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Heat Transfer

Heat Transfer. Physics 202 Professor Lee Carkner Lecture 12. PAL #11 First Law. Final temperature of 20 g, 0 C ice cube dropped into 300 g of hot tea at 90 C. Add up all heats (Q = cm D T and Q = Lm) Heat 1: melt ice Q 1 = (333000)(0.02) = 6660 J Heat 2: warm up now melted ice cube

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Heat Transfer

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  1. Heat Transfer Physics 202 Professor Lee Carkner Lecture 12

  2. PAL #11 First Law • Final temperature of 20 g, 0 C ice cube dropped into 300 g of hot tea at 90 C. • Add up all heats (Q = cm DT and Q = Lm) • Heat 1: melt ice Q1 = (333000)(0.02) = 6660 J • Heat 2: warm up now melted ice cube Q2 = (4190)(0.02)(Tf-0) • Heat 3: cool down tea Q3 = (4190)(0.3)(Tf-90) • Step 4: add up heat Q1 + Q2 + Q3 = 0 6660 + 83.8Tf + 1257Tf –113130 = 0 1340.6Tf = 106470 Tf = 79.4 C

  3. Heat Transfer • What is moving? • In mechanics energy can be transferred through a particle (e.g. a bullet) or a wave (e.g. a sound wave) • Heat can also be transferred by radiation • both a particle and a wave

  4. Conduction • The end in the fire experiences a large vibration of the molecules of the metal • This is called conduction • The movement of heat from a high temperature region to a low temperature region through another material

  5. Conductive Heat Transfer • The rate at which heat is transferred by conduction is given by H = Q/t = kA (TH - TC)/L • Where: • Q is heat and t is time • A is the cross sectional area of the material (in the direction of heat transfer) • T is the temperature (hot or cold)

  6. Thermal Conductivities • For Al, k=235 for Cu, k=428 (W/ m K) • Materials with low k are good thermal insulators • We use foam to insulate our houses • Down filled winter coats trap air for insulation

  7. Composite Slabs H = Q/t = A (TH - TC)/S (L/k) • Where S (L/k) is the sum of the ratios of the thickness and thermal conductivity of each layer of the slab

  8. Radiation • This is how you are warmed by sunlight • The power (in Watts) that is emitted by an object depends on its temperature (T), its area (A) and it emissivity (e) Pr = seAT4 • Where s is the Stefan-Boltzmann constant = 5.6703 X 10-8 W/m2 K4 • T must be in absolute units (Kelvin)

  9. Absorption of Radiation Pa =seATenv4 • Where Tenv is the temperature of the environment Pn = Pa -Pr = seA(Tenv4 - T4)

  10. Emissivity • What is the value of emissivity? • Objects with an emissivity of 1 are called blackbody emitters or absorbers • Every object whose temperature is above 0 K emits thermal radiation • People emit thermal radiation at infrared wavelengths and thus can be detected at night with IR goggles

  11. Convection • The hot air is thus lighter and rises • If the hot air cools as it rises it will eventually fall back down to be re-heated and rise again • Examples: baseboard heating, boiling water, Earth’s atmosphere

  12. Convection Rate Factors • Fluidity • Energy exchange with environment • How rapidly will the material lose heat? • A small temperature difference may result in not enough density difference to move

  13. Next Time • Test #2 • For Monday January 9: • Read: 19.1-19.7 • Do Webassign homework

  14. Water condenses out of the air onto a cold piece of metal. Due to this condensation, the temperature of the air around the metal, • Increases • Decreases • Stays the same • Fluctuates unpredictably • It depends on the temperature of the metal

  15. Ten joules of heat are added to a cylinder of gas causing the piston at the top to rise. How much work does the piston do? • 0 Joules • 5 Joules • 10 Joules • -10 joules • You cannot tell from the information given

  16. Which of the processes in the diagram produces the least work? • 1 • 2 • 3 • 4 • All are the same

  17. Which of the processes in the diagram involves the least change in internal energy? • 1 • 2 • 3 • 4 • All are the same

  18. Which of the processes in the diagram must have net heat output? • 1 • 2 • 3 • 4 • 3 and 4

  19. Which of the processes in the diagram must have net heat input • 1 • 2 • 3 • 4 • 1 and 2

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