4” by 6” Cards Today’s assignment Name, SID – legible penmanship What worked What didn’t and needs clarification

NATS 101 Lecture 4 Temperature, Heat Transfer

What is Temperature? Microscopic View Energy due to random jiggling of molecules Related to average molecular speed; 500 m/s (=1100 mph) at room temperature for air

Temperature and Density Consider volume of air If air is warmed: The molecules will move faster, have “stronger” collisions, and tend to become spaced farther apart Volume increases, so density decreases Warmer  less dense

Temperature Scales Fahrenheit ( o F) - relative US public standard Celsius ( o C) - relative Freezing point 0 o C Boiling point 100 o C o C= 5/9 ( o F-32) Kelvin (K) - absolute K= o C+273 Ahrens p27

What is Heat? Heat-Energy in the process of being transferred from a warmer object to a cooler object Consider a pot of water on a hot burner. Consider the following questions: Williams, p. 19

Heat Transfer Questions What causes the… Pan bottom and handle to get warmer? Top of the water to become warmer? Water temperature to not exceed 100 o C? Region away from side of pan to feel warm? Williams, p. 19

Conduction Heat transfer due to collision of molecules. Conduction warms the bottom of the pan! Conductivity - rate of heat transfer across a 1 cm thick slab of material if one side is kept 1 o C warmer than the other Cheap Experiment: Touch your chair! 1 cm Metal WaterAir Heat Transfer 1oC1oC 0oC0oC

Heat Conductivity

Specific Heat Capacity Heat required to raise temperature of 1 gm of substance 1 o C. Metal has lower heat capacity than water!

Convection Heat transfer due to vertical exchange of mass Occurs in fluids (liquids, gases) because of gravity Warm, buoyant air rises - Cool, dense air sinks Convection warms top of liquid! Warm Cool Warm Cool Warm Cool heat below - convectionheat side - convectionheat top - no convection gravity

Convection Movies 2D Convection Tank Animation2D Convection Tank Animation  2D Convection Model Ra=10**6 2D Convection Model Ra=10**7 IC1 2D Convection Model Ra=10**7 IC2 3D Rayleigh-Benard Convection Model3D Rayleigh-Benard Convection Model  Last hour movie of clouds and moist convection over Catalina mountains

Energy States and Water Phases water molecules are tightly packed in a crystal alignment that prevents them from changing shape LOW ENERGY STATE attractive forces btw molecules weaken and individual molecules can move around each other, but they can not break away SLIGHTLY HIGHER ENERGY STATE water molecules move very rapidly and are not bound together EXTEMELY HIGH ENERGY STATE Small Energy Change Large Energy Change

Latent Heat Ahrens, p 28 Weak attraction between molecules +540 cal/gm +80 cal/gm -540 cal/gm -80 cal/gm -620 cal/gm +620 cal/gm Strong attraction between molecules Some attraction More Ordered Phase Less Ordered Phase Surrounding air warms Surrounding air cools

Latent Heat Energy associated with phase of matter. Must be either added to or taken from a substance when it changes its phase. –To turn liquid water into solid ice, must remove energy from the liquid water. –To turn liquid water into vapor, must add a lot of energy to the liquid water.

Heat Transfer Questions What causes the… Pan bottom and handle to get warmer? Top of the water to become warmer? Water temperature to not exceed 100 o C? Region away from side of pan to feel warm? Williams, p. 19

Modes of Heat Transfer Williams, p. 19 Latent Heat

Take Home Concepts Heat-Energy transfer due to temperature differences Three modes of heat transfer Conduction – molecule to molecule Convection – transport of fluid Radiation – electromagnetic waves Latent Heat – energy of phase changes

Modes of Heat Transfer Energy is only converted from one form to another or transferred from one place to another. Energy is transferred from hot to cold. Conduction - Molecules colliding; most efficient at interface. Convection - Requires movement of a fluid or gas.

Reading Assignment Ahrens Pages Problems 2.7, 2.9