Lect_26_HeatEngines-Refrigerators-Entropy

Lect_26_HeatEngines-Refrigerators-Entropy - Lecture 33 Heat...

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Unformatted text preview: Lecture 33 Heat engines and refrigerators. Heat engine = device with a working substance (eg. gas) that operates in a thermodynamic cycle. In each cycle, the net result is that the system absorbs heat (Q > 0) and does work (W > 0). Examples:- Car engine: burns fuel, heats air inside piston. Piston expands, does mechanical work to move car- Animal: burns food to be able to move Hot and cold reservoirs Stages of the cycle Absorb heat from hot reservoir (Q H ) Perform mechanical work (W ) Dump excess heat into cold reservoir (Q L < 0) Reservoir = large body whose temperature does not change when it absorbs or releases heat. L L Energy flow of Heat engines Working substance in engine completes a cycle, so U = 0: This relation follows naturally from the diagram (Q H splits). Draw it every time! ( Q H + Q L ) W = W = Q H + Q L = Q H Q L L L Energy flow diagrams in heat engines L L Limitations We are not saying that you can absorb 10 J of heat from a hot source (a burning fuel) and produce 10 J of mechanical work... You can absorb 10 J of heat from a hot source (a burning fuel) and produce 7 J of mechanical work and release 3 J into a cold source (cooling system). so at the end you absorbed 10 J but used (= converted to work) only 7 J. (Well see later that it is impossible to make Q H = W, or Q L = 0) Efficiency what you use Efficiency what you pay for = For a heat engine: H W e Q = Example: A heat engine does 30 J of work and exhausts 70 J by heat transfer. What is the efficiency of the engine? 1 e < < W = 30 J Q L = 70 J Q L = 70 J e = W Q H = 0.3 ( or30% ) Q H = W Q L = 100 J L L Carnot engine e = W Q H = Q H Q L Q H = T H T L T H e = Q H Q L Q H = 1 Q L Q H = 1 T L T H Q H Q L Another way to write it: Carnot engine is ideal (reversible) ACT: Two engines Two engines 1 and 2 with efficiencies e 1 and e 2 work in series as shown. Let e be the efficiency of the combination. Which of the shown....
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Lect_26_HeatEngines-Refrigerators-Entropy - Lecture 33 Heat...

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