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

2. PAL #2 Galileo Thermometer  How does it work?   Limitations 

3. Heat  What is heat?   Same temperature, no heat   Heat used to be thought of a fluid (caloric) that could flow to change temperature  Heat is represented by the letter Q

4. Measuring Heat   Common unit of heat is the calorie:  Amount of heat necessary to increase the temperature of 1 gram of water by 1 C   In nutrition the Calorie is used   Case sensitive!  For rates of heat transfer (Q/t), unit is the Watt (W) 

5. Heat and Temperature   If you heat a metal spoon and a wooden spoon for the same time, which will have a higher T?   The specific heat

6. Specific Heat  The specific heat is defined as:  c has units of J/kg C   Need to know the mass of the stuff (m) and the change in temperature (  T)  Q =mc  T

7. Today’s PAL  A certain amount of heat Q will warm 1 g of material A by 3 degrees C and 1 g of material B by 4 degrees C. Which material has the greater specific heat? Explain.

8. Calorimetry   Insulated container that prevents heat transfer from outside   Since calorimeter is insulated, negative heat lost cancels out positive heat gained  Q 1 + Q 2 + Q 3 … = 0   Heat gained always positive, heat lost always negative   Make sure units for T and m match units for c

9. Example: Quenching a Dagger  Suppose a silver dagger of mass m s at T s is immersed in a mass m w of water at T w. What is the final temperature of the water? Q silver + Q water = 0 c s m s  T + c w m w  T = 0 c s m s (T f - T s ) + c w m w (T f - T w ) = 0 c s m s T f -c s m s T s + c w m w T f - c w m w T w = 0 c s m s T f + c w m w T f = c s m s T s + c w m w T w T f = (c s m s T s + c w m w T w )/(c s m s + c w m w )

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

11. Conduction   Why?   They interact and collide with other atoms and electrons and pass the energy on 

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. Radiation  How does the energy from the Sun get to Earth?  How can energy be transported with no physical contact?   Photons are emitted by the Sun and absorbed by you   All objects emit photons 

14. Radiation Rate Factors  Surface area   Emissivity    Radiation is strongly dependant on T

15. The Surface of the Sun

16. Convection   Hot air is less dense than the cooler air above it   After cooling the air may fall back down   Examples: baseboard heating, boiling water, Earth’s atmosphere

17. Convection Rate Factors  Fluidity   Energy exchange with environment  ??  How rapidly will the material lose heat?   Small temperature difference, not enough density difference to move

18. Today’s PAL  A hot piece of metal is at the bottom of a canister that can be completely filled with:  solid iron  liquid water  air  a vacuum  Consider the heat flow from the bottom to the top.  In which situation(s) would there be no conduction?  In which situation(s) would there be no convection?  In which situation(s) would there be no radiation?

19. Conduction Diagram L A T1T1 T2T2 Q

20. Conductive Heat Transfer  The rate of heat transfer via conduction is:  where:  T 1 is the temperature of the hot side and T 2 is the temperature of the cold side  A is the cross sectional area  L is the thickness  k is the thermal conductivity   High k = large heat transfer  Low k = small heat transfer

21. Radiative Heat Transfer  The amount of heat radiated out from an object is called the power (P):   where   = the Stefan-Boltzmann constant  5.6696 X 10 -8 W/m 2 K 4  A is the surface area  e is the emissivity (number between 0 and 1)  0 =  perfect reflector  1 =  perfect absorber or black body

22. Radiation Exchange  All objects emit and absorb radiation  P net =  Ae  T 4 -T 4 2 )  Where T 2 is the temperature of the surroundings  Note that T must be in Kelvin 

23. Next Time  Read: 13.6-13.11  Homework: CH 14, P: 13, 47, CH 13, P: 29, 48  Help sessions start next week  Tuesday and Thursday 6-8pm Science 304