Heat Transfer and Thermodynamics      Chapter 7  Joseph F. Alward, PhD  Department of Physics  University of the Pacific

  Heat Conduction

Air in bathroom is at the same temperature as the wooden floor and the tile. Why does the tile feel colder? ----------------------------------------- Heat is "conducted" when atoms collide and momentum is transferred to the cooler object.

Water is a Very Poor Conductor of Heat

Steel wool jammed into test tube traps ice. Will the ice ever melt?

  Heat Conduction over Hot Coals

Wood is used for cookware handles.   Is wood a good conductor of heat? If the feet are sweaty, will that help, or hurt?

   Heat Conductivity in Metals

Paper will ignite when its temperature exceeds 233 C (451 F). Why doesn't the paper catch fire right away?

  Heat Conduction is Slowed by Insulation

Which home has more insulation in the attic?

  Conductivity of Ice and Snow

Vegetables can survive 0 C temperatures, but not -10 C temperatures.   Before nightfall, the farmer sprayed his crops with water.

   Convection

Warm fluids are less dense than cooler fluids. Thermal energy moves en masse, rather than by momentum transfer between individual atoms. ------------------------------------- Hot radiator warms air in contact with the radiator by conduction, and this warm air move to the ceiling by convection (i.e, because of buoyancy).

  Heated Water Rises

 Rising Smoke and Cooling
 by Expansion

Rising smoke from a barbeque continues to rise until its density matches that of the surrounding air. The rising air also becomes cooler.   ------------------------------------------------------------------ What two reasons can one give for the cooling? When a faster moving object collides with a slower-moving one, which one loses speed?

 Cooling by Expansion

Expanding steam cools rapidly for two reasons: (1) Mixing with cooler air (2)  Fluids cool when        they expand (just        as for the rising        smoke ----------------------------------- Air in mouth when lips are pursed is at higher pressure.  Open-mouthed breath is warm, while narrow stream is cooled.

   Large Bodies of Water Moderate Land Temperature

Because of high specific heat capacity (ability to accept heat without a large temperature   increase) water during the day is cooler than land.  Rising air above warm land is replaced   by cooler air pushed in from the lake.  The reverse happens at night, when the land's   temperature has fallen below that of the lake; the lake's temperature drops, too, at night,   but not as much as the land's.

    Radiation

Oscillation of electric charges (Chapter 8) give off energy--electromagnetic energy.   Infrared waves are given off by all objects; infrared is not visible to humans.

    Wave Length versus Temperature

The hotter the object, the more rapidly the charges in the object oscillate. Cold:  Long wavelength Hot:   Short wavelength

    Emission Spectra 

The hotter the object, the greater the average frequency of oscillation of the object's charges.

   Solar versus Terrestrial Radiation

Visible light comes from the hotter object, but very little from the cooler one. Short wavelength solar radiation penetrates atmosphere, while longer- wavelength terrestrial radiation is reflected back to earth by the atmosphere. -------------------------------------- The Greenhouse Effect Carbon dioxide:  CO2

 Absorption of Radiant Energy

Objects are black because they absorb all of the visible light striking the surface. Infrared (heat radiation) is generally equally well- absorbed. Good absorbers are also good emitters and poor absorbers are poor emitters.

What would happen to a good absorber placed in the sun on an insulating surface if it didn't emit thermal radiation as efficiently as it absorbed it?

   Reflection of Radiant Energy

Opening is a poor reflector of radiant energy.

   Openings are Black Bodies

   Cooling at Night by Radiation

Poor conductors of heat lose thermal energy quickly at night. At night, radiant energy lost from grass is not made up by absorbed radiant energy. Grass becomes colder than air; water moisture condenses on grass.

 Thermos Bottle

Outer silvered wall reflects radiant energy coming from surroundings, helping to keep cold fluids cold. Vacuum prevents heat conduction in either direction, helping to keep hot fluids hot, and cold fluids cold. Inner silvered wall reflects radiant energy back to fluid.  Coffee, for example is kept hot.

   First Law of Thermodynamics

thermo:     heat       dynamics:  force ------------------------------------------------------ Work done on water by paddle-wheel is converted to thermal energy. ------------------------------------------------------ First Law: Heat added = Work done                    or Heat added = Thermal energy increase                                      +                    = Work done by system

 Adiabatic Processes

adiabatos: Greek, "not passable" ---------------------------------------------- If no thermal energy enters or leaves a system through its walls during some process, the process is said to be adiabatic. ---------------------------------------------- Pump exerts a force on the air in the tire, doing work on the air. What happens to the thermal energy--and therefore the temperature--of the air?  Does it rise, or fall?

 Adiabatic Expansion and Compression
 in the Internal Combustion Engine