Second Law Of Thermodynamic: The Concept Of Entropy

As we know that thermodynamic deals with the transformation of energy from one form to another and the laws of thermodynamics describe the limits within which these transformations occur. 

The first law of thermodynamics tells us that energy is a conserved quantity and it always changes from one form to another. But the first law of thermodynamics does not tell in which direction, these changes might occured. This is where the second law of thermodynamics comes into the picture. Because the second law of thermodynamics explains the flow of heat along its direction. 

!!!The Second Law Of Thermodynamics Overcomes All The Limitations Of The First Law Of Thermodynamics!!!

The Second Law was originally formulated in terms of the fact that heat does not spontaneously flow from a colder body to a hotter body.

Why Second Law Of Thermodynamics Is Needed

☛The second law of thermodynamics helps us to predict whether a process is possible or not. And also it tells us about the direction of process (such as heat flow, water flows, etc).

☛ The second law of thermodynamics tells us that energy conversion is never 100% efficient. Because some energy is wasted in increasing the disorder and randomness of a process. Therefore, the energy before the transformation and the energy after the transformation are not the same. And this disorder or random behavior of process is called "Spontaneity" or "Entropy" of that process.

!!!!Entropy Is The Degree Of Disorder Or Randomness Of A System. It Can Also Be Said To The Measure Of Total Unavailable Energy In A System. Thus, It Gives Us An Idea About The Useful Convertible Energy. More The Randomness OF A System, Lesser Will Be The Total Useful Energy Available To A System!!!!

Statements Of Second Law Of Thermodynamics

The second law of thermodynamics generally tells us about the Concept Of Entropy. And this concept of entropy gives us a more quantitative way to describe the tendency for energy to flow in a particular direction. Thus, entropy tell us the direction of processes. 

There are many statements of the second law of thermodynamics. Taking a closer look at each of them individually-

1. "......This laws tells us that the total entropy of an isolated system can never decrease over time. The total entropy can remain constant in ideal cases where the system is in a steady state (equilibrium), or is undergoing a reversible process........"

Therefore, the entropy of the entire universe will always increase with time.

          ➩ ∆(Entropy)universe ＝ 0

!!!!!No Process Is Possible That Results In A Decrease In Entropy!!!!!

2."The second law of thermodynamics tells us that processes occur only in a certain direction and not in any direction". For example- heat always flows from a body of higher temperature to a body of lower temperature, water always flows from a higher water level to a lower water level, etc.

3. "The second law of thermodynamics tells us that all spontaneously occurring processes in nature are to some extent irreversible and are accompanied by degradation of energy". 

4. "A system and its surroundings always proceed in a state of maximum disorder or maximum entropy which implies more stability".

5. "Any chemical reaction automatically changes in the direction in which there is a decrease in energy".

☛ This is what the second law thermodynamics precisely states:

........It is not possible for you to completely convert the heat supplied to a system into work done without introducing a change in the system......

......You now understand it's not possible to completely bring back a disordered system back to order.........

!!!!This Is How Simple Is The Second Law Of Thermodynamics!!!

.......The second law of thermodynamics also tells us that all processes that occur in nature are Irreversible or Spontaneous.......

....Heat (a low grade energy) cannot be completely converted into work (a high grade energy). So if you have the same quantity of both heat and work, work will have a higher value.......

DAILY LIFE EXAMPLES

Actually the second law of thermodynamics occurs all around us and you already have experienced all these things. I have mentioned a few examples of the second law of thermodynamics through which you will come to know about its real life applications.

a) Water flowing over a dam means from high level to a lower level.

b) A hot coffee becomes cold on it’s own. While it can not become hot again on its own.

c) Air leaks from the balloon on its own

The air never goes inside the balloon on its own.

 
d) Two gases will mix automatically on its own

e) Ice melts automatically

You have already seen this at least once in your life.

f) A gas takes the entire volume of the container
(What will happen if you insert some gas in the closed container. Obviously it will spread throughout the container (on its own) and it will occupy the entire space of the container. This is exactly the same as applying perfume on your shirt and the fragrance spreads in the entire room).

All these processes occur spontaneously (i.e on its own) shown above examples and because of this spontaneous process, the entropy of the universe always increases and never decreases.

SIGNIFICANCE

1. The second law of thermodynamics signifies that heat does not flow spontaneously from a colder region to a hotter region, To make it possible, you have to spend some work. Or, equivalently, heat at a given temperature cannot be converted entirely into work.

2. It also signifies that energy is transferred by natural processes in such a way that the entropy of an isolated system never decreases- it either remains the same or increases.

The Formal Statement Of Second Law Of Thermodynamics

There are two important formal statements to describe the old form of the second law of thermodynamics. First, CLAUSIUS statement and the second, KELVIN-PLANK statement. Both statements are based on Irreversible Process.

CLAUSIUS STATEMENT

It states that, "It is impossible for a self acting machine working in a cyclic process without any external force, to transfer heat from a body at a lower temperature to a body at higher temperature". 

This means that Heat can not itself flow from a colder body to a hotter body.

For example- The best example of this statement can be a hot cup of coffee kept on a table. The coffee will eventually cool down showing that heat only flows from high temperature to low temperature without the aid of any external agent. Coffee doesn't get warmer itself as surroundings are at lower temperature.

☛ In Simple words......

!!!!It Is Impossible For Any System To Operate In Such A Way That The Sole Result Would Be An Energy Transfer By Heat From A Cooler To A Hotter Body!!!!

KELVIN-PLANK STATEMENT

It states that, "It is impossible to construct an engine that operates in a cyclic process and produces no effect other than absorbing heat from a single thermal reservoir and convert it into equivalent amount of work". 

This means that there are no engines available yet whose sole result is to convert heat into work completely without any external damage. Thus, Kelvin Planck’s statement of second law of thermodynamics says that there must be at least two thermal reservoirs to operate the engine.

"A perfect engine with 100% efficiency is impossible". 

For example- the best example of this statement is human body. We eat food (high temperature reservoir). The chemical energy of food is utilised by body to maintain its temperature and to do work. When we do some work our body worms up and rejects heat into the environment (low temperature reservoir). Even if we are not doing any work our body still rejects a lot of heat to the environment due to metabolism (digestion respiration etc).

☛ In Simple words......

!!!! No Process Is Possible Whose Sole Effect Is To Take Heat From A Reservoir And Converted It Completely Into Mechanical Work!!!

The Equation Of The Second Law Of Thermodynamics

According to the second law of Thermodynamics, "The entropy of entire universe is always increases". Thus, the change in the entropy of the universe is always positive. In a mathematical  form;

FAQs

Why is the Second Law of Thermodynamics so important? What does it explain?

Second law of thermodynamics is very important because it talks about entropy and as we have discussed, ‘entropy dictates whether or not a process or a reaction is going to be spontaneous’. !!I want you to realise that any natural process happening around you is driven by entropy!!

What Are The Applications Of Second Law Of Thermodynamics??

Second law of thermodynamics is applicable in every aspect of our life in any field, but it has a larger industrial applications to account the energy and entropy generation and also some are direct applications based on the law, like heat engines and in nano fluid heat transfer.

And also the evolution of this entire universe is based on this second law of thermodynamics!

Was The Second Law Of Thermodynamics Practical??

Yes. Refrigerators, Air Conditioners and Hydro-coolants are all implemented using the Second Law of Thermodynamics. It is a very practical law because it gives us an accurate measure of the minimum temperature difference required between a source and a sink in a cooling process and gives us the means to estimate process efficiency.

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