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

## 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
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
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
• 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
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
• 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
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
• 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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## Thermodynamics

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