SECOND THE LAW OF THERMODYNAMICS -2
Additional observations on the nature of processes and cycles.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 1 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Overview
General features of spontaneous processes Over arching fundamental ideas Observational basis for the Second Law Clausius statement of the Second Law Kelvin-Planck statement of the Second KelvinLaw
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 2 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Some general features of spontaneous or natural processes
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 3 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Natural processes
Unconstrained expansion without work Expansion with external work Steady heat transfer without work
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 4 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Unconstrained expansion
& Insulation Q = 0
Frictionless, diathermal piston
Air
CO2
AIR 1 KG 5 BARS 350 K
Partition Movement with Heat Transfer
Carbon Dioxide 3 kg 2 bars 450 K
Module 23 Slide 5 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Air
CO2
p Air > p CO 2 TAir < TCO 2 Q AIR CO 2 < 0
p Air = pCO2 T Air = TCO2 Q AIRCO2 = 0
Air Air
CO2
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 6 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Air Air
CO2
The final equilibrium state
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
{
p Air = pCO2 T Air = TCO2 Q AIRCO2 = 0
Module 23 Slide 7 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Two observations
Once expansion has taken place, the reverse process, i.e., a spontaneous compression does not take place. The expansion cannot be reversed without the action of an outside device.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 8 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
A proposed external device to restore the initial state
Air Air
CO2
The weight falls to rotate the paddles and heat the CO2. An internal work transfer will take place between the air and the CO2 to restore the initial condition.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 9 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Air Air
CO2
To raise the weight, an external source of work is required. A cyclic heat engine raises the weight.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 10 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
The external cyclic device
The external engine that raises the weight causes a heat rejection to the environment. Its (First Law) thermal efficiency is less than unity ( < 1)
& Wcycle
& Q In ,cycle
& Q Out ,cycle
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 11 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Expansion with external work
Air Supply 100 psi 80 oF
The initial state of the system
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Spring, k = 700 lbf/ft
Module 23 Slide 12 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Air Supply 100 psi 80o F
Final state of the system
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Spring, k = 700 lbf/ft
Module 23 Slide 13 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
A process to recover the energy stored in the spring
Same gas at initial T and P of the gas as in the filling process.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 14 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Steady heat transfer without work transfer
THot
TCold
& Q
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
& Q
Module 23 Slide 15 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
THot
TCold
& Q
& Q
Addition of a cyclic heat pump to bring heat back to the high temperature reservoir. External work is needed. needed.
& & Q+W
& Q
& Wexternal
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 16 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Over arching fundamental ideas
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 17 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Central ideas & observations
There is a natural direction of all processes towards a state of lower potential to do work.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 18 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Central ideas & observations
There is tendency in all processes and cyclic devices to produce unrecoverable energy in either doing or consuming work.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 19 The Second of Law Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Central ideas & observations
Thermal inefficiency is a part of all processes and cycles. There is always some wasted heat or work.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 20 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Central ideas and observations Perpetual motion machines are not possible, specifically machines that utilize all of the energy from a reservoir and convert it into work.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 21 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
What can produce work?
Temperature differences. Pressure differences. Electric charge differences. Electrical potential differences Difference of chemical composition (chemical potential).
Module 23 Slide 22 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
The expenditure of energy
Once the work is done, we cannot recreate these differences (i.e., the ability to do work) without providing additional energy (work or heat).
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 23 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Cyclic heat engines
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 24 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
The cyclic heat engine analysis p
S1
S2
V
cycle
W =
Q
cycle
Module 23 Slide 25 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
For each process in the cycle
p
p1
1 Expansion Process
p
p2
2 V1 V2 V
1
Compression Process
p1
p2
2
V1
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
V2
Module 23 Slide 26 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
V
Surroundings
Q 2-1 System W1-2 +W2-1 = WCycle
Surroundings
Q1-2
Wcycle > 0 Q1-2 > Q 2-1 > 0
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki Module 23 Slide 27 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
"First Law" efficiency
Q1- 2 - Q 2-1 = Q1- 2 = Wcycle Q1- 2 <1
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 28 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
High Temperature Reservoir, TH
Q1
WCycle
Q2
Low Temperature Reservoir, TL
Observation: Practical cyclic heat engines always have some heat rejection to the surroundings.
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 29 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Observational basis of the Second Law
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Kulacki
Module 23 Slide 30 The Second Law of Thermodynamics - Additional Observations on the Nature of Processes and Cycles
Observation
Heat will not flow from a cooler to a hotter temperature spontaneously.
THot
TCold
& Q
Heat, Work and Energy. A First Course in Thermodynamics 2009, F. A. Ku...