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Conservation

Conservation. Mechanical work involves a force acting through a distance. Work can involve a change in internal energy. Temperature may change Temperature did not cause the work. Work Expanded. Work is an energy transfer that is not due to a temperature difference. .

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Conservation

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  1. Conservation

  2. Mechanical work involves a force acting through a distance. Work can involve a change in internal energy. Temperature may change Temperature did not cause the work Work Expanded Work is an energy transfer that is not due to a temperature difference. Heat is an energy transfer that is due to a temperature difference.

  3. First Law of Thermodynamics • Internal energy can only change by heat transferred to and work done by a system. • Heat added is + • Heat lost is – • Work by a system is + • Work on a system is - When work is defined as positive on a system it is DU = Q + W.

  4. PV Diagram • The ideal gas law links pressure, volume, and temperature. • PV = nRT • For fixed quantity of gas, any two of the three determine the third. • Graph of pressure vs volume describes the system. P V

  5. Slow Motion • During rapid change the macroscopic state variables are not well defined. • On a PV diagram it is assumed that the system changes slowly. • This is a quasi-static change.

  6. Measuring Work • The mechanical work can be measured on the PV diagram. • The work done is the area under the PV curve describing the system. P V work done

  7. Constant Temperature • Changes often involve holding one variable constant. • Moving a piston slowly changes pressure and volume. • Temperature remains the same. Walls and top are insulating Base is kept at a constant temperature

  8. Isothermal Process • For an ideal gas at constant temperature DT = 0, P is inversely proportional to V. • P = nRT(1/V) • The work done by an ideal gas can be found with calculus. P V

  9. Internal Energy Conserved • An ideal gas only changes internal energy with temperature change. • DU = 0 • From the first law, work done equals heat flow. • DU = Q – W • Q = W Work done by gas Heat flow into gas

  10. A scuba diver is swimming at a depth of 25 m where the pressure is 3.5 atm. The exhaled air forms bubbles 8.0 mm in radius. The bubbles stay at 300 K as they rise. How much work is done by each bubble as it expands reaching the surface? The ideal gas law is used to find the amount of gas. P and V are related. This is an isothermal process. W = 0.95 J. Underwater next

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