# Announcements 1/26/11 - PowerPoint PPT Presentation

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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

• 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

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?

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

• CP + CV

• CP - CV

• CV - CP

• CV / CP

• CP / CV

• Isothermal:

### 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

• more than 10 degrees C

### Demo/Video

• Demo: freeze spray

### Derivation of PVg (for Monatomic)

(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