engines. second law i real processes often make sense in only one direction in time. the second...
DESCRIPTION
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|TRANSCRIPT
EnginesEngines
Second Law ISecond Law I Real processes often make sense in only one Real processes often make sense in only one
direction in time.direction in time.
The second law of thermodynamics states this:The second law of thermodynamics states this:
Heat flows naturally from a hot object to a cold object; Heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to heat will not flow spontaneously from a cold object to a hot object. a hot object.
Heat EngineHeat Engine A device that changes A device that changes
thermal energy into thermal energy into mechanical work is a heat mechanical work is a heat engine.engine.• Requires two temperature Requires two temperature
reservoirsreservoirs• Takes heat in and exhausts Takes heat in and exhausts
heat outheat out Conservation of energy Conservation of energy
applies.applies.• |Q|QHH| = || = |QQLL| + || + |WW||
Steam EngineSteam Engine Steam is produced setting Steam is produced setting
the high temperature.the high temperature.
Expanding steam pushes a Expanding steam pushes a piston doing work.piston doing work.
Steam is condensed setting Steam is condensed setting the low temperature.the low temperature.
Steam TurbineSteam Turbine A steam turbine replaces a A steam turbine replaces a
piston with a paddlewheel.piston with a paddlewheel.
Most power plants use Most power plants use steam turbines.steam turbines.
Fuel is used to make steamFuel is used to make steam• CoalCoal• OilOil• Natural gasNatural gas• UraniumUranium
EfficiencyEfficiency The efficiency of a heat The efficiency of a heat
engine compares the work engine compares the work done to the heat into the done to the heat into the engine.engine.• Measures fuel and workMeasures fuel and work
Efficiency also can compare Efficiency also can compare heat in and out.heat in and out.
HQW
e
H
L
H
LH
H
e
QQQ
QW
e
1
ExhaustExhaust An auto engine has an An auto engine has an
efficiency of 20% and efficiency of 20% and produces 23 kW of produces 23 kW of mechanical power.mechanical power.
How much heat is How much heat is discharged per second?discharged per second?
The efficiency is 0.20.The efficiency is 0.20. The output work in 1 s is The output work in 1 s is
WW = 23 kJ. = 23 kJ.
Find the input heat.Find the input heat. QQHH = = W W / / ee = 115 kJ = 115 kJ
Find the output heat.Find the output heat. QQLL / / QQHH = 1 – = 1 – ee QQLL = = QQHH (1 – (1 – ee) = 92 kJ) = 92 kJ QQLL / / tt = 92 kW = 92 kW
Second Law IISecond Law II Efficiency can only equal 1 when QL = 0.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:The second law of thermodynamics can also read:
No device is possible whose sole effect is to No device is possible whose sole effect is to transform a given amount of heat into work. transform a given amount of heat into work.
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