Engines. Second Law I  Real processes often make sense in only one direction in time.  The second law of thermodynamics states this: Heat flows naturally.

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Presentation transcript:

Engines

Second Law I  Real processes often make sense in only one direction in time.  The second law of thermodynamics states this: Heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.

Heat Engine  A device that changes thermal energy into mechanical work is a heat engine. Requires two temperature reservoirsRequires two temperature reservoirs Takes heat in and exhausts heat outTakes heat in and exhausts heat out  Conservation of energy applies. |Q H | = |Q L | + |W||Q H | = |Q L | + |W|

Steam Engine  Steam is produced setting the high temperature.  Expanding steam pushes a piston doing work.  Steam is condensed setting the low temperature.

Steam Turbine  A steam turbine replaces a piston with a paddlewheel.  Most power plants use steam turbines.  Fuel is used to make steam Coal Oil Natural gas Uranium

Efficiency  The efficiency of a heat engine compares the work done to the heat into the engine. Measures fuel and workMeasures fuel and work  Efficiency also can compare heat in and out.

Exhaust  An auto engine has an efficiency of 20% and produces 23 kW of mechanical power.  How much heat is discharged per second?  The efficiency is  The output work in 1 s is W = 23 kJ.  Find the input heat.  Q H = W / e = 115 kJ  Find the output heat.  Q L / Q H = 1 – e  Q L = Q H (1 – e) = 92 kJ   Q L /  t = 92 kW

Second Law II  Efficiency can only equal 1 when QL = 0. Perfect conversion to mechanical workPerfect conversion to mechanical work Not possible with real systemsNot possible with real systems  The second law of thermodynamics can also read: No device is possible whose sole effect is to transform a given amount of heat into work. next