Entropy
Fundamentals of Thermal-Fluid Sciences
Yunus A. Çengel, Robert H. Turner, John M. Cimbala McGraw-Hill
Week 6-7 (Chapter 8) ENTROPY
KHAIRUL FADZLI BIN SAMAT Faculty of Manufacturing Engineering Universiti Teknikal Malaysia Melaka
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
WHAT IS ENTROPY?
Boltzmann relation
A pure crystalline substance at absolute zero temperature is in perfect order, and its entropy is zero (the third law of thermodynamics).
The level of molecular disorder (entropy) of a substance increases as it melts or evaporates.
Disorganized energy does not create much useful effect, no matter how large it is.
2
The paddle-wheel work done on a gas increases the level of disorder (entropy) of the gas, and thus energy is degraded during this process. In the absence of friction, raising a weight by a rotating shaft does not create any disorder (entropy), and thus energy is not degraded during this process. During a heat transfer process, the net entropy increases. (The increase in the entropy of the cold body more than offsets the decrease in the entropy of the hot body.) 3
ENTROPY
Clausius inequality
Formal definition of entropy
The system considered in the development of the Clausius inequality.
The equality in the Clausius inequality holds for totally or just internally reversible cycles and the inequality for the irreversible ones.
4
A quantity whose cyclic integral is zero (i.e., a property like volume) Entropy is an extensive property of a system.
The net change in volume (a property) during a cycle is always zero. A Special Case: Internally Reversible The entropy change between two specified states is the same whether the process is reversible or irreversible.
Isothermal Heat Transfer Processes
This equation is particularly useful for determining the entropy changes of thermal energy reservoirs.
5
Example 1: Entropy Change during an Isothermal
Yunus A. Çengel, Robert H. Turner, John M. Cimbala McGraw-Hill
Week 6-7 (Chapter 8) ENTROPY
KHAIRUL FADZLI BIN SAMAT Faculty of Manufacturing Engineering Universiti Teknikal Malaysia Melaka
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
WHAT IS ENTROPY?
Boltzmann relation
A pure crystalline substance at absolute zero temperature is in perfect order, and its entropy is zero (the third law of thermodynamics).
The level of molecular disorder (entropy) of a substance increases as it melts or evaporates.
Disorganized energy does not create much useful effect, no matter how large it is.
2
The paddle-wheel work done on a gas increases the level of disorder (entropy) of the gas, and thus energy is degraded during this process. In the absence of friction, raising a weight by a rotating shaft does not create any disorder (entropy), and thus energy is not degraded during this process. During a heat transfer process, the net entropy increases. (The increase in the entropy of the cold body more than offsets the decrease in the entropy of the hot body.) 3
ENTROPY
Clausius inequality
Formal definition of entropy
The system considered in the development of the Clausius inequality.
The equality in the Clausius inequality holds for totally or just internally reversible cycles and the inequality for the irreversible ones.
4
A quantity whose cyclic integral is zero (i.e., a property like volume) Entropy is an extensive property of a system.
The net change in volume (a property) during a cycle is always zero. A Special Case: Internally Reversible The entropy change between two specified states is the same whether the process is reversible or irreversible.
Isothermal Heat Transfer Processes
This equation is particularly useful for determining the entropy changes of thermal energy reservoirs.
5
Example 1: Entropy Change during an Isothermal