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The Zeroth and First Laws

The Zeroth and First Laws. Introduction. Mechanical energy includes both kinetic and potential energy. Kinetic energy can be changed to potential energy and vice versa. Introduction. Total mechanical energy (E) is the sum of kinetic and potential energies.

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The Zeroth and First Laws

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  1. The Zeroth and First Laws

  2. Introduction • Mechanical energy includes both kinetic and potential energy. • Kinetic energy can be changed to potential energy and vice versa.

  3. Introduction • Total mechanical energy (E) is the sum of kinetic and potential energies. • Changes in a system’s total mechanical energy (ΔE) are important.

  4. Thermal Energy • due to the rapid, random motion of the molecular, atomic, and subatomic particles of matter • can be subdivided into kinetic energy and potential energy

  5. Thermal Energy • average kinetic energy is proportional to the temperature of a substance • Internal energy (U): sum of the particle kinetic and potential energies

  6. Zeroth Law • adiabatic boundary: no thermal energy can pass through • diathermic: ideal conductor of thermal energy

  7. Zeroth Law • thermal equilibrium: objects have reached the same temperature

  8. Zeroth Law Two systems that are in thermal equilibrium with a third must be in thermal equilibrium with each other

  9. Zeroth Law • If no net energy exchange occurs in (a), then none will occur in (b).

  10. First Law • The General Law of Conservation of Energy • in general: Q + Wncf = ΔU + ΔE

  11. First Law • mathematical statement of the first law of thermodynamics: Q = ΔU + W

  12. First Law The heat transferred to or from a system is equal to the sum of the change of the system’s internal energy and the work the system does on its surroundings.

  13. can do mechanical work by absorbing and discharging heat • the simplest example is an expanding gas • cylinder with piston

  14. quasi-static process: gas expands without ever being far from thermal equilibrium • gas pressure inside cylinder is in equilibrium with external pressure

  15. work is done on the gas when it is compressed from V1 to V2 • gas warms when it is compressed • work done by gas on surroundings is negative

  16. work done by gas when expanding or contracting against a constant pressure: W = P(V2 – V1)

  17. P-V Diagrams • pressure against a gas is not always constant • graphing pressure versus volume (P-V diagram) makes some equations easier to solve

  18. P-V Diagrams Notice that the area under the curve representing the process on a P-V diagram is equal to the absolute value of the work done by the gas during the process!

  19. P-V Diagrams The sign of the work depends on whether the gas gains or loses energy. Gas expands → does work on surroundings → sign is positive Gas contracts → surroundings do work on it → sign is negative

  20. Expansion Cycles • If a gas is to be useful as a machine, it must be able to expand repeatedly, following a cycle.

  21. Expansion Cycles • For a cycle, the absolute value of the work done is equal to the area enclosed by the path of the cycle on a P-V diagram. • Clockwise path: + work • CCW path: – work

  22. Expansion Cycles • The work done by a gas depends on the path of the process in a P-V diagram. • Heat engines: positive • Refrigerators: negative

  23. State Variables • Internal energy is path-independent: its change does not depend on the way the energy is added. • Path-independent quantities are called state variables.

  24. Thermodynamic Systems • a piece of the universe isolated for study • if it is not part of the system, it is part of the surroundings

  25. Open System • can exchange both matter and energy with its surroundings • Ex.: ice cube resting on a kitchen counter

  26. Closed System • can exchange energy but not matter with its surroundings • Ex.: expanding gas in a thermally conducting cylinder with a gas-tight piston

  27. Isolated System • cannot exchange energy or matter with its surroundings • Ex.: liquid in a perfectly insulated vacuum flask

  28. Isolated System • energy is conserved • energy may be converted but none leaves or enters • universe is the only true isolated system • no practical system is isolated

  29. Isolated System • The First Law of Thermodynamics is a conservation law • It can be stated as...

  30. Isolated System In an isolated system, the total quantity of energy is constant, neither being created nor destroyed.

  31. Thermodynamic Processes • a change in the thermodynamic state of a system • often categorized by which variables are held constant

  32. Thermodynamic Processes • Adiabatic process: exchanges no thermal energy between system and its surroundings • Q = 0 ΔU = -W

  33. Thermodynamic Processes • Isothermal process: temperature of the system is constant • no phase changes • ΔU = 0 J Q = W

  34. Thermodynamic Processes • Isochoric process: volume of the system is constant • W = 0 J Q = ΔU

  35. Thermodynamic Processes • Isobaric process: pressure of the system is constant • W = PΔV Q = ΔU + PΔV

  36. Thermodynamic Processes • A process that allows the use of ideal gas relationships is known as an ideal gas process.

  37. The Second and Third Laws

  38. Heat Engines • The surroundings must contain either a source for thermal energy, a sink (receiver) for thermal energy, or both. • Heat reservoir—temperature cannot be changed significantly

  39. Heat Engines • Hot reservoir • higher temperature than the system • source of thermal energy for the system

  40. Heat Engines • Cold reservoir • lower temperature than the system • thermal energy sink for the system • Both types are used to operate a heat engine.

  41. Second Law of Thermodynamics Energy flows from an area of higher concentration to an area of lower concentration.

  42. Heat Engines • Requirements: • hot reservoir • cold reservoir • working fluid (liquid or gas)

  43. Heat Engines • Overview: • thermal energy absorbed from hot reservoir causes fluid to expand • expansion causes mechanical work

  44. Heat Engines • Overview: • fluid gives up thermal energy to cold reservoir and contracts • fluid is heated to expand again

  45. Early Steam Engines • aeolipile • Hero of Alexandria • not cyclic • Thomas Savery • first practical steam engine—water pump

  46. Early Steam Engines • Thomas Newcomen • James Watt • used separate chambers to heat and cool steam • helped begin the Industrial Revolution

  47. Early Steam Engines • James Watt • double-acting piston • additional mechanical improvements

  48. The Carnot Cycle • Reversible process: quasi-static process that leaves the system in exactly the same state after occurring twice, once normally and once in reverse

  49. The Carnot Cycle • Reversible cycle: leaves the system in the same state as it was before the entire process occurred • most efficient means of converting thermal energy to mechanical work

  50. The Carnot Cycle • Carnot cycle is the most efficient cycle that can operate between two temperatures • four-step, reversible cycle

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