Physics 101: Lecture 24 Conduction, Convection+Radiation Final Physics 101: Lecture 24 Conduction, Convection+Radiation 1

Review Heat is FLOW of energy Specific Heat Latent Heat Today: Heat Flow of energy may increase temperature Specific Heat DT = Q / (c m) Latent Heat heat associated with change in phase Today: Heat Conduction Convection Radiation 07

Question Touch base of chair, and desk top, which feels colder? A) Base B) Same C) Desk Both must be the same temperature (room temperature), but metal feels colder because it conducts heat better/faster. 10

Heat Transfer: Conduction Hot molecules have more KE than cold molecules High-speed molecules on left collide with low-speed molecules on right energy transferred to lower-speed molecules heat transfers from hot to cold I = rate of heat transfer = Q/t [J/s] I = k A (TH-TC)/L Q/t = k A T/x k = “thermal conductivity” Units: J/s-m-C good thermal conductors…high k good thermal insulators … low k R = L/(Ak) = thermal resistance L = Dx TH Hot TC Cold Area A demos 13

Question If you want to maximize the amount of Heat flow which substance should you choose? 1. Aluminum 2. Copper 3. Steel 15

Example: Boiling water A steel pot with an area of 0.05m2 and thickness of 0.006m is placed on top of a burner so that its bottom surface remains at 1500C. How much water boils away each minute?

Question If you want to limit the amount of Heat flow (Insulate) which substance should you choose? 1. Air 2. Wood 3. Glass 15

How does an oven work?

Heat Transfer Convection Air heats at bottom Thermal expansion…density gets smaller Lower density air rises Archimedes: low density floats on high density Cooler air pushed down Cycle continues with net result of circulation of air Practical aspects heater ducts on floor A/C ducts on ceiling stove heats water from bottom “riding the thermals” heater demos 27

Heat Transfer: Radiation All things radiate and absorb electromagnetic energy Iemit = eAT4 Iabsorb = eAT04 Inet = Iemit - Iabsorb = eA(T4 - T04) if T > T0, object cools down if T < T0, object heats up T Surroundings at T0 Hot stove HW 38

Earth Homework The Earth has a surface temperature around 270 K and an emissivity of 0.8, while space has a temperature of around 2 K. What is the net power radiated by the earth into free space? (Radii of the Earth and the Sun are Re = 6.38×106 m, Rs = 7×108 m.) Average consumption of US= 1.3x108 Watts 42

Summary Conduction - contact Convection - fluid motion Radiation Ch 14 Problems 1, 11, 19, 29, 35, 39, 41, 49, 51, 59, 63 50