Assignment__5 - Assignment#5 PROBLEM 5.1 PROBLEM STATEMENT...

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Assignment #5 PROBLEM 5.1 PROBLEM STATEMENT: Sketch and consider the following processes, and decide whether they are reversible or irreversible. If you believe they are irreversible, describe the source or sources of the irreversibility. (a) Water is pumped from a river to a lake at higher elevation. When hydroelectric power is needed, water flows from the lake through turbogenerators and back to the river. Will the generated energy be equal to the energy used for pumping? (b) Your new Jaguar automobile uses an alternator to charge the battery. The alternator is driven by a belt from the engine. If the battery is entirely discharged, will the electrical energy discharged from the battery equal to the energy taken from the engine to drive the alternator? Will the energy from the battery equal to the energy provided to the battery by the alternator? (c) A power plant burns natural gas to produce steam, which in turn is used to turn a turbo-generator to produce electricity. Will the amount of energy produced as electricity equal to the chemical energy released by the natural gas? SOLUTIONS: (a) Irreversible, due to mechanical friction in the pump and generator, turbulence and viscous fluid friction in piping, and Ohmic (electric resistance) losses in generator and wires. , , , , , , = ∆ + = ∆ - < P IN WATER IRREV PUMP G OUT WATER IRREV GEN G OUT P IN W PE W W PE W so W W (b) Irreversible due to mechanical friction in the engine, pulley belt,and alternator, Ohmic (electric resistance) losses in alternator, battery and wires, and chemical reaction losses in battery. Thus W BAT always < W ALT W P,IN W G,OUT Pump Generator Engine Alternator Battery
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Assignment #5 (c) Irreversible due to heat transfer across finite temperature difference in steam generator, mechanical and fluid friction in the turbine, Ohmic (electric resistance) losses in generator and wires. Thus W GEN always < E GAS . DISCUSSION OF RESULTS : All real processes have irreversibilities associated with them that result in irrecoverable energy losses to the environment. E STM W TURB W GEN E GAS Steam generator Turbine Generator
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Assignment #5 PROBLEM 5.3 PROBLEM STATEMENT: Which of the following do you think are possible given your understanding of dissipative processes. Do they violate the First Law? (a) An electric motor is used to spin up a large flywheel that stores kinetic energy.
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