Announcements 1/26/11

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?

a. Increaseb. Decreasec. First increase, then

decreased. First decrease, then

increasee. Stay the same

Reading quiz

What is “CV”?

a. heat capacity b. mass-pacityc. molar heat capacityd. molar heat capacity, but only for

constant volume changese. 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.)

a. constant volumeb. constant pressurec. they are the samed. it depends on the temperature

CV and CP

Constant volume change (monatomic):W = 0Eint = Qadded

(3/2)nRT = Qadded

Compare to definition of C: Qadded = nCVT

CV = (3/2)R (monatomic)

Constant pressure changea. What’s different?b. 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:

a. CP + CV

b. CP - CV

c. CV - CP

d. CV / CP

e. CP / CV

constantPV

Isothermal vs Adiabatic

Isothermal: Adiabatic:

steeper curves for adiabatic

constantPV constantPV

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)

a. less than 0.1 degree Cb. about 0.1 degrees Cc. about 1 degree Cd. about 10 degrees Ce. more than 10 degrees C

Demo/Video

Demo: freeze spray Video: adiabatic expansion Demo: adiabatic cotton burner

Derivation of PV (for Monatomic)

Eint = Qadded + Won

(3/2) nRT = - PdV(3/2) nRdT = -PdV(3/2) nR d(PV/nR) = -PdV(3/2) (PdV + VdP) = -PdV(3/2) VdP = -(5/2) PdVdP/P = -(5/3) dV/VlnP = (-5/3)lnV + constantlnP = ln(V-5/3) + constantP = constant V-5/3 (it’s a different

constant)P V5/3 = constant

What’s differentif 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?

a.

b.

c.

d.

e.

Thought questions What would be the molar specific heat for an

adiabatic process? (Hint: think of Q = nCT.)a. CV

b. CV + Rc. CV + 2Rd. CV - Re. none of the above

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

a. CV

b. CV + Rc. CV + 2Rd. CV - Re. none of the above

Water/steam “saturation curve”: ideal gas?

0.00 0.01 0.02 0.03 0.04 0.05 0.060

5000

10000

15000

20000

25000

30000

Part Liquid, Part GasAll Liquid

Pre

ssu

re (

kPa

)

Volume, divided by mass (m3/kg)

bk = actual data for water/steam

All Gas

adding heatenergy, T=325C

1 atm= 101 kPa


0.00 0.01 0.02 0.03 0.04 0.05 0.060

5000

10000

15000

20000

25000

30000


Pre

ssu

re (

kPa

)


bk = actual data for water/steamrd = ideal gas, T=400bl = ideal gas, T=500gn = ideal gas, T=600

All Gas


1 atm= 101 kPa


0.00 0.01 0.02 0.03 0.04 0.05 0.060

5000

10000

15000

20000

25000

30000


Pre

ssu

re (

kPa

)



All Gas

"critical point"


1 atm= 101 kPa


0.00 0.01 0.02 0.03 0.04 0.05 0.060

5000

10000

15000

20000

25000

30000

Part Liquid, Part Gas

????

All Liquid

Pre

ssu

re (

kPa

)



All Gas

"critical point"


1 atm= 101 kPa

Documents

Announcements 1/26/11