Suppose in a football game a player always makes foul. So the other good players cannot concentrate in the game and referee also. The foul making player breaks the discipline of playing. So the total game cannot be conducted properly.

Similarly increase or decrees of oxygen level in the environment both are difficult for human being. In that two cases are disorderliness or entropy increases. Disorderliness of any system is called entropy. In other words entropy is a measure of disorderliness.

In a system if gas compressed in adiabatic process, heat energy and temperature both increases. Again if we expand the gas with same process heat energy and temperature both changes anything does not remain same. Gas does work done with the help of internal energy.

Scientist Clausius realized that at the experiment of second law of thermodynamics temperature remains constant in isothermal process similarly there is a term in adiabatic process which remains same. Clausius named the term *entropy*.

In adiabatic process the term of thermodynamic process remains constant called entropy. In another words entropy is that physical property which remains same in adiabatic process.

Entropy is a physical property of material. It is very important in thermodynamic. It indicates of flow of heat and determines the conduction of heat. It is called thermal inertia. Entropy is measurable. Change of entropy can be determined in a system. Entropy is natural quantity like pressure, volume and temperature.

Suppose, T is absolute temperature of any system which takes or releases dQ heat.

Then entropy is dS = dQ/T

Unit of T is Kelvin and dQ is Joule. So the unit of entropy is Joule/Kelvin (J/K).

**Calculation of entropy**

For small change in temperature, temperature is considered constant.

In that case entropy dS = dQ/T

If temperature does not remain same, For big change in temperature entropy change

Temperature T cannot be negative. Absorb heat is considered as positive and release heat considered negative.

**Examples of entropy calculation**

**Example 1: Find change entropy for 100 degree Celsius 2kg water into 100 degree Celsius vapor. **

**Answer:** Here,

T = 100+273 = 373K

In this situation temperature remains same as

We know,

**Example 2: Find change of entropy for 0 degree Celsius 5kg water into 100 degree Celsius vapor.**

Answer: In this situation temperature is not fixed it changes o to 100 degree Celsius. We have to use integral formula of entropy.

Here,

T_{1} = 273K

T_{2} = 100+273 = 373K

**Significance of entropy**

- Entropy cannot be felt like temperature and pressure.
- Increase of entropy means ordered state to disordered state for a system.
- It helps to determination of thermodynamic state of a material.
- Entropy is a physical quantity which value depends upon heat and absolute temperature.
- It is a thermodynamic property which determines the direction of conduction of heat.
- Entropy indicates the state of any material or environment as temperature, pressure, volume, internal energy etc.

**Entropy in reversible process**

**Figure: **Reversible process entropy

Reversible process is the process which can run backward with same quantity heat. The state of entropy in reversible process can be explained easily by Carnot cycle. Carnot cycle works into four steps. First and second step are isothermal expansion and adiabatic expansion. Third and fourth step are isothermal compression and adiabatic compression.

Q_{1} heat and T_{1} temperature for AB isothermal expansion. So the entropy of AB = dQ_{1} / T_{1}

For BC and DA there is no change in entropy because it is done with adiabatic process, there no change in entropy in adiabatic process. We know heat cannot go or out inside into the system or outside from the system, entropy remains same in adiabatic process. Entropy only changes between AB and CD due to isothermal process.

In CD isothermal compression Q_{2} heat and T_{2} temperature entropy is = dQ_{2} / T_{2}

Hence the process is reversible process in Carnot cycle AB isothermal expansion entropy and CD isothermal compression entropy is same but opposite value.

dQ_{1} / T_{1} = dQ_{2} / T_{2}

Total entropy change, dS = (dQ_{1} / T_{1}) – (dQ_{2} / T_{2}) = 0

System entropy in reversible process remains constant. There is no change in entropy for reversible process.

**Entropy in irreversible process**

**Figure: **Irreversible process entropy

Irreversible process is the process where forward and backward heat or temperature is not same. All process on the earth is normally irreversible process.

Suppose A is hot substance with T_{1} temperature and B is cool with T_{2} temperature. If we touch two substances very closely then heat flow A to B.

Entropy of A decreases with time and entropy of B increases.

For that we can write entropy of A is dS_{1} = – dQ/T_{1}

Entropy of B is dS_{2} = dQ/T_{2}

Same heat dQ from A object to B so same heat dQ flows between two sides but temperatures are different.

Now total entropy change, dS = dQ/T_{2} – dQ/T_{1}

We will get a positive value of the result because B object entropy is greater than A with time.

In aforesaid it is proven that change in entropy in irreversible process is possible.

Entropy increases in irreversible process. All work done in the earth is naturally irreversible process. Irreversible process will continue until the different of temperature of different substances. When entropy rising limit will go maximum limit then temperature of all substances will be same. On that time temperature cannot be converted into work done. Increasing of entropy means decreasing of temperature to work done conversion. According to Scientist Clausius entropy is rising globally and going to maximum limit. Heat energy conversion into work is going to zero value. Scientist named this situation heat death of the universe. The energy which cannot be formed as work is called unavailable form of energy. This is all about information of entropy.