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Heat

Physics 102 Professor Lee Carkner Lecture 3. “If you can’t stand the heat, get out of the kitchen.” -Harry S. Truman. Heat. PAL #2 Galileo Thermometer. How does it work? Limitations

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Heat

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  1. Physics 102 Professor Lee Carkner Lecture 3 “If you can’t stand the heat, get out of the kitchen.” -Harry S. Truman Heat

  2. PAL #2 Galileo Thermometer • How does it work? • Limitations • Not very accurate, limited range, needs to be kept upright, won’t work in free fall, delicate, can freeze solid

  3. If two objects are in thermal equilibrium with each other • a) They are at the same pressure • b) They have the same volume • c) They are not in contact with each other • d) They are not exchanging heat • e) Their temperatures cannot be measured

  4. Which of the following places the temperatures in the correct rank, highest to lowest? • a) 50 X, 50 W, 50 Y • b) 50 X, 50 Y, 50 W • c) 50 W, 50 X, 50 Y • d) 50 Y, 50 W, 50 X • e) 50 Y, 50 X, 50 W

  5. Four metal plates all made of the same material are increased in temperature by the same amount. Rank the height increase from most to least. • a) 1, 2, 3, 4 • b) 1 and 4 tie, 2 and 3 tie • c) 1, 4, 2 and 3 tie • d) 2 and 3 tie, 1, 4 • e) All tie

  6. Heat • What is heat? • Same temperature, no heat • Heat is not a “thing”, it is a transfer of energy • Units: • Joules • calories (cal) = 4.186 J • For rates of heat transfer (Q/t), unit is the Watt (W) = J/s

  7. Specific Heat • The specific heat (c): c = Q/mDT • c has units of J/kg C • Need to know the mass of the stuff (m) and the change in temperature (DT) • Can rewrite as: Q =mcDT

  8. Calorimetry • To do experiments with heat we use a calorimeter • The total heat exchange is the sum of the heat from all processes • Q1 + Q2 + Q3 … = 0 • Always write DT = Tf-Ti • Use consistent units • Make sure units for T and m match units for c

  9. PAL: Quenching a Dagger • Suppose a silver dagger of mass ms at Ts is immersed in a mass mw of water at Tw. Derive an expression for the final temperature of the water when thermal equilibrium is reached.

  10. How Does Heat Move? • Heat (like information) is transferred in different ways • Conduction • Convection • Radiation

  11. Conductive Heat Transfer • The rate of heat transfer via conduction is: Q/t = kA(T1-T2)/L • where: • T1 is the temperature of the hot side and T2 is the temperature of the cold side • L is the thickness • k is in units of W/ m K • Low k = small heat transfer A T2 T1 Q L

  12. Conduction Rate Factors • Free electrons • Density • Cross sectional area • Large window loses more heat than small • Temperature difference • Thickness • Heat takes less time to move through thinner material

  13. Convection Rate Factors • Fluidity • Energy exchange with environment • How easy is it to heat (by conduction or radiation) the material in the first place? • Temperature difference

  14. Radiative Heat Transfer • All objects emit photons • The amount of heat radiated out from an object is called the power (P): DQ/Dt = Pr = sAeT4 • where • s = • 5.6696 X 10-8 W/m2 K4 • A is the surface area • e is the emissivity (number between 0 and 1) • 0 = • perfect reflector • 1 = • perfect absorber or black body

  15. Radiation Rate Factors • Surface area • Emissivity • Dark objects emit and absorb more than light ones • Temperature

  16. Radiation Exchange • Total power transferred is power absorbed minus power emitted Pnet = sAe(T4-T42) • Note that T must be in Kelvin • No radiation at 0 K

  17. Next Time • Read: 13.6-13.11 • Homework: CH 14, P: 13, 37, CH 13, P: 29, 48 • Help sessions start next week • Tuesday and Thursday 6-7pm Science 120

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