Conduction, Convection+Radiation Conduction, Convection+Radiation

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

Heat Transfer: Conduction l Hot molecules have more KE than cold molecules l High-speed molecules on left collide with low-speed molecules on right è energy transferred to lower-speed molecules è heat transfers from hot to cold l I = rate of heat transfer = Q/t [J/s]  I =  A (T H -T C )/L »Q/t =  A  T/  x   = “thermal conductivity” »Units: J/s-m-C »good thermal conductors…high  »good thermal insulators … low   R = L/(A  = thermal resistance demos T H Hot T C Cold L =  x Area A 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 l A steel pot with an area of 0.05m 2 and thickness of 0.006m is placed on top of a burner so that its bottom surface remains at C. 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 l Air heats at bottom l Thermal expansion…density gets smaller l Lower density air rises è Archimedes: low density floats on high density l Cooler air pushed down l Cycle continues with net result of circulation of air l Practical aspects è heater ducts on floor è A/C ducts on ceiling è stove heats water from bottom è “riding the thermals” heater demos 27

Heat Transfer: Radiation l All things radiate and absorb electromagnetic energy è I emit = eA  T 4 è I absorb = eA  T 0 4 è I net = I emit - I absorb = eA  (T 4 - T 0 4 ) »if T > T 0, object cools down »if T < T 0, object heats up T Surroundings at T 0 Hot stove HW 38

Summary l Conduction - contact l Convection - fluid motion l Radiation – Electromagnetic waves 50