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Brief Further Discussion of the 1 st Law: The Signs of Work & Heat

Learn about the signs of work and heat in thermodynamics, common properties, reversible and irreversible processes, and examples. Understand how heat and work interact in systems.

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Brief Further Discussion of the 1 st Law: The Signs of Work & Heat

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  1. Brief Further Discussion of the 1st Law: The Signs of Work & Heat

  2. The Signs of Work & Heat W > 0, Q > 0 A' A W < 0, Q < 0 • By convention, Heat Q & Work W are • both taken aspositivewhen their • direction isintothe system of interest. • Q & W are negativewhen their • direction is out of the system of interest.

  3. The Signs of Work & Heat

  4. Common Properties of Heat & Work 1. Both are only recognized at the boundaries of a system as they cross the boundaries. 2. Systems possess energy, but notheat or work. We can say a process has done some work, but we can’t say that a state has done some work.

  5. Common Properties of Heat & Work 3.Neither heat nor work are state functions. They can’t be expressed in total differential form. Infinitesimal amounts are denoted by đQ& đWinstead of dQ and dW, respectively. Their magnitudes depend on the path.

  6. Reversible Pressure-Volume Work Reversible (Quasi-Static) Processes  Processes for which the timescale is assumed to be so slow that every intermediate state deviates only infinitesimally from equilibrium. • After a reversible process has happened, the process can be reversed & causes no change in either the system or its surroundings. That is, all properties of both system & surroundings must be recovered to their original states. There must be no net work or heat. These are only ideal processes, & do not exist in real world.

  7. Example of a Reversible Process A Reversible Process could be realized in the situation in the figure by removing the sand one grain at a time!

  8. Irreversible Processes  Processeswhichcan’t return both the system & the surroundings to their original conditions without extra work from the outside being done. That is, the system & the surroundings would not return to their original conditions if the process was reversed, unless work is done from the outside. • In irreversible processes, the intermediate states can’t be specified by any set of macroscopic variables. All real processes are irreversible!!

  9. An Irreversible Process

  10. Reversible Pressure-Volume Work For the Ideal Gas ONLY

  11. Expansion Work • 3-step expansion of an ideal gas at constant temp

  12. 3-step compression of an ideal gas at constant temp

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