Thursday, June 9, 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 1:00 pm on Monday in Phillips 215.
• Matt will hold an extended SI session on Monday from 4:00 - 6:00 pm in Phillips 208.
• Web Projects are being published.  Check the list and let me know if I should add or modify your information.
• If you did not take the post-concepts survey yesterday, you can do so today immediately after class.
• Grades for the Flywheel RWP for Exam 2 are now posted on Blackboard.  Consult my solution and grading rubric, and let me know if you believe that your score is not correct.
• Several students still have not signed the honor pledge for this exam problem, and thus no score is posted.  It is not too late to correct this omission.
• Today is the last day to withdraw from summer school.
• Remember to bring your laptop computer with you to take the lab exam today or tomorrow.

Assignments:

• HW16 is due tonight at midnight.
• HW17 is due Sunday at midnight.
• HW18 is due Monday at midnight.

Chapter 16:  Temperature and Heat

Temperature scales:
T(Kelvin) = T(Celsius) + 273.15
    T(Fahrenheit) = (9/5)T(Celsius) + 32

Heat is a transfer of energy due to a temperature difference.
A calorie is the amount of heat needed to raise the temperature of 1 g of water from 14.5^o to 15.5^o C.
1 Calorie = 1 kcal
The mechanical equivalent of heat is 4.186 J/cal.
The heat (Q) required to change the temperature of a substance by (dT) is Q = mc(dT), where m is the mass of the substance and c is its specific heat.
Water has one of the highest specific heats of any substance.  This means it can store a lot of thermal energy, and it also takes a lot of heat to change its temperature compared with other materials.

Heat can be transferred by 3 processes:

conduction - requires contact between materials:  Q/dt = kA(T2-T1)/L
convection - requires fluid, occurs naturally due to buoyant forces from density variations
radiation - all objects absorb and emit electromagnetic radiation (infrared)
The rate of radiation (radiating power) is given by Stefan's law:  P = sAeT^4.

Newton's law of cooling:  The rate that a substance cools (or warms) is proportional to the temperature difference between its temperature and the ambient temperature.

Most substances expand when heated, and the rate of expansion depends on the temperate difference and the linear, area, or volume expansion coefficient.

Ponderables:

• If you heat up a metal plate with a hole in it, what will happen to the size of the hole?
• Why does water at 75 degrees Farenheit feel cold but air at this same temperature feels warm?
• If you get a hot cup of coffee but don't want to drink it right away, should you add cold dairy creamer right away or just before you drink it for the hottest cup of coffee?  Which method of heat transfer is most significant for this problem?
• How many Calories does your body expend when you drink a glass of ice water?  Answer to this nagging question.
• What procedure should be used to make a pitcher of freshly-brewed iced tea in the shortest time possible?
• How should gloves be constructed to minimize heat loss from your hands on a cold winter day?
• Why is it difficult to get a sunburn from sunlight passing through a window?
• If you are in a cool, dark room, would you be warmer wearing a white or black T-shirt?
• A copper cube is colored black on one side and filled with water at 52 deg. C.  Why does an infrared thermometer show a reading of 48 deg. C on the black side and 28 deg. C on the copper sides?  (Hint:  The thermometer assumes an emissivity of 0.95.)

Problems:

• How does specific heat correspond to thermal conductivity?
• In terms of energy, how many candy bars does it take to get to the top of Mt. Mitchell?
• What is the final temperature when 100 mL of water at 10^oC is mixed with 100 mL of water at 50^oC in a styrofoam cup? [30^oC]
• What is the final temperature when 100 g of ice at -5^oC is mixed with 100 mL of water at 50^oC in a styrofoam cup? [19.7^oC] 
• What if only 20 mL of water at 50^oC is added to 100 g of ice at -5^oC?  What happens then?  Will all of the ice melt? [solution]
• These problems are similar to Example 17-6 in your textbook.
• How long does it take for a cup of water to boil when placed in a solar cooker?
• How much fluid does a runner loose through perspiration?
• Estimate the power of a microwave oven if a cup of water at room temperature boils in 2 min. on high.
• You look "radiant"!  How much?

Demos:

• Torch juggling
• Leslie cube
• Infrared thermometer
• Thermal expansion

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)?