K.J. INSTITUTE OF ENGINEERING AND TECHNOLOGY FACULTIES :- Santosh Varma Bhavin Sir By :- Parth tavrawala ( ) BRANCH :MECHANICAL B (D2D) Engineering Thermodynamics Second Law Of Thermodynamics

SECOND LAW OF THERMODYNAMICS

SECOND LAW (CONT) A process can not happen unless it satisfies both the first and second laws of thermodynamics. The first law characterizes the balance of energy which defines the “quantity” of energy. The second law defines the direction which the process can take place and its “quality”. Define a “Heat Engine”: A device that converts heat into work while operating in a cycle.Heat Engine Heat engine QHQH QLQL THTH TLTL W net  Q-W net =  U (since  U=0 for a cycle)  W net =Q H -Q L Thermal efficiency,  th is defined as  th =W net /Q H =(Q H -Q L )/Q H =1-(Q L /Q H ) Question: Can we produce an 100% heat engine, i.e. a heat engine where Q L =0?

Identifies the direction of a process. (e.g.: Heat can only spontaneously transfer from a hot object to a cold object, not vice versa) Used to determine the “Quality” of energy. (e.g.: A high- temperature energy source has a higher quality since it is easier to extract energy from it to deliver useable work.) Used to exclude the possibility of constructing 100% efficient heat engine and perpetual-motion machines. (violates the Kevin- Planck and the Clausius statements of the second law) Used to introduce concepts of reversible processes and irreversibilities. Determines the theoretical performance limits of engineering systems. (e.g.: A Carnot engine is theoretically the most efficient heat engine; its performance can be used as a standard for other practical engines) Second-law.ppt Modified 10/9/02

STEAM POWER PLANT A steam power plant is a good example of a heat engine where the working fluid, water, undergoes a thermodynamic cycle W net = W out - W in = Q in -Q out Q in is the heat transferred from the high temp. reservoir, and is generally referred to as Q H Q out is the heat transferred to the low temp. reservoir, and is generally referred to as Q L Thermal efficiency  th = W net /Q H = (Q H -Q L )/Q H =1-(Q L /Q H ) Typical Efficiency of a large commercial steam power plant  40% Thermal Reservoir  hypothetical body with a very large thermal capacity (relative to the system beig examined) to/from which heat can be transferred without changing its temperature. E.g. the ocean, atmosphere, large lakes. Back

KEVIN-PLANCK STATEMENT The Kelvin-Planck Statement is another expression of the second law of thermodynamics. It states that: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce net work. This statement is without proof, however it has not been violated yet. Consequently, it is impossible to built a heat engine that is 100%. Heat engine QHQH THTH W net A heat engine has to reject some energy into a lower temperature sink in order to complete the cycle. T H >T L in order to operate the engine. Therefore, the higher the temperature, T H, the higher the quality of the energy source and more work is produced. Impossible because it violates the Kelvin-Planck Statement/Second Law

HEAT PUMPS AND REFRIGERATORS A “heat pump” is defined as a device that transfers heat from a low- temperature source to a high-temperature one. E.g. a heat pump is used to extract energy from outside cold outdoor air into the warm indoors. A refrigerator performs the same function; the difference between the two is in the type of heat transfer that needs to be optimized. The efficiencies of heat pumps and refrigerators are denoted by the Coefficient of Performance (COP) where Heat pump/ Refrigerator QHQH QLQL THTH TLTL W net For a Heat Pump: COP HP =Q H /W net =Q H /(Q H -Q L ) = 1/(1-Q L /Q H ) For a Refrigerator: COP R =Q L /W net =Q L /(Q H -Q L ) = 1/(Q H /Q L -1) Note: COP HP = COP R + 1 COP HP >1, ex: a typical heat pump has a COP in the order of 3 Question: Can one build a heat pump operating COP= , that is W net = 0 and Q H =Q?

CLAUSIUS STATEMENT The Clausius Statement is another expression of the second law of thermodynamics. It states that: It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower- temperature body to a higher-temperature body. Similar to the K-P Statement, it is a negative statement and has no proof, it is based on experimental observations and has yet to be violated. Heat can not be transferred from low temperature to higher temperature unless external work is supplied. Heat pump QHQH QLQL THTH TLTL Therefore, it is impossible to build a heat pump or a refrigerator without external work input.

EQUIVALENCE OF THE TWO STATEMENTS It can be shown that the violation of one statement leads to a violation of the other statement, i.e. they are equivalent. A 100% efficient heat engine; violates K-P Statement Heat pump QLQL QLQL THTH TLTL Heat transfer from low-temp body to high-temp body without work; A violation of the Clausius statement Heat pump Q H +Q L QLQL THTH TLTL W net =Q H Heat engine QHQH

PERPETUAL-MOTION MACHINES (YAC: 5-5) Imagine that we can extract energy from unlimited low-temperature energy sources such as the ocean or the atmosphere (both can be thought of as thermal reservoirs). Heat engine Heat pump QLQL QHQH QHQH W in = Q H -Q L W net =Q L THTH Ocean T L It is against the Kevin-Planck statement: it is impossible to build an 100% heat engine. Perpetual Motion Machines, PMM, are classified into two types: PMM1- Perpetual Motion Machines of the First Kind: They violate the First Law of Thermodynamics PMM2 - Perpetual Motion Machines of the Second Kind : Violate the Second Law of Thermodynamics

REVERSIBLE PROCESSES AND IRREVERSIBILITIES (YAC: 5-6) A reversible process is one that can be executed in the reverse direction with no net change in the system or the surroundings. At the end of a forwards and backwards reversible process, both system and the surroundings are returned to their initial states. No real processes are reversible. However, reversible processes are theoretically the most efficient processes. All real processes are irreversible due to irreversibilities. Hence, real processes are less efficient than reversible processes. Common Sources of Irreversibility: Friction Sudden Expansion and compression Heat Transfer between bodies with a finite temperature difference. A quasi-equilibrium process, e.g. very slow, frictionless expansion or compression is a reversible process.

REVERSIBLE PROCESSES AND IRREVERSIBILITIES (CONT’D) A work-producing device which employs quasi-equlibrium or reversible processes produces the maximum amount of work theoretically possible. A work-consuming device which employs quasi-equilibrium or reversible processes requires the minimum amount of work theoretically possible. One of the most common idealized cycles that employs all reversible processes is called the Carnot Cycle proposed in 1824 by Sadi Carnot.

THE SECOND LAW OF THERMODYNAMICS CAN BE UNDERSTOOD THROUGH CONSIDERING THESE PROCESSES: A rock will fall if you lift it up and then let go Hot pans cool down when taken out from the stove. Ice cubes melt in a warm room.

WHAT’S HAPPENING IN EVERY ONE OF THOSE? Energy of some kind is changing from being localized (concentrated) somehow to becoming more spreed out. i.e in example 1: The potential energy localized in the rock is now totally spread out and dispersed in: A little air movement. Little heating of air and ground.

IN THE PREVIOUS EXAMPLE System: rock above ground then rock on ground. Surroundings: air + ground

The second law of thermodynamics states that energy (and matter) tends to become more evenly spread out across the universe.second law of thermodynamics i.e to concentrate energy (or matter) in one specific place, it is necessary to spread out a greater amount of energy (as heat) across the remainder of the universe ("the surroundings").

WHAT IS ENTROPY? Entropy just measures the spontaneous dispersal of energy: or how much energy is spread out in a process as a function of temperature.

FOLLOW THE ENTROPY Entropy a measure of disorder in the physical system. the second law of thermodynamics – the universe, or in any isolated system, the degree of disorder (entropy) can only increase. the movement towards a disordered state is a spontaneous process.

SO IN A SIMPLE EQUATION: Entropy = “ energy dispersed”/ T Entropy couldn't be expressed without the inclusion of absolute temperature. Entropy change Δ S shows us exactly how important to a system is a dispersion of a given amount of energy.

i.e you can pump heat out of a refrigerator (to make ice cubes), but the heat is placed in the house and the entropy of the house increases, even though the local entropy of the ice cube tray decreases.

ENTROPY CHANGE Δ S In chemical terms entropy is related to the random movements of molecules and is measured by T Δ S. When a system is at equilibrium, no net reaction occurs and the system has no capacity to do work. Q = T Δ S This is a condition of maximum entropy.

Work can be done by system proceeding to equilibrium and measure of the maximum useful work is given by the following equation W = - Δ H + T Δ S

Is the second law of thermodynamics violated in the living cells? Cell is not an isolated system: it takes energy from its environment to generate order within itself. Part of the energy that the cell uses is converted into heat. The heat is discharged into the cell's environment and disorders it. The total entropy increases NO!

Part of the energy that the cell uses is converted into heat. The heat is discharged into the cell's environment and disorders it ► ► ►► The total entropy increases

ENTROPY AND LIFE For example, living things are highly ordered, low entropy, structures, but they grow and are sustained because their metabolism generates excess entropy in their surroundings. For living systems, approaching chemical equilibrium means decay and death.

ENTROPY AND LIFE For living systems, approaching equilibrium means decay and death.

Building blocks The apparent paradox: S Life Equilibrium

GIBBS FREE ENERGY Gibbs introduced the concept of free energy as an another measure of the capacity to do useful work. Free energy G is defined as Δ G = Δ H- T Δ S & W = - Δ H + T Δ S Note that Δ G= -W So that when the measure of W is positive (i.e the system is doing useful work), the measure of Δ G is negative and vice versa.

Gibbs’ free energy (G) change in free energy endergonic - any reaction that requires an input of energy. exergonic - any reaction that releases free energy Reactant Product Energy must be supplied. Energy supplied Energy released Reactant Product Energy is released.

Glucose-1-p Glucose-6-p Since changes in free energy and enthalpy are related only to the difference between the free energies and enthalpies of reactants and products, so we can characterize the above reaction as: Δ G = G g-6-p - G g-1-p or Δ H= H g-6-p - H g-1-p

If the algebraic sign is: 1- negative, the reaction is exergonic (i.e it will proceeds spontaneously from left to right as written). 2-Positive, the reaction is endergonic, (i.e it will not proceeds spontenously. 3- Zero, the reaction is at equilibrium.

When Δ H is: 1- negative, the reaction is exothermic (i.e it gives off heat to its surroundings). 2-Positive, the reaction is endothermic (i.e it take heat from its surroundings). 3- Zero, the reaction is isothermic ( no net exchange of heat occurs with the surroundings).

STANDARD FREE ENERGY “ Δ G°” “ Δ G°” of a chemical reaction are calculated at 25 C° and at 1 atmospheric pressure. The biological standard free energy Δ G° − ” is more useful in biochemistry, here the standard conditions are: pH = 7 Temp = 37 C° 1 M concentrations of reactants and products.

THANKS..... HAPPY DIWALI