Friday, 26 August 2016

WHAT ARE THE LIMITATIONS OF FIRST LAW OF THERMODYNAMICS

WHAT ARE THE LIMITATIONS OF FIRST LAW OF THERMODYNAMICS


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

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