transcript:

1 11/21/2013PHY 113 C Fall Lecture 241 PHY 113 C General Physics I 11 AM – 12:15 PM MWF Olin 101 Plan for Lecture 24: Review: Chapters 17-18, 14, Sound; Doppler effect & standing waves 2.Physics of fluids; pressure, buoyant force, Bernoulli’s equation 3.Temperature & heat & ideal gas law 4.First law of thermodynamics 5.Cycles and their efficiency

2 11/21/2013 PHY 113 C Fall Lecture 242

3 11/21/2013PHY 113 C Fall Lecture 243 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

4 11/21/2013PHY 113 C Fall Lecture 244 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

5 11/21/2013PHY 113 C Fall Lecture 245 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?

6 11/21/2013PHY 113 C Fall Lecture 246 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?

7 11/21/2013PHY 113 C Fall Lecture 247 Webassign – Assignment #21 A gasoline engine has a compression ratio of 6.00 and uses a gas for which γ = (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= =0.35

8 11/21/2013PHY 113 C Fall Lecture 248 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.

9 11/21/2013PHY 113 C Fall Lecture 249 Comment on adiabatic process (Q=0) --

10 11/21/2013PHY 113 C Fall Lecture 2410 Comment on adiabatic process (Q=0) -- continued

11 11/21/2013PHY 113 C Fall Lecture 2411 Comment on adiabatic process (Q=0) – continued Suppose you were asked to calculate the final pressure for an expansion process where V i /V f =1/10 when P i =1 atm. and when  =1.3?

12 11/21/2013PHY 113 C Fall Lecture 2412 Review of main ideas from Chapters: – Sound waves Physics of fluids – Temperature, heat, thermodynamics

13 11/21/2013PHY 113 C Fall Lecture 2413 Physics of sound waves Sound waves are described by the wave equation Change of average air density or pressure position time

14 11/21/2013PHY 113 C Fall Lecture 2414 Standing wave : Standing waves. Two sinusoidal waves, same amplitude, same f, but opposite directions

15 11/21/2013PHY 113 C Fall Lecture 2415 Standing waves between reflecting walls

16 11/21/2013PHY 113 C Fall Lecture 2416 Doppler effect

17 11/21/2013PHY 113 C Fall Lecture 2417 toward away Relative velocity of source toward observer

18 11/21/2013PHY 113 C Fall Lecture 2418 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?

19 11/21/2013PHY 113 C Fall Lecture 2419 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/m 2 ) = 1 Pascal  Density:  =mass/volume 1 kg/m 3 = gm/ml

20 Buoyant force for fluid acting on a solid: F B =  fluid V displaced g 11/21/2013PHY 113 C Fall Lecture 2420 General relationship between P and  mg A yy

21 11/21/2013PHY 113 C Fall Lecture 2421 Bernoulli’s equation:

22 11/21/2013PHY 113 C Fall Lecture 2422 Bernoulli’s equation:

23 11/21/2013PHY 113 C Fall Lecture 2423 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 = m 2, and the needle has a cross- sectional area a = 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. Webassign questions on fluids (Assignment #17)

24 11/21/2013PHY 113 C Fall Lecture 2424 Effects of temperature on materials – continued -- ideal gas “law” (thanks to Robert Boyle ( ), Jacques Charles ( ), and Gay- Lussac ( ) pressure in Pascals volume in m 3 # of moles temperature in K J/(mol K) 1 mole corresponds to x molecules Notion of temperature:

25 11/21/2013PHY 113 C Fall Lecture 2425 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 1 o C). Q = C  T Heat capacity per mass: C=mc Heat capacity per mole (for ideal gas): C=nC v C=nC p

26 11/21/2013PHY 113 C Fall Lecture 2426 Some typical specific heats MaterialJ/(kg· o C)cal/(g· o C) Water (15 o C) Ice (-10 o C) Steam (100 o C) Wood Aluminum Iron Gold

27 11/21/2013PHY 113 C Fall Lecture 2427 Heat and changes in phase of materials Example: A plot of temperature versus Q added to 1g = kg of ice (initially at T=-30 o C)

28 11/21/2013PHY 113 C Fall Lecture 2428 Typical question concerning heat: Suppose you have a well-insulated cup of hot coffee (m=0.3kg, 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?

29 11/21/2013PHY 113 C Fall Lecture 2429 Important equations for macroscopic and microscopic descriptions of thermodynamic properties of matter

30 11/21/2013PHY 113 C Fall Lecture 2430 Question from previous exam:

31 11/21/2013PHY 113 C Fall Lecture 2431 FBFB mg T

32 11/21/2013PHY 113 C Fall Lecture 2432 Question from previous exam:

33 11/21/2013PHY 113 C Fall Lecture 2433


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