G.K.BHARAD INSTITUTE OF ENGINEERING(059) Subject:-Engineering Thermodynamics(2131905) Topic:- Entropy Prepared By:-Savaliya Subhash P.(130590119087) Guidance By:- Suchak sir Lathigara sir

Entropy:- The entropy is a thermodynamics property of a working substance and serves as a valuable tool in the second law analysis of engineering devices. Entropy is a function of heat which shows the possibility of conversion of that heat into work. Entropy is defined as: dS = (δQ/T) S is entropy per unit mass; T=Absolute temprature. Entropy is a property of a state not a process Change of entropy:

Entropy:- Entropy: a thermodynamic property, can be used as a measure of disorder. The more disorganized a system the higher the entropy. Defined using Clausius inequality This inequality is valid for all cycles, reversible and irreversible. Consider a reversible Carnot cycle Define a thermodynamic property entropy (S), such that

Entropy – A Property:- Defined using Clausius inequality Entropy is a thermodynamic property; it can be viewed as a measure of disorder. i.e. More disorganized a system the higher its entropy. Defined using Clausius inequality where Q is the differential heat transfer & T is the absolute temperature at the boundary where the heat transfer occurs Clausius inequality is valid for all cycles, reversible and irreversible. Consider a reversible Carnot cycle:

it must be a property, by defintion: Since, i.e. it does not change if you return to the same state it must be a property, by defintion: Let’s define a thermodynamic property entropy (S), such that True for a Reversible Process only

A Special Case: Internally Reversible A quantity whose cyclic integral is zero (i.e., a property like volume) Entropy is an extensive property of a system. The net change in volume (a property) during a cycle is always zero. The entropy change between two specified states is the same whether the process is reversible or irreversible. A Special Case: Internally Reversible Isothermal Heat Transfer Processes This equation is particularly useful for determining the entropy changes of thermal energy reservoirs.

Entropy:-

Since entropy is a thermodynamic property, it has fixed values at a fixed thermodynamic states. 1 2 reversible process any T S

Entropy Changes in a System Qualitative:- Ssolid < Sliquid

Entropy:- Sliquid < Svapour

Entropy Increase Principle:-

Entropy Increase Principle:- A process can take place only in the direction that complies with the increase of entropy principle, that is, Sgen0. Entropy is non-conservative since it is always increasing. The entropy of the universe is continuously increasing, in other words, it is more disorganized and is approaching chaotic. The entropy generation is due to the existence of irreversibilities. Therefore, the higher the entropy generation the higher the irreversibilities and, accordingly, the lower the efficiency of a device since a reversible system is the most efficient system.

Increase of entropy principle:- For any process in which an irreversibility exists,the net effect is the increase of the entropy of the universe, or

Increase of entropy principle:- Some entropy is generated or created during an irreversible process and it is called entropy generation or Sgen. Sgen is always positive or zero.

Entropy Generation Example:- Example: Show that the heat can not transfer from the low-temperature sink to the high-temperature source based on the increase of entropy principle. DS(source) = 2000/800 = 2.5 (kJ/K) DS(sink) = -2000/500 = -4 (kJ/K) Sgen= DS(source)+ DS(sink) = -1.5(kJ/K) < 0 It is impossible based on the entropy increase principle Sgen0, therefore, the heat can not transfer from low-temp. to high-temp. without external work input If the process is reversed, 2000 kJ of heat is transferred from the source to the sink, Sgen=1.5 (kJ/K) > 0, and the process can occur according to the second law Source 800 K Sink 500 K Q=2000 kJ

If the sink temperature is increased to 700 K, how about the entropy generation? DS(source) = -2000/800 = -2.5(kJ/K) DS(sink) = 2000/700 = 2.86 (kJ/K) Sgen= DS(source)+ DS(sink) = 0.36 (kJ/K) < 1.5 (kJ/K) Entropy generation is less than when the sink temperature is 500 K, less irreversibility. Heat transfer between objects having large temperature difference generates higher degree of irreversibilities

The increase of entropy principle (closed system):- For an isolated (or simply an adiabatic closed system), the heat transfer is zero, then This means that the entropy of an adiabatic system during a process always increases or, In the limiting case of a reversible process, remains constant. In other words, it never decreases. This is called Increase of entropy principle. This principle is a quantitative measure of the second law.

The increase of entropy principle (closed system):- The entropy change can be evaluated independently of the process details. However, the entropy generation depends on the process, and thus it is not a property of the system. The entropy generation is always a positive quantity or zero and this generation is due to the presence of irreversibilities. The direction of entropy transfer is the same as the direction of the heat transfer: a positive value means entropy is transferred into the system and a negative value means entropy is transferred out of the system.

The Increase Of Entropy Principle:- The equality holds for an internally reversible process and the inequality for an irreversible process. A cycle composed of a reversible and an irreversible process. Some entropy is generated or created during an irreversible process, and this generation is due entirely to the presence of irreversibilities. The entropy generation Sgen is always a positive quantity or zero. Can the entropy of a system during a process decrease?

Third Law of Thermodynamics:- If increasing temperature increases entropy, then the opposite should be true also. Is it possible to decrease the temperature to the point that the entropy is zero? At what T does S = 0? If entropy is zero, what does that mean?

Second Law of Thermodynamics:- The second law of thermodynamics: The entropy of the universe does not change for reversible processes and increases for spontaneous processes. Reversible (ideal): Irreversible (real, spontaneous):

Property Diagrams Involving Entropy:- On a T-S diagram, the area under the process curve represents the heat transfer for internally reversible processes.

Entropy:- Clausius Inequality: For internally reversible cycles:

Used in your book to demonstrate the validity of the Clausius inequality

Reversible Processes:- In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process. Changes are infinitesimally small in a reversible process.

Irreversible Processes:- Irreversible processes cannot be undone by exactly reversing the change to the system. All Spontaneous processes are irreversible. All Real processes are irreversible.

Third Law of Thermodynamics:- Entropy of a perfect crystalline substance at 0 K is zero No entropy = highest order possible Purpose of 3rd Law Allows S to be measured for substances S = 0 at 0 K S = standard molar entropy The entropy of the pure substance is our zero (reference point)

Application of third law of thermodynamics:- 1) Provides an absolute reference point for the determination of entropy. 2) Explaining the behavior of solids at very low temperature . 3) Measurement of action forces of the reacting substances. 4) Analysing the chemical and phase equilibrium.

THANK YOU