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Announcements 1/26/11. Prayer Please do this “Quick Writing” assignment while you’re waiting for class to start:

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Announcements 1/26/11

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Announcements 1/26/11

  • Prayer

  • Please do this “Quick Writing” assignment while you’re waiting for class to start:

    Ralph is confused because he knows that when you compress gases, they tend to heat up (think of a bicycle pump nozzle getting hotter as you force the gas from the pump to the tire). So, how are “isothermal” processes possible? How can you compress a gas without its temperature increasing?


Demo

  • Constant volume change, aka “alcohol rocket”


Thought question

  • How will the temperature of the gas change during this process from A to B?

    • Increase

    • Decrease

    • First increase, then decrease

    • First decrease, then increase

    • Stay the same


Reading quiz

  • What is “CV”?

    • heat capacity

    • mass-pacity

    • molar heat capacity

    • molar heat capacity, but only for constant volume changes

    • your “curriculum vitae”, a detailed resumé


Thought question

  • Which will be larger, the molar heat capacity for constant volume changes or the molar heat capacity for constant pressure changes? (Hint: Think of the First Law.)

    • constant volume

    • constant pressure

    • they are the same

    • it depends on the temperature


CV and CP

  • Constant volume change (monatomic):

    W = 0

    Eint = Qadded

    (3/2)nRT = Qadded

    Compare to definition of C: Qadded = nCVDT

    CV = (3/2)R (monatomic)

  • Constant pressure change

    • What’s different?

    • result: CP = (5/2)R (monatomic)

  • What would be different for gases with more degrees of freedom?


Reading quiz (graded)

  • What does gamma equal in the equation for an adiabatic process:

    • CP + CV

    • CP - CV

    • CV - CP

    • CV / CP

    • CP / CV


Isothermal vs Adiabatic

  • Isothermal:

  • Adiabatic:

     steeper curves for adiabatic


Thought question

  • How much do you think the temperature of the air in this room would change by if I compressed it adiabatically by a factor of 10? (Vf = V0/10)

    • less than 0.1 degree C

    • about 0.1 degrees C

    • about 1 degree C

    • about 10 degrees C

    • more than 10 degrees C


Demo/Video

  • Demo: freeze spray

  • Video: adiabatic expansion

  • Demo: adiabatic cotton burner


Derivation of PVg (for Monatomic)

Eint = Qadded + Won

(3/2) nRT = - PdV

(3/2) nRdT = -PdV

(3/2) nR d(PV/nR) = -PdV

(3/2) (PdV + VdP) = -PdV

(3/2) VdP = -(5/2) PdV

dP/P = -(5/3) dV/V

lnP = (-5/3)lnV + constant

lnP = ln(V-5/3) + constant

P = constant  V-5/3 (it’s a different constant)

P V5/3 = constant

What’s different

if diatomic?


Thought question

  • Which of the curves on the PV diagram below is most likely to represent an isothermal compression, followed by an adiabatic expansion back to the initial volume?


Thought questions

  • What would be the molar specific heat for an adiabatic process? (Hint: think of Q = nCDT.)

    • CV

    • CV + R

    • CV + 2R

    • CV - R

    • none of the above

  • What would be the molar specific heat for an isothermal process? (Same hint.)

    • CV

    • CV + R

    • CV + 2R

    • CV - R

    • none of the above


Water/steam “saturation curve”: ideal gas?


Water/steam “saturation curve”: ideal gas?


Water/steam “saturation curve”: ideal gas?


Water/steam “saturation curve”: ideal gas?


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