02.energyfundm.aos104f08.sld9

# 02.energyfundm.aos104f08.sld9 - Energy Fundamentals Open...

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Energy Fundamentals Open and Closed Systems First Law of Thermodynamics Second Law of Thermodynamics ! Examples of heat engines and efficiency Heat Transfer ! Conduction, Convection, Radiation Radiation and Blackbodies ! Electromagnetic Radiation ! Wien’s Law, Stefan-Boltzmann Law 1 Energy Fundamentals To analyze energy flows, define systems and use 1st and 2nd Laws of Thermodynamics Open System: energy or matter flow across boundaries ! Ex. Lake—water flows in and out Closed System: only energy flows across boundaries ! Ex. Closed bottle—only heat flows through 2 First Law of Thermodynamics “Energy cannot be created or destroyed” Energy balance equation: Energy in = Energy out + Change in internal energy ! Change in internal energy " U commonly due to change in temperature: " U = m c " T m = mass, c = specific heat, " T = temperature change 3 Units of specific heat c (definitions): 1 BTU is the energy required to raise the temperature of 1 lb of water by 1°F. 1 calorie is the energy required to raise the temperature of 1 gram of water by 1°C. 1 kilojoule (preferred): For water, c = 1 cal / g·°C = 4.184 kJ / kg·°C 4 When a substance changes phase by freezing or boiling, " U = mH L H L = latent heat, and m is the mass of substance Internal energy changes due to phase changes: Changing from solid ! liquid is the latent heat of fusion . Changing from liquid ! gas is the latent heat of vaporization . 5 H L (fusion of 0°C water) = 333 kJ/kg H L (vaporization of 100°C H 2 O) = 2257 kJ/kg 6 7 Example: Global Precipitation Over entire globe (area of globe 5.1x10 14 m 2 ), precipitation averages 1 m/yr. What energy is required to evaporate all of the precipitation if the temperature of the water is 15 °C? Specific heat (15°C) 4.184 kJ/kg Heat of Vaporization (100°C) 2257 kJ/kg Heat of Vaporization (15°C) 2465 kJ/kg Heat of Fusion 333 kJ/kg 8 Use First Law of Thermodynamics: Energy = Energy + Change in In Out Internal Energy In this case, assume “energy out” = 0 (no losses, and all energy put into the system is used for evaporation) ! Energy In = Change in Internal Energy = mH L 9

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m = (1m/yr)(5.1 # 10 14 m 2 )(1000 kg/m 3 ) = 5.1 # 10 17 kg/yr Energy = (5.1 # 10 17 kg/yr)(2465 kJ/kg) = 1.3 # 10 21 kJ/yr
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