Friday, June 10, 2011

Announcements:

• Our final exam is scheduled for next Tuesday from 8:00 - 11:00 am.  I will hold an optional review session for the final exam next Monday during our regular class time and also at 12:00 pm on Monday in Phillips 215.
• Dmitry will hold an extended SI session on Monday from 5:00 - 7:00 pm in Phillips 220?
• Scanned copies of my solutions for our two midterm exams are now posted in the Course Documents section of the Blackboard site.

Assignments:

• HW17 is due Sunday at midnight
• HW18 is due Monday at midnight
• Web Projects are due today.  Even if you did not publish one, you can receive extra credit for reviewing two sites according to the review guidelines.
• Optional Web Project reviews are due Tuesday morning either on paper or via the Digital Drop Box on Blackboard.
• All coursework is due Tuesday at 8:00 am.  Any work submitted after the final exam will not be graded, so be sure to complete any remaining homework, test corrections, or extra credit before then.

Chapter 17 - Phases and Phase Changes

Ideal gas law:  PV = nRT   (Note specific cases:  Boyle's Law:  PV = constant, and Charles' Law: V/T = constant)

    Note:  Remember that T must be in Kelvin, and n = number of moles, and R is the universal gas constant.

The pressure of a gas is proportional to the average kinetic energy of the gas molecules (since the molecules are bouncing off the walls of the container), which is directly proportional to the absolute (Kelvin) temperature of the gas.  From this relationship, the average (rms) speed of the gas molecules can be calculated, and this is useful for determining whether a planet has an atmosphere based on the escape speed of each type of gas molecule.

Average kinetic energy of a gas:  Kav = (3/2)kT

RMS speed of a gas molecule:  v(rms) = sqrt(3kT/m) = sqrt(3RT/M)

Internal energy of a monatomic ideal gas: U = (3/2)NkT = (3/2)nRT

The heat required to change the phase of a mass is Q = mL, where L is the latent heat of fusion (Lf), vaporization (Lv), or sublimation (Ls).

Solids and Elastic Deformation:

Elastic modulus = stress/strain
Stress is proportional to force exerted and has same units as pressure.
Strain is a measure of degree of deformation.
1D - Young's modulus, Y
2D - Shear modulus, S
3D - Bulk modulus, B
Elastic deformation is where the solid returns to its original size and shape when the stress is removed.  Most materials have an elastic range, but when stresses exceed the elastic limit, the object becomes permanently deformed or breaks.

Review question:  A Galileo thermometer consists of several sealed glass spheres of various specific gravities in a water column.  What will happen when the temperature rises?

Balloon demo and questions relating the ideal gas law with fluid physics:

• Compared with atmospheric pressure, what is the pressure inside an inflated rubber balloon?
• When a balloon is placed in liquid nitrogen, what happens to the pressure and weight?
• Is the buoyant force on a balloon inflated with air more or less than the weight of the air inside the balloon?
• When a deflated balloon is placed on an electronic scale, the scale reads 0.5 g.  When the balloon is inflated, the scale reads 0.7 g.  When the balloon is placed in LN2, its volume is almost zero, and the scale reads 1.4 g.  From these values, determine the magnitude of all the forces acting on an inflated balloon.
• From this example, what can you learn about measuring devices (digital balances in particular)?

Chapter 18 - The Laws of Thermodynamics

The work done by a gas is W = P(Vf - Vi),  where W is positive if the gas expands.
Work done on a gas has the opposite sign:  W = -P(Vf - Vi)

First Law of Thermodynamics:
The change in internal energy of a system is: Uf - Ui = Q - W
    where Q is positive when heat is added to the system, and W is the work done by the gas.
Note: some textbooks state:  Uf - Ui = Q + W, with W as the work done on the gas.

A cyclic process is one in which Uf = Ui, so that Q = W. 
An adiabatic process has no heat transfer into or out of the system: Q = 0.
An isobaric process occurs at constant pressure: dP = 0.
An isothermal process has no change in temperature:  dT = 0.
An isochoric process has no change in volume: dV = 0.  Therefore, W = PdV = 0.
The first law of thermodynamics is an extension of the law of conservation of energy.
The second law of thermodynamics tells us that heat flows from hot to cold objects and that no heat engine can operate at 100% efficiency.  This law also tells us that all natural processes tend toward states of disorder, and that the entropy (disorder) of the Universe is always increasing.

Ponderables:

• When a gas is compressed quickly, what is dU, dQ, and W?
• Fresh air is pumped into the cabin of an airplane flying at 30,000 feet.  Does this air need to be warmed or cooled?  What is the air temperature and pressure at 30,000 ft?
• What is the maximum efficiency of an automobile engine?
• Is it possible for an electric heater to be 100% efficient?
• Is conservation of energy consistent with the Second Law of Thermodynamics?
• Is it theoretically possible to put your hand through a book?
• Why is it impossible to build a perpetual motion machine?
• What is meant by the ultimate heat death of the universe?

Demos:

• Sterling engine
• Thermoelectic motor