PPT: Second Law Of Thermodynamics Mechanical Engineering Notes | EduRev

Thermodynamics

Mechanical Engineering : PPT: Second Law Of Thermodynamics Mechanical Engineering Notes | EduRev

 Page 1


Limitation of “FIRST LAW”
• The first law of thermodynamic states that a certain 
energy flow takes place when a system undergoes a 
process or change of state is possible or not.
– According to first law in ‘cyclic process’
• Work is completely converted into heat or heat is 
completely converted into work.
• “HEAT” & “WORK” are mutually converted into each 
other.  
• But from experience this is NOT TRUE!
Page 2


Limitation of “FIRST LAW”
• The first law of thermodynamic states that a certain 
energy flow takes place when a system undergoes a 
process or change of state is possible or not.
– According to first law in ‘cyclic process’
• Work is completely converted into heat or heat is 
completely converted into work.
• “HEAT” & “WORK” are mutually converted into each 
other.  
• But from experience this is NOT TRUE!
Limitation of “FIRST LAW”
• First law does not help 
to predict whether the 
certain process is 
possible or not.
• The first law does not 
give info about 
Direction.
• It does not provide and 
specify sufficient 
condition to process 
take place.
Work(W) System Heat(Q)
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Not possible
Page 3


Limitation of “FIRST LAW”
• The first law of thermodynamic states that a certain 
energy flow takes place when a system undergoes a 
process or change of state is possible or not.
– According to first law in ‘cyclic process’
• Work is completely converted into heat or heat is 
completely converted into work.
• “HEAT” & “WORK” are mutually converted into each 
other.  
• But from experience this is NOT TRUE!
Limitation of “FIRST LAW”
• First law does not help 
to predict whether the 
certain process is 
possible or not.
• The first law does not 
give info about 
Direction.
• It does not provide and 
specify sufficient 
condition to process 
take place.
Work(W) System Heat(Q)
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Not possible
The “SECOND LAW” of 
thermodynamic:
• The second low of thermodynamic gives more 
information about thermodynamic processes.
• Second law may be defined as
– “Heat can not flow itself from colder body to a 
hotter body”. 
• The Second law is also used to determine the 
theoretical limits for the performance of 
mostly used engineering systems like heat 
engines and heat pump…. 
Page 4


Limitation of “FIRST LAW”
• The first law of thermodynamic states that a certain 
energy flow takes place when a system undergoes a 
process or change of state is possible or not.
– According to first law in ‘cyclic process’
• Work is completely converted into heat or heat is 
completely converted into work.
• “HEAT” & “WORK” are mutually converted into each 
other.  
• But from experience this is NOT TRUE!
Limitation of “FIRST LAW”
• First law does not help 
to predict whether the 
certain process is 
possible or not.
• The first law does not 
give info about 
Direction.
• It does not provide and 
specify sufficient 
condition to process 
take place.
Work(W) System Heat(Q)
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Not possible
The “SECOND LAW” of 
thermodynamic:
• The second low of thermodynamic gives more 
information about thermodynamic processes.
• Second law may be defined as
– “Heat can not flow itself from colder body to a 
hotter body”. 
• The Second law is also used to determine the 
theoretical limits for the performance of 
mostly used engineering systems like heat 
engines and heat pump…. 
“Kelvin-Plank” statement:
• The Kelvin-Plank statement of the second law of 
thermodynamic is states that
– “It is impossible to for any devise as heat engine that 
operates on a cycle to receive heat from a single 
reservoir and produce net amount of work”.
• This statement means that only part of total heat 
absorbed by heat engine from a high 
temperature is converted to work, the remaining 
heat must be rejected at a low temperature. 
Page 5


Limitation of “FIRST LAW”
• The first law of thermodynamic states that a certain 
energy flow takes place when a system undergoes a 
process or change of state is possible or not.
– According to first law in ‘cyclic process’
• Work is completely converted into heat or heat is 
completely converted into work.
• “HEAT” & “WORK” are mutually converted into each 
other.  
• But from experience this is NOT TRUE!
Limitation of “FIRST LAW”
• First law does not help 
to predict whether the 
certain process is 
possible or not.
• The first law does not 
give info about 
Direction.
• It does not provide and 
specify sufficient 
condition to process 
take place.
Work(W) System Heat(Q)
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Hot Reservoir T1
Cold Reservoir T2
Heat 
Flow
Not possible
The “SECOND LAW” of 
thermodynamic:
• The second low of thermodynamic gives more 
information about thermodynamic processes.
• Second law may be defined as
– “Heat can not flow itself from colder body to a 
hotter body”. 
• The Second law is also used to determine the 
theoretical limits for the performance of 
mostly used engineering systems like heat 
engines and heat pump…. 
“Kelvin-Plank” statement:
• The Kelvin-Plank statement of the second law of 
thermodynamic is states that
– “It is impossible to for any devise as heat engine that 
operates on a cycle to receive heat from a single 
reservoir and produce net amount of work”.
• This statement means that only part of total heat 
absorbed by heat engine from a high 
temperature is converted to work, the remaining 
heat must be rejected at a low temperature. 
Continued…..
• The heat engine as shown 
in fig.1is converting 100 % 
of heat into 100% work.
• This system is not 
satisfying second law.
• So, this statement can be 
also expressed as “No 
heat engine has a 100 % 
of thermal efficiency”.
Q=100 kj
w=100 kj
Q=100 kj
w=40kj
Q2=60 kj
Heat Source
Heat 
Engine
Heat Source 
T1
Heat 
Engine
Sink T2
1
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