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Packet 3.4 Thermodynamics

Packet 3.4 Thermodynamics. Internal Energy W = P Δ V 1 st Law of Thermodynamics: Δ U = Q – W Define: Adiabatic, isothermal, isobaric & isochoric and show these on a P-V diagram Irreversibility & disorder Entropy is a measure of disorder State 2 nd Law of Thermodynamics

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Packet 3.4 Thermodynamics

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  1. Packet 3.4Thermodynamics • Internal Energy • W = PΔV • 1st Law of Thermodynamics: ΔU = Q – W • Define: Adiabatic, isothermal, isobaric & isochoric and show these on a P-V diagram • Irreversibility & disorder • Entropy is a measure of disorder • State 2nd Law of Thermodynamics • Heat engine efficiency, η = W/Qh • Carnot Engine • Energy Degradation

  2. Internal Energy • Is the total potential and kinetic energy of the molecules in a substance. • Potential energy is associated with intermolecular forces. • Kinetic energy includes both translational and rotational motion. • When we consider an ideal gas, the intermolecular forces are assumed to be zero! • Internal energy of a gas comes only from the random kinetic energy of the atom of the gas.

  3. Ideal Gas & Internal Energy • Ek = 1/2mv2 = 3/2kT so, U = 3/2 NkT • U =3/2nRT • Δ U = 3/2 nR Δ T

  4. Ideal Gas & Internal Energy • What is the internal energy of 30 moles of oxygen gas at room temperature? • U =3/2nRT

  5. Ideal Gas & Internal Energy • If the room were moving at a high velocity would that mean the internal energy of the gas would be greater?

  6. Work done on or by a gas • Imagine compressing a gas by exerting a force on the piston from the outside. • Consider heating the piston and it expands to perform work. • W = F x D & F = PA • W = P (A x D) • W = P Δ V

  7. Example • A gas is compressed at constant pressure 2.00 x 105 Pa from a volume of 2.00 m3 to a volume of 0.500 m3. • What is the work done on the gas. • If the temperature initially was 40˚ C what is the final temperature of the gas?

  8. Thermodynamic Processes Isochoric Isobaric Isothermal • CLICK HERE – TO GO OVER EACH PROCESS!! Const. Volume Const. Pressure Const. Temp • Remember! Area under curve is Work Done!

  9. Adiabatic Expansion • Rapid expansion or compression of a gas. • No Heat (Q) can flow in or out of the system. ΔQ = 0 • Any work done equals a direct change in internal energy. ΔU = ΔW • Bottle Rockets • Diesel Engines

  10. Figure 18-11Adiabatic Heating

  11. The First Law of Thermodynamics (FLT) -- Heat and work are forms of energy transfer and energy is conserved. U = Q - Wby change in total internal energy work done by the system heat added to system State Function Process Functions or U = Q + Won

  12. "Process Problems" For which process is W the largest? smallest?For which process is Q the largest? smallest?

  13. Physics Joke • Once all the scientists die and go to heaven. They decide to play hide-n-seek and Einstein goes first. Einstein counts up to 100 and then start searching. Everyone starts hiding except Newton. Newton just draws a square of 1 meter and stands in it, right in front of Einstein. Einsteins counting ....97,98,99,100, opens his eyes and finds Newton standing in front. Einstein says "Newtons out, Newtons out." Newton denies and says I am not out. He claims that he is not Newton. All the scientists come out and he proves that he is not Newton. How??

  14. His proof: • Newton says: I am standing in a square of area 1m square.. That means I am Newton per meter square.. Hence I am Pascal. Since newton per meter square = Pascal

  15. Conceptual Checkpoint 18-2Which is the adiabatic curve?

  16. The second law of thermodynamics When objects of different temperatures are brought into thermal contact, the spontaneous flow of heat that results is always from the high temperature object to the low temperature object.

  17. Heat Engines • Max Efficiency: TH – Tc/ Th= x 100 • Energy goes in • Useful Work taken out • Some gets wasted

  18. The 2nd Law of Thermodynamics • The second law of thermodynamics deals with the limitations imposed on heat engines: that is on devices whose aim is to covert heat (disordered energy) into mechanical energy (ordered energy).

  19. The 2nd Law of Thermodynamics • The Entropy of an isolated system never decreases. • It is impossible for heat to (spontaneously) flow from a cold to a hot object. • It is impossible for a heat engine working in a cycle to absorb heat and perform an equal amount of work. • The most efficient heat engine operating between two given temperatures is the Carnot Engine.

  20. Entropy • Entropy like Internal energy is a State Function! • Entropy Is a measure of the disorder of a system. • ΔS = ΔQ/T • If ΔQ > 0 entropy increases • If ΔQ < 0 entropy decreases.

  21. Philosophy • Three Laws of Thermodynamics (paraphrased): First Law: You can't get anything without working for it. Second Law: The most you can accomplish by work is to break even. Third Law: You can't break even.

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