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This ppt based on 2nd law of thermodynamics.

- 1. WHY 2ND LAW OF THERMODYNAMICS:- The 1st law of thermodynamics states that a certain energy flow takes place when a system under goes a process or change of state. δW=δQ But , it does not give any information on whether that process or change of state is possible or not.
- 2. ACCORDING TO 1ST LAW OF THERMODYNAMICS:- Work is completely converted in to heat or heat is completely converted into work. (δW = δQ and δQ=δW) Potential energy can be transformed into kinetic energy or kinetic energy can be transformed into Potential energy. (PE→KE and KE→PE) Heat flows from hot to cold or from cold to hot. (Th → Tl and Tl → Th) Gas expands from high pressure to low pressure or from low pressure to high
- 3. LIMITATION OF 1ST LAW OF THERMODYNAMICS:- Heat is not completely converted into work.(Q ˃ W) kinetic energy can not be transformed into Potential energy.(KE ≠> PE) Heat flow from cold to hot is not possible.(Tl ≠> Th) Gas expands from low pressure to high pressure is not possible.
- 4. LIMITATION OF 1ST LAW OF THERMODYNAMICS:- 1st law does not help to predict whether the certain process is possible or not. A process can be proceed in particular direction only , but 1st law does not gives information about direction. 1st law not provides sufficient condition for certain process to take place.
- 5. 2ND LAW OF THERMODYNAMICS:- Heat can not flow itself from cold body to hot body. The 2nd law of thermodynamics is also used to determine the theoretical limits for the performance of mostly used in engineering systems like heat engines and refrigerators.
- 6. DIRECTION OF CHANGE:- The second law of thermodynamics asserts that processes occur in a certain direction and that the energy has quality as well as quantity. In above example, two bodies are different temperature are brought into contact, heat energy flows from the body at high temperature to that at low temperature. Heat energy never flow from lower temperature level to higher temperature without applying external work. Hot Container Cold Surroundings Possible Impossible
- 7. KELVIN PLANK STATEMENT:- It is impossible to construct a heat engine that operates on cycle to receive heat from single reservoir and produce equivalent amount of work. It implies that it is impossible to build a heat engine that has 100% thermal efficiency .
- 8. CLAUSIUS STATEMENT:- It is impossible to construct a device as heat pump that operates in a cycle and produces no effect other then the transfer of heat from a lower temp body to higher temp body. Heat can not itself flow from colder body to hater body.
- 9. 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. 1. Violation of Kelvin-plank statement leading to violation of Clausius statement . 2. Violation of Clausius statement leading to violation of Kelvin-plank statement .
- 10. VIOLATION OF KELVIN-PLANK STATEMENT LEADING TO VIOLATION OF CLAUSIUS STATEMENT:-
- 11. VIOLATION OF CLAUSIUS STATEMENT LEADING TO VIOLATION OF KELVIN-PLANK STATEMENT :-
- 12. ENTROPY:- Is a thermodynamic function of the state of the system Can be interpreted as the amount of order or disorder of a system As with internal energy, it is the change in entropy is important and not its absolute value. Change in Entropy:- The change in entropy S of a system when an amount of thermal energy Q is added to a system by a reversible process at constant absolute temperature T is given by: S = Q / T
- 13. EXAMPLE:- 1. A heat engine removes 100 J each cycle from a heat reservoir at 400 K and exhausts 85 J of thermal energy to a reservoir at 300 K. Compute the change in entropy for each reservoir. Since the hot reservoir loses heat, we have that: S = Q / T = -100 J / 400 K = -0.25 JK-1 For the cold reservoir we have: S = Q / T = 85 J / 300 K = 0.283 JK-1 Therefore: The increase in entropy of the cold reservoir is greater than the decrease for the hot reservoir.
- 14. ENTROPY OF IDEAL GAS:- An expression for the entropy change of an ideal gas can be obtained from Eq. ds = 𝑑𝑢 𝑇 + 𝑃 𝑑𝑣 𝑇 ----- Eq. 1 Or ds = 𝑑ℎ 𝑇 - 𝑣 𝑑𝑃 𝑇 ----- Eq. 2 by employing the property relations for ideal gases. By substituting du = cv dT and P = 𝑹𝑻 𝑽 into Eq. 1, the differential entropy change of an ideal gas becomes ds = cv 𝑑𝑇 𝑇 + R 𝑑𝑣 𝑣
- 15. The entropy change for a process is obtained by integrating this relation between the end states: ΔS = 𝑐 𝑣 𝑑𝑇 𝑇 + 𝑅 ln 𝑣2 𝑣1 ------ Eq. 3 A second relation for the entropy change of an ideal gas is obtained in a similar manner by substituting dh = cpdT and v = 𝑅𝑇 𝑃 into Eq. 2 and integrating. The result is ΔS = 𝑐 𝑝 𝑑𝑇 𝑇 + 𝑅 ln 𝑝1 𝑝2 ------ Eq. 4
- 16. From Eq.(3) and (4), we see that for constant volume process, ΔS = 𝑐 𝑣 𝑑𝑇 𝑇 = 𝑐 𝑣 ln T2 T1 For constant pressure process, we have ΔS = 𝑐 𝑝 𝑑𝑇 𝑇 = 𝑐 𝑝 ln T2 T1 For isothermal process, ΔS = R ln 𝑣2 𝑣1 = R ln 𝑝1 𝑝2
- 17. THE CARNOT PRINCIPLE:- The Carnot cycle is a thermodynamic process that describes how a fluid is used to convert thermal energy into work. Characteristics:- High Efficiency Multi-Source Engine Better reliability and easier maintenance Reversible Safe, discrete and oxygen-free Modularity and flexibility
- 18. P-V DIAGRAM:- A pressure volume diagram is used to describe corresponding changes in volume and pressure in a system. The PV diagram, called an indicator diagram, was developed by James Watt and his employee John Southern (1758–1815) to improve the efficiency of engines.
- 19. This is the PV diagram….. isothermal segments (AB and CD) occur when there is perfect thermal contact between the working fluid and one of the reservoirs, so that whatever heat is needed to maintain constant temperature will flow into or out of the working fluid, from or to the reservoir.
- 20. adiabatic segments (BC and DA) occur when there is perfect thermal insulation between the working fluid and the rest of the universe, including both reservoirs, thereby preventing the flow of any heat into or out of the working fluid.
- 21. THERMAL EFFICIENCY:- The thermal efficiency of any engine working between the temperatures of T1 and T2 is i.e. increase the temperature difference under which the engine works.