Short Notes: Second Law of Thermodynamics | Thermodynamics - Mechanical Engineering PDF Download

 Page 1


Second Law of Thermodynamics 
• The second law of thermodynamics asserts that processes occur in a certain 
direction and that the energy has quality as well as quantity.The first law 
places no restriction on the direction of a process, and satisfying the first law 
does not guarantee that the process will occur. Thus, we need another general 
principle (second law) to identify whether a process can occur or not.
fig. above shows Heat transfer froA process can occur when and only when it 
satisfies both the first and the second laws of thermodynamics. •
• The second law also asserts that energy has a quality. Preserving the quality 
of energy is a major concern of engineers. In the above example, the energy 
stored in a hot container (higher temperature) has higher quality (ability to 
work) in comparison with the energy contained (at lower temperature) in the 
surroundings.
Page 2


Second Law of Thermodynamics 
• The second law of thermodynamics asserts that processes occur in a certain 
direction and that the energy has quality as well as quantity.The first law 
places no restriction on the direction of a process, and satisfying the first law 
does not guarantee that the process will occur. Thus, we need another general 
principle (second law) to identify whether a process can occur or not.
fig. above shows Heat transfer froA process can occur when and only when it 
satisfies both the first and the second laws of thermodynamics. •
• The second law also asserts that energy has a quality. Preserving the quality 
of energy is a major concern of engineers. In the above example, the energy 
stored in a hot container (higher temperature) has higher quality (ability to 
work) in comparison with the energy contained (at lower temperature) in the 
surroundings.
• The second law is also used in determining the theoretical limits for the 
performance of commonly used engineering systems, such as heat engines 
and refrigerators etc.m a hot container to the cold surroundings is possible; 
however, the reveres process (although satisfying the first law) is impossible.
Thermal Energy Reservoirs
• Thermal energy reservoirs are hypothetical bodies with a relatively large 
thermal energy capacity (mass x specific heat) that can supply or absorb 
finite amounts of heat without
undergoing any change in temperature. Lakes, rivers, atmosphere, oceans are 
example of thermal reservoirs.
• A two-phase system can be modeled as a reservoir since it can absorb and 
release large guantities of heat while remaining at constant temperature.
• A reservoir that supplies energy in the form of heat is called a source and one 
that absorbs energy in the form of heat is called a sink.
Heat Engines
• Heat engines convert heat to work. There are several types of heat engines, 
but they are characterized by the following:
° They all receive heat from a high-temperature source (oil furnace, 
nuclear reactor, etc.)
° They convert part of this heat to work
° They reject the remaining waste heat to a low-temperature sink
° They operate in a cycle
• Thermal efficiency: is the fraction of the heat input that is converted to the net 
work output (efficiency = benefit/cost).
W
= - “ and = & , - & „
in
Page 3


Second Law of Thermodynamics 
• The second law of thermodynamics asserts that processes occur in a certain 
direction and that the energy has quality as well as quantity.The first law 
places no restriction on the direction of a process, and satisfying the first law 
does not guarantee that the process will occur. Thus, we need another general 
principle (second law) to identify whether a process can occur or not.
fig. above shows Heat transfer froA process can occur when and only when it 
satisfies both the first and the second laws of thermodynamics. •
• The second law also asserts that energy has a quality. Preserving the quality 
of energy is a major concern of engineers. In the above example, the energy 
stored in a hot container (higher temperature) has higher quality (ability to 
work) in comparison with the energy contained (at lower temperature) in the 
surroundings.
• The second law is also used in determining the theoretical limits for the 
performance of commonly used engineering systems, such as heat engines 
and refrigerators etc.m a hot container to the cold surroundings is possible; 
however, the reveres process (although satisfying the first law) is impossible.
Thermal Energy Reservoirs
• Thermal energy reservoirs are hypothetical bodies with a relatively large 
thermal energy capacity (mass x specific heat) that can supply or absorb 
finite amounts of heat without
undergoing any change in temperature. Lakes, rivers, atmosphere, oceans are 
example of thermal reservoirs.
• A two-phase system can be modeled as a reservoir since it can absorb and 
release large guantities of heat while remaining at constant temperature.
• A reservoir that supplies energy in the form of heat is called a source and one 
that absorbs energy in the form of heat is called a sink.
Heat Engines
• Heat engines convert heat to work. There are several types of heat engines, 
but they are characterized by the following:
° They all receive heat from a high-temperature source (oil furnace, 
nuclear reactor, etc.)
° They convert part of this heat to work
° They reject the remaining waste heat to a low-temperature sink
° They operate in a cycle
• Thermal efficiency: is the fraction of the heat input that is converted to the net 
work output (efficiency = benefit/cost).
W
= - “ and = & , - & „
in
• The thermal efficiencies of work-producing devices are low. Ordinary spark-
o o
ignition automobile engines have a thermal efficiency of about 20%, diesel
engines about 30%,
and power plants in the order of 40%.
The Second Law: Kelvin-Planck Statement
• It is impossible for any device that operates on a cycle to receive heat from a 
single reservoir and produce a net amount of work. In other words, no heat 
engine can have a
thermal efficiency of 100%.
fig above shows A heat engine that violates the Kelvin-Planck statement of the 
second law cannot be built.
Refrigerators and Heat Pumps
• In nature, heat flows from high-temperature regions to low-temperature ones. 
The reverse process, however, cannot occur by itself.
• The transfer of heat from a low-temperature region to a high-temperature one 
requires special devices called refrigerators.
• Refrigerators are cyclic devices, and the working fluids used in the cycles are 
called refrigerant.
• Heat pumps transfer heat from a low-temperature medium to a high- 
temperature one.
• Refrigerators and heat pumps are essentially the same devices; they differ in 
their objectives only. Refrigerator is to maintain the refrigerated space at a 
low temperature.
• On the other hand, a heat pump absorbs heat from a low-temperature source 
and supplies the heat to a warmer medium.
Page 4


Second Law of Thermodynamics 
• The second law of thermodynamics asserts that processes occur in a certain 
direction and that the energy has quality as well as quantity.The first law 
places no restriction on the direction of a process, and satisfying the first law 
does not guarantee that the process will occur. Thus, we need another general 
principle (second law) to identify whether a process can occur or not.
fig. above shows Heat transfer froA process can occur when and only when it 
satisfies both the first and the second laws of thermodynamics. •
• The second law also asserts that energy has a quality. Preserving the quality 
of energy is a major concern of engineers. In the above example, the energy 
stored in a hot container (higher temperature) has higher quality (ability to 
work) in comparison with the energy contained (at lower temperature) in the 
surroundings.
• The second law is also used in determining the theoretical limits for the 
performance of commonly used engineering systems, such as heat engines 
and refrigerators etc.m a hot container to the cold surroundings is possible; 
however, the reveres process (although satisfying the first law) is impossible.
Thermal Energy Reservoirs
• Thermal energy reservoirs are hypothetical bodies with a relatively large 
thermal energy capacity (mass x specific heat) that can supply or absorb 
finite amounts of heat without
undergoing any change in temperature. Lakes, rivers, atmosphere, oceans are 
example of thermal reservoirs.
• A two-phase system can be modeled as a reservoir since it can absorb and 
release large guantities of heat while remaining at constant temperature.
• A reservoir that supplies energy in the form of heat is called a source and one 
that absorbs energy in the form of heat is called a sink.
Heat Engines
• Heat engines convert heat to work. There are several types of heat engines, 
but they are characterized by the following:
° They all receive heat from a high-temperature source (oil furnace, 
nuclear reactor, etc.)
° They convert part of this heat to work
° They reject the remaining waste heat to a low-temperature sink
° They operate in a cycle
• Thermal efficiency: is the fraction of the heat input that is converted to the net 
work output (efficiency = benefit/cost).
W
= - “ and = & , - & „
in
• The thermal efficiencies of work-producing devices are low. Ordinary spark-
o o
ignition automobile engines have a thermal efficiency of about 20%, diesel
engines about 30%,
and power plants in the order of 40%.
The Second Law: Kelvin-Planck Statement
• It is impossible for any device that operates on a cycle to receive heat from a 
single reservoir and produce a net amount of work. In other words, no heat 
engine can have a
thermal efficiency of 100%.
fig above shows A heat engine that violates the Kelvin-Planck statement of the 
second law cannot be built.
Refrigerators and Heat Pumps
• In nature, heat flows from high-temperature regions to low-temperature ones. 
The reverse process, however, cannot occur by itself.
• The transfer of heat from a low-temperature region to a high-temperature one 
requires special devices called refrigerators.
• Refrigerators are cyclic devices, and the working fluids used in the cycles are 
called refrigerant.
• Heat pumps transfer heat from a low-temperature medium to a high- 
temperature one.
• Refrigerators and heat pumps are essentially the same devices; they differ in 
their objectives only. Refrigerator is to maintain the refrigerated space at a 
low temperature.
• On the other hand, a heat pump absorbs heat from a low-temperature source 
and supplies the heat to a warmer medium.
Expansion
Valve
Coefficient of Performance (COP)
• The performance of refrigerators and heat pumps is expressed in terms of the 
coefficient of performance (COP) which is defined as
coP ' = *? & L = 2 ± . coph ,, = H e n e f " = 3 * -
Cost w. Cost
• It can be seen that
COPhp=COPr+ 1
• Air conditioners are basically refrigerators whose refrigerated space is a room 
or a building.
• The Energy Efficiency Rating (EER): is the amount of heat removed from the 
cooled space in BTU’s for 1 Wh (watt-hour)
EER = 3.412 COPR
• Most air conditioners have an EER between 8 to 12 (COP of 2.3 to 3.5).
The Second Law of Thermodynamics: Clausius Statement
• It is impossible to construct a device that operates in a cycle and produces no 
effect other than the transfer of heat from a lower-temperature body to 
higher-temperature body.
• In other words, a refrigerator will not operate unless its compressor is driven 
by an external power source.
• Kelvin-Planck and Clausius statements of the second law are negative 
statements, and a negative statement cannot be proved. So, the second law, 
like the first law, is based on experimental observations
• The two statements of the second law are equivalent. In other words, any 
device violates the Kelvin-Planck statement also violates the Clausius 
statement and vice versa
Page 5


Second Law of Thermodynamics 
• The second law of thermodynamics asserts that processes occur in a certain 
direction and that the energy has quality as well as quantity.The first law 
places no restriction on the direction of a process, and satisfying the first law 
does not guarantee that the process will occur. Thus, we need another general 
principle (second law) to identify whether a process can occur or not.
fig. above shows Heat transfer froA process can occur when and only when it 
satisfies both the first and the second laws of thermodynamics. •
• The second law also asserts that energy has a quality. Preserving the quality 
of energy is a major concern of engineers. In the above example, the energy 
stored in a hot container (higher temperature) has higher quality (ability to 
work) in comparison with the energy contained (at lower temperature) in the 
surroundings.
• The second law is also used in determining the theoretical limits for the 
performance of commonly used engineering systems, such as heat engines 
and refrigerators etc.m a hot container to the cold surroundings is possible; 
however, the reveres process (although satisfying the first law) is impossible.
Thermal Energy Reservoirs
• Thermal energy reservoirs are hypothetical bodies with a relatively large 
thermal energy capacity (mass x specific heat) that can supply or absorb 
finite amounts of heat without
undergoing any change in temperature. Lakes, rivers, atmosphere, oceans are 
example of thermal reservoirs.
• A two-phase system can be modeled as a reservoir since it can absorb and 
release large guantities of heat while remaining at constant temperature.
• A reservoir that supplies energy in the form of heat is called a source and one 
that absorbs energy in the form of heat is called a sink.
Heat Engines
• Heat engines convert heat to work. There are several types of heat engines, 
but they are characterized by the following:
° They all receive heat from a high-temperature source (oil furnace, 
nuclear reactor, etc.)
° They convert part of this heat to work
° They reject the remaining waste heat to a low-temperature sink
° They operate in a cycle
• Thermal efficiency: is the fraction of the heat input that is converted to the net 
work output (efficiency = benefit/cost).
W
= - “ and = & , - & „
in
• The thermal efficiencies of work-producing devices are low. Ordinary spark-
o o
ignition automobile engines have a thermal efficiency of about 20%, diesel
engines about 30%,
and power plants in the order of 40%.
The Second Law: Kelvin-Planck Statement
• It is impossible for any device that operates on a cycle to receive heat from a 
single reservoir and produce a net amount of work. In other words, no heat 
engine can have a
thermal efficiency of 100%.
fig above shows A heat engine that violates the Kelvin-Planck statement of the 
second law cannot be built.
Refrigerators and Heat Pumps
• In nature, heat flows from high-temperature regions to low-temperature ones. 
The reverse process, however, cannot occur by itself.
• The transfer of heat from a low-temperature region to a high-temperature one 
requires special devices called refrigerators.
• Refrigerators are cyclic devices, and the working fluids used in the cycles are 
called refrigerant.
• Heat pumps transfer heat from a low-temperature medium to a high- 
temperature one.
• Refrigerators and heat pumps are essentially the same devices; they differ in 
their objectives only. Refrigerator is to maintain the refrigerated space at a 
low temperature.
• On the other hand, a heat pump absorbs heat from a low-temperature source 
and supplies the heat to a warmer medium.
Expansion
Valve
Coefficient of Performance (COP)
• The performance of refrigerators and heat pumps is expressed in terms of the 
coefficient of performance (COP) which is defined as
coP ' = *? & L = 2 ± . coph ,, = H e n e f " = 3 * -
Cost w. Cost
• It can be seen that
COPhp=COPr+ 1
• Air conditioners are basically refrigerators whose refrigerated space is a room 
or a building.
• The Energy Efficiency Rating (EER): is the amount of heat removed from the 
cooled space in BTU’s for 1 Wh (watt-hour)
EER = 3.412 COPR
• Most air conditioners have an EER between 8 to 12 (COP of 2.3 to 3.5).
The Second Law of Thermodynamics: Clausius Statement
• It is impossible to construct a device that operates in a cycle and produces no 
effect other than the transfer of heat from a lower-temperature body to 
higher-temperature body.
• In other words, a refrigerator will not operate unless its compressor is driven 
by an external power source.
• Kelvin-Planck and Clausius statements of the second law are negative 
statements, and a negative statement cannot be proved. So, the second law, 
like the first law, is based on experimental observations
• The two statements of the second law are equivalent. In other words, any 
device violates the Kelvin-Planck statement also violates the Clausius 
statement and vice versa
Source (T|)
liJlHlfenfi
w „, = 0
Refrigerator <
Q h + Q c
qL= o
Source (Ti)
fig. above shows The violation of the Kelvin-Planck statement leads to violation of 
Clausius
Note:
• Any device that violates the first law of thermodynamics (by creating energy) 
is called a perpetual-motion machine of the first kind (PMM1)
• The device that violates the second law is called a perpetual-motion machine 
of the second kind (PMM2).
Reversible and Irreversible Process
• A reversible process is defined as a process that can be reversed without 
leaving any trace on the surroundings.
• It means both system and surroundings are returned to their initial states at 
the end of the reverse process. Processes that are not reversible are called 
irreversible.
• Reversible processes do not occur and they are only idealizations of actual 
processes.
• Some factors that cause a process to become irreversible:
° Friction
° Unrestrained expansion and compression 
° mixing
° Heat transfer (finite AT)
° Inelastic deformation 
° Chemical reactions
Internally Reversible process
• If no irreversibilities occur within the boundaries of the system. In these 
processes a system undergoes through a series of equilibrium states, and 
when the process is reversed, the system passes through exactly the same 
equilibrium states while returning to its initial state.