Friday, November 9, 2001

• RWP11 has been cancelled
• Exam 3 is scheduled for Monday, November 19.
• Format will be similar to Exam 2:  20 or 25 MC questions, 1 take-home RWP, 1 orig. RWP

The pressure of a gas is proportional to the average kinetic energy of the gas molecules, 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.

Chapter 11 - Heat

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.

The heat required to change the phase of a mass is Q = mL, where L is the latent heat of fusion or vaporization.

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.

Ponderables:

• 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 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?

• Table 11.1 and 11.3 - How does specific heat correspond to thermal conductivity?
• (P.2) In terms of energy, how many candy bars does it take to get to the top of Mt. Mitchell?
• P.7 on page 363:  What is the temperature change in the water from the top to the bottom of Niagara Falls (h = 50 m)?
• What is the final temperature when 100 mL of water at 10^oC is mixed with 100 mL of water at 50^oC?
• What is the final temperature when 100 g of ice at -5^oC is mixed with 100 mL of water at 50^oC?
• P.42:  How long does it take for a cup of water to boil when placed in a solar cooker?

P.61:  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?

• HW11b - due Monday
• RWP11 - cancelled (sorry!)
• Web Projects - due Wednesday, November 28.