We were discussing various basic
concepts of thermodynamics such as “Thermodynamic system, boundary and surrounding” in our recent post. We have also discussed the first law of
thermodynamics in the field of thermal engineering.
Today we will see here the
limitations of first law of thermodynamics with the help of this post. After
this post, we will see the second law of thermodynamics in our next
post.
Before discussing the limitations of
first law of thermodynamics, let us brief here the first law of thermodynamics
and after that we will discuss the limitations of first law of thermodynamics.
As we have already discussed
that first law of thermodynamics deals with the law of
conservation of energy and according to law of conservation of energy, energy
can’t be created or destroyed but also it could be converted from one form of
energy to another form of energy.
Heat and work, these are two
different forms of energy. If heat energy is provided to the system by the
surrounding then system may provide the work energy to the surrounding and
similarly if work is being done upon the system then system may deliver the
energy in terms of heat to the surrounding.
Hence, this is the basic concept of
first law of thermodynamics. Now we will come to the point without wasting your
time.
Let us see here the limitations of first law of thermodynamics
Limitation 1st
First law of thermodynamics has
explained that energy of a system will be conserved and will be converted from
one form of energy to another form of energy during a process and the complete
energy of the system before the process and after the process will be remaining
constant.
But first law of thermodynamics has
not provided the information about the direction of the process. For example
first law of thermodynamics has not provided the information about the
direction of flow of heat i.e. weather heat can flow from colder object to a
hotter object or not.
Hence, this is the first limitation
of the first law of thermodynamics that it does not provide any restriction on
the direction of the process.
Let us consider one example to
understand it clearly. We have following figure as shown here. Hot cup of tea
is placed in a cold room and hot tea will be cooled after some time because
heat energy will be lost here by hot tea and same quantity of heat energy will
be absorbed here by the surrounding of cold room and hence first law of
thermodynamics is applicable here.
But let me ask one question, reverse
of this process is possible or not. Surely reverse of this process is not
possible. This concept of was not explained in first law of thermodynamics and
second law of thermodynamics has explained this concept.
Limitation 2nd
We have seen in first law of
thermodynamics, energy transformation through the system will take place in the
form of heat energy or work energy. First law of thermodynamics has not
provided the information that how much quantity of complete energy of the
system will be converted in to the work energy.
According to first law of
thermodynamics for a cyclic process, work energy will be converted completely
in heat energy and vice-versa i.e. heat energy will be converted completely in
to work energy.
But is it possible? My answer will
be no and if i am wrong please write in comment.
Limitation 3rd
First law of thermodynamics has not
provided any information about the conditions in which conversion of heat
energy in work energy is possible.
So, these are the limitations of
first law of thermodynamics and therefore second law comes and explains these
limitations of first law of thermodynamics. Before going for second law of
thermodynamics, we must see the very important post i.e. thermal reservoir in
our next post.
Do you have suggestions? Please
write in comment box
Reference:
Engineering thermodynamics by P.K.
Nag
Engineering thermodynamics by R. K.
Rajput
Image courtesy: Google
Why the heat energy will not be converted completely into the work energy? Please explain it with some example.
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