2. Physics 101: Lecture 26, Pg 1 Physics 101: Lecture 26 Conduction, Convection, Radiation Today’s lecture will cover Textbook Chapter 14.4-14.9 Final

3. Physics 101: Lecture 26, Pg 2 Calendar Today: HE 3 at 7pm, conflict in 151 Loomis at 5:15pm Wed Apr 28 lecture: Thermodynamics I Mon May 3 lecture: Thermodynamics II Wed May 5 lecture: no lecture, just review Lab 9: Wed Apr 28 – Mon May 3 Disc 14: this week, no quiz. No disc next week. Honors talks Tu Apr 27, W Apr 28 8pm+, 251 Loomis Check your grades in gradebook! Finals: Wed May 12 1:30pm Thurs May 13 8am MUST select final by TOMORROW, Tu Apr 27.

4. Physics 101: Lecture 26, Pg 3 Review l Heat is FLOW of energy è Flow of energy may increase temperature l Specific Heat   = Q / (c m) l Latent Heat è heat associated with change in phase l Today: Heat è Conduction è Convection è Radiation 07

5. Physics 101: Lecture 26, Pg 4 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 teaspoons 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: Then I =  T/R demos T H Hot T C Cold L =  x Area A 13

6. Physics 101: Lecture 26, Pg 5 Conduction ACT l On a cold winter night, which will keep you warmer in bed. A) A thin cotton sheet B) A thick wool blanket C) Either one

7. Physics 101: Lecture 26, Pg 6 Preflight 1 On a cool night you make your bed with a thin cotton sheet covered by a thick wool blanket. As you lay there all covered up, heat is leaving your body, flowing though the sheet and the blanket and into the air of the room. Compare the amount of heat that flows though the sheet to the amount of heat that flows through the blanket. 1. More heat flows through sheet than through the blanket. 2. More heat flows through blanket than through the sheet. 3. The same amount of heat flows through sheet as the blanket. More heat flows through the sheet because it is thinner. the heat leaves your body and goes into the surrounding air, thus whatever leaves your body goes through both the sheet and blanket I need to go do some further research to get a real answer, sleep time... correct 15

8. Physics 101: Lecture 26, Pg 7 Conduction w/ 2 layers ACT l Compare the heat flow through material 1 and 2. A) H 1 > H 2 B) H 1 =H 2 C) H 1 < H 2 l Estimate T 0 the temperature between the two A) 5 CB) 12.5 CC) 20 C Inside: T H = 25C Outside: T C = 0C H1H1 H2H2 T0T0  x 1 = 0.02 m A 1 = 35 m 2 k 1 = 0.080 J/s-m-C  x 2 = 0.075 m A 2 = 35 m 2 k 2 = 0.030 J/s-m-C 19

9. Physics 101: Lecture 26, Pg 8 l Find H=Q/t in J/s è Key Point: Continuity (just like fluid flow) »H 1 = H 2 »  1 A(T 0 -T C )/  x 1 =  2 A(T H -T 0 )/  x 2 » solve for T 0 = temp. at junction »then solve for H 1 or H 2 n answers: T 0 =2.27 C H=318 Watts  x 1 = 0.02 m A 1 = 35 m 2 k 1 = 0.080 J/s-m-C  x 2 = 0.075 m A 1 = 35 m 2 k 1 = 0.030 J/s-m-C Inside: T H = 25C Outside: T C = 0C H1H1 H2H2 T0T0 Conduction w/ 2 layers 22

10. Physics 101: Lecture 26, Pg 9 Conduction ACT l Which marbles will fall last? 1) Copper 2) Steel3) Aluminum 24

11. Physics 101: Lecture 26, Pg 10 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

12. Physics 101: Lecture 26, Pg 11 Heat Transfer: Radiation l All things radiate electromagnetic energy è I emit = Q/t = eA  T 4 »e = emissivity (between 0 and 1) n perfect “black body” has e=1 »T is temperature of object in Kelvin »  = Stefan-Boltzmann constant = 5.67 x 10 -8 J/s-m 2 -K 4 è No “medium” required l All things absorb energy from surroundings è I absorb = eA  T 0 4 »T is temperature of surroundings in Kelvin »good emitters (e close to 1) are also good absorbers T Surroundings at T 0 Hot stove DEMO 35

13. Physics 101: Lecture 26, Pg 12 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

14. Physics 101: Lecture 26, Pg 13 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 R e = 6.38×10 6 m, R s = 7×10 8 m.) I net = I emit - I absorb = eA  (T 4 - T 0 4 ) 42

15. Physics 101: Lecture 26, Pg 14 Preflight One day during the winter, the sun has been shining all day. Toward sunset a light snow begins to fall. It collects without melting on a cement playground, but it melts immediately upon contact on a black asphalt road adjacent to the playground. How do you explain this. The asphalt is darker and therefore absorbs more heat 45 There is salt on the asphalt. I want summer! physics

16. Physics 101: Lecture 26, Pg 15 Summary l Conduction - contact l Convection - fluid motion l Radiation 50