Chapters_17_and_18

Chapters_17_and_18 - (the Earth environment energy system)...

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(the Earth environment energy system) The phenomena for the phases of water, heat and energy transfer
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possible heat engine impossible heat engine
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), ( 1 2 T T Cm T Cm Q - = Δ = , Lm Q = , 2 3 2 3 2 1 2 0 nRT T k N m N U B = = = υ dW dQ dU + = W Q U + = , pdV dW - = , tan t cons PV γ , V P C C = , R C C V P - , 1 H H L Q W Q Q e - = = Outline ---- 1. Heat Q , specific heat C , latent heat L : --- 1) heat Q : concept and units: 1 Cal = 4.186 J --- 2) specific heat C : applying to the same phase only (m --- mass of the gas) --- 3) latent heat L : applying to the phase transition process at the same temperature only 2. Internal energy U : applying to the single-atom molecules ( N ---number of molecules, n --- number of moles, R ---general gas constant) 3. The 1st law of thermodynamics: --- 1) expression and meaning: or the energy is conservative. --- 2) definition of work W : --- 3) determinations of Q, U and W in typical processes -- equal V , or equal P , or equal T. --- 4) adiabatic process : where 4. The 2nd law of thermodynamics: --- 1) expression and meaning: --- 2) engine efficiency e : ! 1 , 1 be NEVER can e e < 5. Ability to solve real problems Chapters 17&18: Laws of Thermodynamics
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1. Heat Q , specific heat C , latent heat L (1) Heat Q : . 186 . 4 1 J Cal = (a) definition: ), ( 1 2 T T Cm T Cm Q - = Δ = (b) unit of C : J /kg.K SI system , or cal/g. o C non-SI system. --- hi, how much is the C for water? --- for water liquid (from 0 o C to +100 o C), C liquid = 1 cal/g. o C . --- for water steam (from 100 o C and above), C steam = 0.48 cal/g.oC . (3) Latent heat L (constant) : (a) definition: , Lm Q = (b) unit of L : J /kg SI system , or cal/g – non-SI system. --- hi, how much is the L for water? --- from water ice (at 0 o C) to water liquid (at 0 o C), L ice-liquid = 80 cal/g . Be careful ! -- this applies ONLY in the same phase . Be careful ! -- this applies ONLY at the phase transition (at the same temperature, but different phases) . --- from water liquid (at +100 o C) to water steam (at +100 o C), L liquid-steam = 540 cal/g . The other way around are correspondingly the same for L . (a) concept: heat is a type of energy that is transferred from one object to the other because of a difference in their temperatures (let’s say one is hot, the other one is cold ). (b) unit of Q : J – Joule (SI system), relation: Cal – tradition unit (still popular in engineering), --- hi, how much is 1 cal heat? --- well, say the energy needed to raise 1 g of water (liquid H 2 O) by 1 o C (near + 10 o C). That is 1 cal. (2) Specific heat C (constant) :
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Solution: The H 2 O heating processes include -- -20 o C ice 0 o C ice 0 o C liquid +100 o C liquid +100 o C steam +120 o C steam Q Q Q Q Q = 0.5 × 1 × 10 3 × [0 – (-20)] + 80 × 1 × 10 3 + 1 × 1 × 10 3 × (100 – 0) + 540 × 1 × 10 3 + 0.48 × 1 × 10 3 × (120 – 100), = ( 10 + 80 + 100 + 540 + 9.6) × 10 3 = 739.6 (kcal) = 739.6 × 4.186 (kJ) = 3.96
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This note was uploaded on 07/13/2008 for the course PHYS 6C taught by Professor Staff during the Spring '01 term at UCLA.

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Chapters_17_and_18 - (the Earth environment energy system)...

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