2nd Law of Thermodynamics LEC-1

ME2041- Thermodynamics LEC-7
Dr. Asiri Indrajith
Asiri.Kulathunga@gmail.com
0716367129
Saturday, 16 October 2021

2nd Law of thermodynamics
The absence of the process illustrated above indicates that conservation of energy is
not the whole story. If it were, movies run backwards would look perfectly normal to
us!
Water flows down a hill, heat flows from a hot body to a cold one, rubber bands
unwind, fluid flows from a high-pressure region to a low-pressure region, and we
all get old! Our experiences in life suggest that processes have a definite
direction.
The first law of thermodynamics relates the several variables involved in a
physical process, but does not give any information as to the direction of the
process. It is the second law of thermodynamics that helps us establish the
direction of a particular process.
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Heat engine, Heat pump and refrigerators
We refer to a device operating on a cycle as a heat engine, a heat pump, or a

refrigerator, depending on the objective of the particular device. If the objective of
the device is to perform work it is a heat engine; if its objective is to transfer heat
to a body it is a heat pump; if its objective is to transfer heat from a body, it is a
refrigerator. Generically, a heat pump and a refrigerator are collectively referred
to as a refrigerator.
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An engine or a refrigerator operates between two thermal energy reservoirs,

entities that are capable of providing or accepting heat without changing
temperatures. The atmosphere and lakes serve as heat sinks; furnaces, solar
collectors, and burners serve as heat sources. Temperatures TH and TL identify the
respective temperatures of a source and a sink.
The net work W produced by the engine in one cycle would be equal
to the net heat transfer, a consequence of the first law
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The thermal efficiency of the heat engine and the coefficients of performance
(COP) of the refrigerator and the heat pump are defined as follows:
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Statement of second law

The second law of thermodynamics is a statement about which processes
occur and which do not. There are many ways to state the second law;
here is one:
Clausius Statement It is impossible to construct a device that operates
in a cycle and whose sole effect is the transfer of heat from a cooler body to
a hotter body.
[This statement relates to a refrigerator (or a heat pump). It states that it
is impossible to construct a refrigerator that transfers energy from a
cooler body to a hotter body without the input of work;]
Kelvin-Planck Statement It is impossible to construct a device that operates

in a cycle and produces no other effect than the production of work and the
transfer of heat from a single body.
In other words, it is impossible to construct a heat engine that extracts energy
from a reservoir, does work, and does not transfer heat to a low-temperature
reservoir.
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Show that the Clausius and Kelvin-Planck statements of the second law
are equivalent.
Consider the system shown. The device in (a) transfers heat and violates the
Clausius statement, since it has no work input. Let the heat engine transfer the
same amount of heat QL. Then Q ′ H is greater than QL by the amount W. If we
simply
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transfer the heat QL directly from the engine to the device, as shown in (b), there
is no need for the low-temperature reservoir and the net result is a conversion of
energy (Q′H− QH) from the high-temperature reservoir into an equivalent amount
of work, a violation of the Kelvin-Planck statement of the second law. Conversely, a
violation of the Kelvin-Planck statement is equivalent to a violation of the
Clausius statement.
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Reversibility
In our study of the first law we made use of the concept of equilibrium and we
defined equilibrium, or quasi equilibrium, with reference to the system only. We
must now introduce the concept of reversibility so that we can discuss the most
efficient engine that can possibly be constructed; it is an engine that operates
with reversible processes only: a reversible engine.
A reversible process is defined as a process which, having taken place, can be
reversed and in so doing leaves no change in either the system or the
surroundings.
Observe that our definition of a reversible process refers to both the system and
the surroundings. The process obviously has to be a quasi equilibrium process;
additional requirements are:
1. No friction is involved in the process.
2. Heat transfer occurs due to an infinitesimal temperature difference only.
3. Unrestrained expansion does not occur. 11
Carnot engine
The heat engine that operates the most efficiently between a high-temperature
reservoir and a low-temperature reservoir is the Carnot engine. It is an
ideal engine that uses reversible processes to form its cycle of operation; thus it
is also called a reversible engine. We will determine the efficiency of the Carnot
engine and also evaluate its reverse operation. The Carnot engine is very useful,
since its efficiency establishes the maximum possible efficiency of any real
engine. If the efficiency of a real engine is significantly lower than the efficiency
of a Carnot engine operating between the same limits, then additional
improvements may be possible.
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1 → 2: Isothermal expansion. Heat is transferred reversibly from the high-
temperature reservoir at the constant temperature TH. The piston in the cylinder
is withdrawn and the volume increases.
2 → 3: Adiabatic reversible expansion. The cylinder is completely
insulated so that no heat transfer occurs during this reversible process. The
piston continues to be withdrawn, with the volume increasing.
3 → 4: Isothermal compression. Heat is transferred reversibly to the low-
temperature reservoir at the constant temperature TL. The piston compresses
the working substance, with the volume decreasing.
4 → 1: Adiabatic reversible compression. The completely insulated
cylinder allows no heat transfer during this reversible process. The piston
continues to compress the working substance until the original volume,
temperature, and pressure are reached, thereby com pleting the cycle.
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A refrigeration unit is cooling a space to − 5°C by rejecting energy to the

atmosphere at 20°C. It is desired to reduce the temperature in the refrigerated
space to − 25°C. Calculate the minimum percentage increase in work
required, by assuming a Carnot refrigerator, for the same amount of energy
removed.
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ME2041- Thermodynamics LEC-7

Dr. Asiri Indrajith

Saturday, 16 October 2021

We refer to a device operating on a cycle as a heat engine, a heat pump, or a

An engine or a refrigerator operates between two thermal energy reservoirs,

Statement of second law

Kelvin-Planck Statement It is impossible to construct a device that operates

A refrigeration unit is cooling a space to − 5°C by rejecting energy to the

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