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Energy Equation 2nd Law of Thermodynamics

Energy Equation 2nd Law of Thermodynamics. Energy cannot be created or destroyed. (see Conservation of Mass for exception) It can, however, change from one form to another; transfer from one system to another. Energy conservation is a Postulate. Closed System.

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Energy Equation 2nd Law of Thermodynamics

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  1. Energy Equation2nd Law of Thermodynamics • Energy cannot be created or destroyed. (see Conservation of Mass for exception) • It can, however, change from one form to another; transfer from one system to another. • Energy conservation is a Postulate

  2. Closed System • In a closed system no mass enters or leaves the system. • However, energy can leave as • Work [W] (organized motion) • Heat [Q] (random motion)

  3. 1st Law for Closed System dECM = Q + W • Work done by system (out) +ve • Heat supplied to system (in) +ve • d - exact differential (state variable) •  - not e. d. (path dependant) • Same per mass, per mole.

  4. Other Forms of 1st Law • In rate form: • In integral form:

  5. Applications of 1st Law • Fixed volume: pdV = 0 only internal energy, E = U

  6. Applications of 1st Law • Constant pressure, slow moving piston • No kinetic energy, no potential energy

  7. Application of 1st Law • Piston with friction: Without (a): With (b): 1st Law still:

  8. Application of 1st Law Assume: ideal gas, same initial/final volume For either a) or b): Then: But:

  9. Application of 1st Law Less energy (heat transfer) required for same expansion process without friction. Show for yourselves: if pa same as pb the gas would expand less since some of the work used to overcome friction

  10. Application of 1st Law • Heat from saturated liquid to saturated vapor at constant p: Q12 = H12 = m(hg - hf) = mhfg hfg - latent heat of vaporization Similarly: latent heat of fusion latent heat of sublimation

  11. 1st Law with Shaft Work

  12. Cycles • A repeated thermodynamic cycle where the initial state equals the final state. • denotes integration over a cycle • By definition:

  13. Cycles First law: Net work out = net heat in: Cycle efficiency:

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