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Physics 52 - Heat and Optics Dr. Joseph F. Becker Physics Department San Jose State University

Physics 52 - Heat and Optics Dr. Joseph F. Becker Physics Department San Jose State University. © 2005 J. F. Becker. Chapter 19 The First Law of Thermodynamics. Ch. 19 First Law of Thermodynamics

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Physics 52 - Heat and Optics Dr. Joseph F. Becker Physics Department San Jose State University

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  1. Physics 52 - Heat and OpticsDr. Joseph F. BeckerPhysics DepartmentSan Jose State University © 2005 J. F. Becker

  2. Chapter 19The First Law of Thermodynamics

  3. Ch. 19 First Law of Thermodynamics 1. Thermodynamic systems2. Work done during volume changes3. Paths between thermodyn. states4. Internal energy and the First Law 5. Kinds of thermodynamic processes6. Internal energy of an ideal gas7. Heat capacities of an ideal gas8. Adiabatic processes for ideal gas

  4. Sign conventions:a) Q is positiveb) Q is negativec) W is positived) W is negative e) In a steam engine the heat in and the work out (done) are both defined to be POSITIVE.

  5. (a) Molecule hits a piston moving away from it; speed and KE of molecule decrease. (expan. -> cooling) (b) Molecule hits a piston moving toward it; speed and KE of molecule increase. (comp. -> heating)

  6. WORK-ENERGY THEOREM When a molecule hits a wall moving away from it, the molecule does work on the wall; the molecule’s speed and kinetic energy decrease. The gas does positive work (on the piston); and the internal energy of the gas decreases. Molecule hits a wall moving toward it, the wall does work on the molecule; the molecule’s speed and kinetic energy increase. The gas does negative work (on the piston); and the internal energy of the gas increases.

  7. The infinitesimal work done by the system (gas) during the small expansion dx is dW = p A dx = F dx so W = òp dV

  8. The work done equals the area under the curve on a p-V diagram.(a) Volume increases: work and area are positive. (b) Volume decreases: work and area are negative. (c) A constant-pressure: Volume increases, W > 0.

  9. (a) Three different paths between state 1 and state 2.(b)-(d) The work done by the system during a transition between two states depends on the PATH (or PROCESSES) chosen.

  10. The final states are the same. The intermediate states (p and V) during the transition from state 1 to 2 are entirely different. Heat, like work, depends on the initial and final states AND the path between the states. (a) Heat is added slowly so as to keep the temperature constant. (b) Rapid, free expansion does no work and there is no heat transfer. HEAT ADDED

  11. The First Law of Thermodynamics DU = Q - W In a thermodynamic process, the internal energy of a system may increase, decrease, or remain the same:(a) DU > 0(b) DU < 0 (c) DU = 0

  12. Daily thermodynamic process of your body (a thermodynamic system)

  13. The net work done by the system in the CYCLIC PROCESSaba is –500 J.

  14. A p-V diagram of a cyclic process.Q = ? <

  15. KINDS OF THERMODYNAMIC PROCESSESADIABATIC – NO HEAT TRANSFER; Q=0; DU=-WISOCHORIC – CONSTANT VOLUME; W=0; DU = Q ISOBARIC – CONSTANT PRESSURE; W=p (V2 – V1)ISOTHERMAL – CONSTANT TEMPERATURE; only for ideal gas DU = 0, Q = W The First Law of Thermodynamics DU = Q - W or Q = DU + W

  16. Processes for an ideal gas:adiabatic Q=0isochoric W=0isothermal DU=0 W = 0 DU = 0 Q = 0

  17. The internal energy of an ideal gas depends only on its temperature, not on its pressure or volume.

  18. Raising the temperature of an ideal gas by various processes: Q = DU + W and DU = f(T)IsochoricW = 0 so Q = DUIsobaricQ = DU + p(V2-V1)

  19. A p-V diagram of an adiabatic process for an ideal gas from state a to state b. pV g = constant (adiabat) pV = constant (isotherm)

  20. ADIABATIC PROCESSES for IDEAL GAS Q=0 so DU = Q – W = 0 – p DV and DU = n Cv dT for an ideal gasDU = n Cv dT = – pDV = -(n R T/V) dV dT/T + (R/ Cv) dV/V = 0dT/T + (g - 1) dV/V = 0 where g – 1 = (Cp/Cv ) –1òdT/T + (g - 1)òdV/V = 0 ; lnT+(g-1) lnV = const ln (TV g-1 ) = constant and T1V1g-1 =T2V2g-1 And since pV = nRT, (pV/nR) V g-1 = constant’ (p/nR)V g= constant’p1V1g=p2V2g

  21. Adiabatic compression of air in a cylinder of a diesel engine.

  22. p Carnot Cycle for an ideal gas HOT COLD

  23. The Carnot cycle for an ideal gas. Isotherms in light-blue. Adiabats in dark-blue.

  24. Along which path is a) work done and b) heat transferred by the system greatest? p Q = DU + W

  25. The absolute value of heat transfer during one cycle is 7200 J. Assigned for HW

  26. A quantity of air is taken from state a to state b along the straight line. Is Tb > Ta? Given values for all p’s and V’s, calculate the work.

  27. Q = 90 J W = 60 J Q = ? |W| = 35 J Q = ? W = 15 J

  28. Ub = 240 J. Uc = 680 J. Qbc = ? Wabc = 450 J Qab = ? Qdc = ? Wadc =120 J Qad = ? Ud = 330 J. Ua = 150 J.

  29. Review

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