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


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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 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

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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

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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

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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

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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

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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

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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

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Comment on adiabatic process (Q=0) --

PHY 113 C Fall 2013 -- Lecture 24

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Comment on adiabatic process (Q=0) -- continued

PHY 113 C Fall 2013 -- Lecture 24

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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

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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

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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

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Standing waves. Two sinusoidal waves, same amplitude, same f, but opposite directions

Standing wave:

PHY 113 C Fall 2013 -- Lecture 24

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Standing waves between reflecting walls

PHY 113 C Fall 2013 -- Lecture 24

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Doppler effect

PHY 113 C Fall 2013 -- Lecture 24

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toward

away

Relative velocity of source toward observer

PHY 113 C Fall 2013 -- Lecture 24

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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

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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

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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

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Bernoulli’s equation:

PHY 113 C Fall 2013 -- Lecture 24

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Bernoulli’s equation:

PHY 113 C Fall 2013 -- Lecture 24

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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

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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

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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

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Some typical specific heats

PHY 113 C Fall 2013 -- Lecture 24

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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

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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

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Important equations for macroscopic and microscopic descriptions of thermodynamic properties of matter

PHY 113 C Fall 2013 -- Lecture 24

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Question from previous exam:

PHY 113 C Fall 2013 -- Lecture 24

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FB

mg

T

PHY 113 C Fall 2013 -- Lecture 24

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Question from previous exam:

PHY 113 C Fall 2013 -- Lecture 24