Thermodynamics

1. Thermodynamics Carlos Silva November 11th 2009

2. The power of heat • From the greek therme (heat) and dynamis (power,force) • The capacity of hot bodies to produce work Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance Sadi Carnot (1796-1832)

3. Laws of thermodynamics • 0th • Definition of temperature • Systems at different temperatures exchange energy until reaching a thermal equilibrium • 1st • Conservation of energy • heat is a form of energy • 2nd • Entropy of an isolated system never decreases • perpetual motions of machines is impossible • 3rd • Entropy at absolute zero temperature (0 K) • it is impossible to cool a system until zero

4. BASIC DEFINITIONS

5. Closed Systems and Control Volume • System • a set of interacting or interdependent entities, real or abstract, forming an integrated whole • Closed System • System that is isolated from its surroundings • In thermodynamics • a closed system can exchange heat and work (energy), but not matter, with its surroundings • Isolated system cannot exchange anything • Control Volume • Region of space through which mass flows

6. Work, Power, Energy • Work (J) • Measure of motion accomplishment of a system due to the action of a force over a distance and time (Dynamics) • (…) work expresses the useful effect that a motor is capable of producing. This effect can always be linked to the elevation of a weight to a certain height(…) the product of the weight multiplied by the height to which it is raised” (Sadi Carnot) • Power (W=J/s) • The rate at which work is done • Energy (J) • Amount of work that can be accomplished by a force • Is the capacity of a system to perform work

7. Tonne of oil equivalent • Energy released by burning one tonne of crude oil (Toe) • Approximately 42 GJ (oil properties can vary) • International Energy Agency: • 10 Gcal • 41,868 GJ • 11,630 MWh • 7,4 barrels of oil

8. Energy: Primary and Final • Primary • Energy contained in raw fuels • Final • Energy available after conversion and transportation systems • Útil • Energy after utilization Sankey diagram for S.Miguel 2007

9. Property, State and Process • Property – macroscopic characteristic of a system • Extensive properties • The value for the overall system is the sum of the values for its parts (mass, volume, energy) • Intensive properties • The values are not additive, may vary from one place to the other at any time (pressure, temperature, specific volume) • State – a condition of a system, described by the properties • usually a snapshot in time x(t)=(P,T) • Process – change of properties and therefore state of the system • brings the system from x(t) to x(t+1)

10. Specific volume, Density • Specific volume • volume occupied by a unit of mass (mass / volume) • water 4º - 1dm3/kg • Iron -128,2 cm3/kg • Density • mass by unit of volume (volume/mass) • Water at 4º - 1000kg/m3 / water at 20º - 998kg/m3 • Iron - 7800kg/m3

11. Temperature and Pressure • Pressure (Pa=N/m2) • Effect of a force in a surface • Caused by the collision of molecules to the boundaries of a system • Temperature (K) • At the microscopic scale, is a measure of the energy of the particles • solid state (vibration of molecules) • liquid (translation movement) • gas (vibration and rotation movements • Thermal equilibrium – system does not change temperature

12. Heat, Specific Heat • Heat (J) • is the process of energy transfer from one body or system due to thermal contact • can be defined as thermal energy • energy of a body that increases with temperature • Specific heat • energy required to increase 1 degree of a 1unit (kg or mol) of a substance • Can be measured at constant pressure (Cp) • Water - 4,186 J/(g·K) (25 º C) / 2,080 J/(g·K) (100º C) • Can be measured at constant volume (Cv)

13. Efficiency • Thermal Efficiency • Heat Engines • Carnot Efficiency

14. Coefficient of Performance • Some devices use work to move heat from one place to other • inverse process of thermal machines • Heat Pumps • Air conditioners

15. Laws of thermodynamics Zeroth LAW

16. Systems thermodynamic equilibrium • When two systems are put in contact with each other, there will be a net exchange of energy between them unless or until they are in thermal equilibrium, that is, they are at the same temperature • "If A and C are each in thermal equilibrium with B, A is also in thermal equilibrium with C.“ • single temperature and pressure can be attributed to the whole system

17. Laws of thermodynamics FiRST LAW

18. Enthalpy (H) • Measure of internal energy of a closed system • sum of internal energy plus the product of pressure and volume • For constant pressure, the enthalpy increases with heat • Specific enthalpy (J/kg) • Energy per unit of mass (PCI) • Low (hidrocarbonets) • Fuel 42MJ/kg • Propane 46 MJ/kg • High

19. Conservation of Energy • The total amount of energy in a closed system remains constant over time (are said to be conserved over time) • The increase in the internal energy of a system is equal to the amount of energy added by heating the system minus the amount lost as a result of the work done by the system on its surroundings. • Energy cannot be created nor destroyed • Energy can change form (for example chemical to thermal)

20. Laws of thermodynamics Second LAW

21. Entropy (S) • Thermodynamics • Measure of uniformity of the distribution (quality) of energy • Information • For a system whose exact description is unknown, its entropy is defined as the amount of information needed to exactly specify the state of the system

22. Entropy increases in nature • Temperature differences between systems in contact with each other tend to even out and that work can be obtained from these non-equilibrium differences, but that loss of heat occurs, in the form of entropy, when work is done • In a system, a process that occurs will tend to increase the total entropy of the universe • Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature (Clausius) • It is impossible to convert heat completely into work in a cyclic process (Kelvin)

23. Reversible and Irreversible Processes • Reversible (ideal) • system and surroundings can be restored to the initial state from the final state without producing any changes in the thermodynamics properties • it should occur infinitely slowly due to infinitesimal gradient • all the changes in state occurred in the system are in thermodynamic equilibrium with each other • Irreversible (natural) • All processes in nature are irreversible • Finite gradient between the two states of the system • heat flow between two bodies occurs due to temperature gradient between the two bodies;

24. Laws of thermodynamics Third Law

25. Entropy at absolute zero (0 K) • As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value • decreasing entropy of a system requires increasing the entropy of surroundings

26. Thermodynamic Processes

27. Boyle’s Law • The absolute pressure and volume of a gas (ideal) are inversely proportional, if the temperature is kept constant within a closed system • Ideal Gas law • k - Boltzman constant (8.314 J·K−1mol-1) • n – number of moles

28. Different Processes Isobaric Isometric Isothermal ΔT = 0 but Q ≠ 0 Adiabatic ΔT ≠ 0 but Q = 0 Cyclic If clockwise – heat engine If counterclockwise – heat pump

29. Ideal (Carnot) Cycle • Carnot Theorem • No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between those same reservoirs Pressure-Volume Temperature-Entropy

30. Real Cycles • There are no ideal cycles • Irreversible systems, losses of heat