lecture 30: 2nd law of thermodynamics - missouri...
TRANSCRIPT
![Page 1: Lecture 30: 2nd Law of Thermodynamics - Missouri S&Tweb.mst.edu/~vojtaa/engphys1/lectures/lec30.pdf · 2014-12-11 · 2nd Law of Thermodynamics Clausius: Heat flows naturally from](https://reader034.vdocuments.mx/reader034/viewer/2022050221/5f665d48e4569545646fa960/html5/thumbnails/1.jpg)
• Thermodynamic cycles
• 2nd law of Thermodynamics
• Carnot Cycle
Lecture 30: 2nd Law of Thermodynamics
![Page 2: Lecture 30: 2nd Law of Thermodynamics - Missouri S&Tweb.mst.edu/~vojtaa/engphys1/lectures/lec30.pdf · 2014-12-11 · 2nd Law of Thermodynamics Clausius: Heat flows naturally from](https://reader034.vdocuments.mx/reader034/viewer/2022050221/5f665d48e4569545646fa960/html5/thumbnails/2.jpg)
Thermodynamic Cycles
System returns to initial state
𝑈𝑓 = 𝑈𝑖∆𝑈 = 𝑄 −𝑊 = 0
𝑄 = 𝑊
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Work in cycles
𝑊= area enclosed in the cycle
Clockwise: more positive 𝑊 than negative 𝑊𝑊𝑛𝑒𝑡 > 0
Counter-clockwise: more negative 𝑊 than positive 𝑊𝑊𝑛𝑒𝑡 < 0
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Heat engine
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Efficiency
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2nd Law of Thermodynamics
Clausius:
Heat flows naturally from a hot object to a cold object;
heat will not flow spontaneously from a cold object to a
hot object.
Kelvin-Planck:
No device is possible whose sole effect is to transform a
given amount of heat completely into work.
impossible to construct perpetual motion machine
of 2nd kind
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Carnot Cycle
1-2 isothermal expansion
2-3 adiabatic expansion
3-4 isothermal compression
4-1 adiabatic compression
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Carnot Cycle
1-2 isothermal expansion
2-3 adiabatic expansion
3-4 isothermal compression
4-1 adiabatic compression
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Efficiency of Carnot Cycle
1-2 isothermal expansion
3-4 isothermal compression
4-1 adiabatic compression
2-3 adiabatic expansion
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Carnot Cycle has maximum efficiency
Heat transfer during isothermal process reversible
No heat transfer during process that involves
temperature change
Carnot cycle is reversible
If more efficient engine existed:
Couple hypothetical engine with reverse Carnot engine
Transforms amount of heat completely into work
Violates 2nd Law
More efficient engine can not exist
𝑒𝐶𝑎𝑟𝑛𝑜𝑡 = 1 −𝑇𝐶𝑇𝐻
Maximum efficiency of any cycle
operating between 𝑇𝐶 and 𝑇𝐻