Engines
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Thermodynamics: Second Law and Heat Engines, Slides of Physics
An introduction to the second law of thermodynamics and its application to heat engines, including steam engines and steam turbines. It explains the concept of efficiency and the limitations imposed by the second law. Examples of calculating heat input and output for a heat engine are also provided.
Typology: Slides
2012/2013
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Engines
Second Law I
Real processes often make sense in only onedirection 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 toa hot object.
Steam Engine
Steam is produced settingthe high temperature.
Expanding steam pushes apiston doing work.
Steam is condensed settingthe low temperature.
Steam Turbine
A steam turbine replaces apiston with a paddlewheel.
Most power plants usesteam turbines.
Fuel is used to make steam•
Coal
-^
Oil
-^
Natural gas
-^
Uranium
Exhaust
An auto engine has anefficiency of 20% andproduces 23 kW ofmechanical power.
How much heat isdischarged per second?
The efficiency is 0.20.
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^
e
Q
= L
Q
H^
e
) = 92 kJ
Q
/ L
t^ = 92 kW
Second Law II
Efficiency can only equal 1 when QL = 0.
-^
Perfect conversion to mechanical work
-^