Environmental Physics PHYS 1149
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This 50 page Class Notes was uploaded by Maudie Larkin on Monday October 12, 2015. The Class Notes belongs to PHYS 1149 at Georgia Southern University taught by Anand Balaraman in Fall. Since its upload, it has received 37 views. For similar materials see /class/222053/phys-1149-georgia-southern-university in Physics 2 at Georgia Southern University.
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Date Created: 10/12/15
Chapter 4 Energy and Heat Observe and Explain A test tube is closed with a rubber stopper The bottom is placed in an open flame After I 7 i 339 ll about 20 seconds the ll J Jl stopper pops off quot39 u f Construct an explanation for how the hot gas pushed out the stopper Observe and Explain A glass full of cold lemonade is placed in a bowl of hot water After about 10 minutes the lemonade in the glass will warm to a new higher temperature and the water in the bowl will cool to that same temperature Consider the lemonade in the glass as the quotsystemquot and explain this observed process using your knowledge of molecules and their motion Observe and Explain A glass full of cold lemonade is placed in a bowl of hot water After about 10 minutes the lemonade in the glass will warm to a new higher temperature and the water in the bowl will cool to that same temperature Consider the water in the bowl as the quotsystemquot and explain this observed process using your knowledge of molecules and their motion 41 Heat and Internal Energy Heat is energy that flows because of a temperature difference Heat usually raises a material s internal energy which is the energy associated with random molecular motion It can also change the phase of the materials Observe and Explain Vigorously rub two pieces of paper together pressing the fingers of each hand firmly on the paper as you rub it Consider one piece of paper as the system Why did the thermal energy of that piece of paper increase 41 Heat and Internal Energy The first law of thermodynamics says that the change in internal energy is the sum of the mechanical work done on an object and the heat that flows 39 t The ame heats the The spinning paddle In 0 water The temperature does work on the increases water The temperature increases H gt r j 42 Temperature Temperature measures the average thermal energy of the molecules that make up a substance lower temperature higher temperature 42 Temperature Common temperature scales include Celsius and Fahrenheit as well as the Kelvin scale which is based on the lowest possible temperature called absolute zero 37315 K h 212 F water boils 27315 K water freezes absolute zero Fahrenheit Kelvin Celsius 42 Temperature Which of these would you rather stick your hand in for a brief moment 42 Temperature Which of these would you rather stick your hand in for a brief moment Thermal energy depends on both temperature and densny a a a f 9 1quot a 39 a atquot 43 Heat Transfer When two objects are in direct contact such as the soldering iron and the circuit board heat is tranferred by conduction nagyngme 2003 Pearson Education In publishing as Pearson AddisnurWesley Air currents near a warm glass of water rise taking thermal energy with them in a process known as convection Guayngm guns Pearson Education Inc publishing as Pearson Aumsnnrwexley The lamp at the top shines on the lambs huddled below warming them The energy is transferred by radiation Downgth 2003 Pearson Earmarle lnc publishing as Pearson Addusanrwssley 43 Heat Transfer Conduction Conduction is heat flow energy transfer by direct molecular collisions Th To It is largely responsible A for energy losses from buildings which has implications for energy use and climate RRRR 03 A E 43 Heat Transfer Conduction Thermal conductivity k is a measure of a material s ability to conduct heat The Th larger k is the more efficiently a substance conducts heat Material Air Aluminum Concrete typical Fiberglass Glass typical Rock granite Steel Styrofoam extruded polystyrene foam Water Wood pine Urethane foam TABLE 0401 Thermal conductivity WmK 0026 237 1 0042 08 337 46 Thermal conductivity Btuinchhourft2 F 018 42 078 013 Energy Environment and Climate Copyright WW Norton amp Company 2008 43 Heat Transfer Conduction In building insulation the R value gives a measure of that material s resistance to the flow of heat R21 k where d is the thickness of the material 43 Heat Transfer Conduction The R value of a composite wall is the sum of the R values of its individual components Here the R value is 216 nearly all of it from the 1 fiberglass insulation Energy Enviro me i and Climate Copyright WWV Norton amp Company 2008 WWWW ll Interior Exterior FIGURE 406 43 Heat Transfer Conduction 0 For a 35 ft x 25 ft window on a day when the temperature difference between inside and outside is 25 F m Heat loss rate Singleglazed 09 13 W wood Doubleglazed 20 56 W wood Argonfilled 29 39 W doubleglazed Argonfilled 55 21 W tripleglazed 43 Heat Transfer Convection Convection is energy transfer by circulation of material important in circulating heat within buildings as well as energy loss associated with windows a b 43 Heat Transfer Convection Warm fluid rises gives up its thermal energy and then sinks to form individual cells of convective motion A top view of convection cells in a laboratory experiment shows striking regularity with fluid rising in the centers of the hexagonal cells and sinking at the edges a 43 Heat Transfer Convection Th0 Curioi5 affect dive3915 air owrng Imrtlrsouth into easrrwesr Winds which causes the single circular0n 59 in each hemisphere to become three cells Northern Hemisphere circulation cells Southern Hemisphere 39 Circulation cells 1 Not to scale Convection carries heated air near Earth s surface higher into the atmosphere On a larger scale it produces largescale atmospheric circulation patterns that transfer energy from the equatorial regions of Earth toward the poles 43 Heat Transfer Radiation Radiation is energy transfer by electro magnetic waves It is especially important for objects such as planets and stars are surrounded by vacuum 43 Heat Transfer Radiation 0 All objects as long as they re above absolute zero temperature emit energy in the form of electromagnetic radiation 0 They may also gain electromagnetic energy from their surroundings so there isn t necessarily a net energy loss 43 Heat Transfer Radiation The emissivity is a number between 0 and 1 that gives the efficiency with which a material radiates electromagnetic energy it is equal to the efficiency of absorption at a given wavelength Estimate the emissivity of A white cue ball a black tshirt Earth 43 Heat Transfer Radiation The power P in watts of an object with emissivity 9 surface area A and temperature T which must be measured in Kelvins is PzeaAT4 note that iS a constant Of nature 43 Heat Transfer Ra d Ian 0 n 120 Infrared Incandescent 100 lamp 3200 K Sun 8 5800 K E 80 2 Earth a 288 K 60 5 g 40 Visible lt lt1 0 20 Ultraviolet O l l l 0 1000 2000 3000 4000 5000 6000 7000 Temperature K FIGURE 409 Energy Environment and Climate Copyright WW Norton amp Company 2008 Observe and Find a Pattern You have a small electric heater and AT0C water m an nsulated container Is there a relationship between 10 1 00 24 the heating 2 of the 20 2000 48 system the water and 30 3000 72 its temperature change 40 4000 96 AT Can we describe 50 5000 120 this relationship mathematically Observe and Find a Pattern In a second set of experiments recorded in the table below insulated water containers with 010 96 different masses of 020 48 030 32 water are heated by the same amount 4000 J Can we describe this relationship mathematically 040 24 Observe and Find a Pattern Identical mass 1 kg systems with different types of matter are W F h t 095 heated by the same res wa er Seawater 103 amount 4000 J s Alcohol 165 there a pattern In the Mercury 2847 data listed in the table Can it be described mathematically Observe and Find a Pattern Combine the results of these three experiments You can account for the type of material using the new quantity c called the specific heat of that particular type of material measured in Jkg C or equivalently JkgK 44 Heat Capacity and Specific Heat Different materials heat up at different rates Heat capacity or specific heat is a parameter that describes the amount of heat needed to effect a given temperature change in a substance If you apply an amount of heat C2 to a substance of mass m and specific heat capacity c its temperature will change by an amount ATZQ WZC Observe and Find a Pattern 0 You have an electric heater and liquid in an insulated container At tlO minutes the water 0 o 220 begins to boil steam is 1000 610 clearly vnsnble 10 2000 1000 0 Why doesn t the temperature keep 15 3000 1000 increasing after 10 20 4000 1000 minutes have passed Where is the energy supplied by the heater going 45 Phase Changes and Latent Heat Sometimes the heat supplied to an object goes into a change of state instead as when ice melts or when water boils Alums ihrulc around Moms are held time together Alums Lll39t l39zir upui39l and lrmcl equilibrium pmilimh h icnk mnlcculur hands hul liu39ly through space CL39Cp liar lhcy can slide around each other occasional COlllHlUlb Me Liquid Boil Freeze Condense or NisanWesley Energy is supplied to the system The system releases energy 45 Phase Changes and Latent Heat Heats oftransformation describe the energy per unit mass required to change a material s state On melting or vaporizing the heat of transformation is stored as latent heat For example water vapor has more energy than liquid water When water vapor condenses energy is released 45 Phase Changes and Latent Heat Hurricanes are powered by the energy released through the condensation of water vapor that rises into the atmosphere above warm tropical oceans A hurricane can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth Energy Quality Reason and Explain On the next page is a list of different types of matter with the same amount of energy Order the matter with respect to their potential to do work with the most useful for doing work listed first To be concrete ask which types of energy could most easily be used to get a generator to turn 1 2 3 4 5 6 Energy Quality Reason and Explain mom of the energy stored in the molecules of gasoline mom of thermal energy in the air of your bedroom mom of gravitational potential energy of water at the top of a waterfall mom of thermal energy in very hot gas in a cylinder closed by a movable piston 1000J ofthermal energy in a cold gas in a cylinder closed by a movable piston mom of chemical energy stored in the bonds of a piece of wood 46 Energy Quality Energy has quality as well as quantity Quality is the ability of a form of energy to do useful work Highest quality Mechanical energy electricity Lowest quality 46 Energy Quality What happened to the quality of energy in this process 1 T2 Can we get any of this 4 energy back into a high I 39 quality form Can we get ALL of this energy back into a high quality form a b FFFFFFFF 10 46 Energy Quality The second law of thermodynamics prohibits a A T2 transformation of low I quality energy into highquality energy with 100 percent efficiency a b 47 Entropy Heat Engines and the Second Law of Thermodynamics The second law of thermodynamics is Highest Low ultlmately about the quality Mechanical entropy energy tendency toward more e ecmc39w disorder Entropy is a measure of that disorder Lowest H High quality EQmBQEQtEEQ entropy Enargy Environment and Cilmala riuii WW Nurl y 47 Entropy Heat Engines and the Second Law of Thermodynamics A heat engine is a device that converts random thermal energy into mechanical energy such as the gasoline engine in most cars or steam turbines in electric power plants Mechanical energy or electricity 47 Entropy Heat Engines and the Second Law of Thermodynamics The second law limits our ability to produce high quality energy efficiently through methods involving heat As a result heat engines inevitably reject some of the energy they extract from their fuels as waste heat Mechanical energy or electricity 47 Entropy Heat Engines and the Second Law ofThennodynan cs The efficiency of a heat engine is a ratio of the mechanical energy delivered to total Wham energy extracted energy or electricity The maximum possible efficiency e of a heat engine is given by e1 C h 47 Entropy Heat Engines and the Second Law of Thermodynamics Consider the maximum possible efficiency of a fossil fuel power plant Th 650K 0 And a nuclear power plant 1 1 2046 enuclear Th 570K 48 Energy Quality End Use and Cogeneration Energy efficiency dictates that we not use high quality energy like electricity for low quality uses such as heating In the efficient process of cogeneration waste heat from electricity generation is put to use for heating 48 Energy Quality End Use and Cogeneration For electric power plants we distinguish between thermal power output the total energy extracted from the fuel including waste heat and the electrical energy that is the T pant s usefu product auvinrgmzizztazzisat 48 Energy Quality End Use and Cogeneration You ll sometimes see the designation MWth for megawatts thermal and MWe for megawatts electric to distinguish between thermal power output and electrical energy produced 49 Refrigerators and Heat Pumps Heat doesn t flow spontaneously from a cooler to a hotter object Electricity in You can make heat flow from cool to hot but only if you supply high quality energy 49 Refrigerators and Heat Pumps Refrigerators and heat pumps transfer heat from cooler to hotter systems Heat pumps can provide efficient heating by using small amounts of highquality energy to transfer large amounts of heat
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