1. 14 Heat Homework: Problems: 3, 5, 13, 21, 33, 47, 49. Internal Energy
Heat Capacity & Specific Heat
Phase Transitions
Thermal Conduction

2. Heat Heat is energy transferred due to temperature difference.
Symbol, Q [J]
Ex. 4186J heat needed to raise 1kg of water one degree C.

3. example c’s
in J/(kg·°C)
aluminum
copper
ice
water

4. specific heat c = Q/mDT [J/(kg·K)]
heat to raise 1kg by 1 degree °C or K.
slope warming curve = DT/Q = 1/(mc)
Q = mcDT

5. Calorimetry
Measure heat lost/gained:

6. Example Calorimetry
2kg of “substance-A” heated to 100C. Placed in 5kg of water at 20C. After five minutes the water temp. is 25C.
heat lost by substance = heat gained water.

7. continued:

8. Phase Transitions: Latent Heat
L = Q/m [J/(kg)]
heat needed to melt (f) or vaporize (v) 1kg

9. example L’s in J/kg: melting (f) vaporization (v)
alcohol , ,000
water , ,226,000

10. Example: How much heat must be added to 0.5kg of ice at 0C to melt it?
Q = mL = (0.5kg)(333,000J/kg)
= 167,000J
same amount of heat must be removed from 0.5kg water at 0C to freeze it.

11. Heat Transfer
Conduction
Convection
Radiation

12. Conduction Heat conduction is the transmission of heat through matter.
dense substances are usually better conductors
most metals are excellent conductors

13. conduction equation heat current = energy/time [watts]
heat current = kADT/L
k = thermal conductivity
& DT = temperature difference, L below

14. conduction example some conductivities in J/(m-s-C°):
silver copper aluminum 240
Ex: Water in aluminum pot. bottom = 101°C, inside = 100°C, thickness = 3mm, area = 280sq.cm.
Q/t = kA(Th-Tc)/L
= (240)(0.028)( )/(0.003)
= 2,240 watts heat current

15. Heat transfer 2m x 1m window, 4mm thick, single pane glass.
Assume temp. difference = 5°C
Q/t = kA(DT)/L = (0.84)(2)(5)/0.004
About 2,000 watts

16. R-Factors and Thermal Resistance

17. Convection Convection – transfer through bulk motion of a fluid.
Natural, e.g. warm air rises, cool falls
Forced, e.g. water-cooled engine

18. Radiation Heat transfer by electromagnetic radiation, e.g. infrared.
Examples:
space heaters with the shiny reflector use radiation to heat.
If they add a fan, they use both radiation and convection

19. Summary Definition of Internal Energy Heat Capacity Specific Heat
Phase Transitions
Latent Heat
Phase Diagrams
Energy Transport by Conduction, Convection, and Radiation

20. Example:
A student wants to check “c” for an unknown substance. She adds 230J of heat to 0.50kg of the substance. The temperature rises 4.0K.

21. Greenhouse Effect
‘dirtier’ air must be at higher temperature to radiate out as much as Earth receives
higher temperature air is associated with higher surface temperatures, thus the term ‘global warming’
very complicated model!

22. Phase Change freeze (liquid to solid) melt (solid to liquid)
evaporate (liquid to gas)
sublime (solid to gas)
phase changes occur at constant temperature

23. Temperature vs. Heat (ice, water, water vapor)

24. Heat and Phase Change
Latent Heat of Fusion – heat supplied to melt or the heat removed to freeze
Latent Heat of Vaporization – heat supplied to vaporize or heat removed to liquify.

25. Newton’s Law of Cooling
For a body cooling in a draft (i.e., by forced convection), the rate of heat loss is proportional to the difference in temperatures between the body and its surroundings
rate of heat-loss ~ DT

26. Real Greenhouse
covering allows sunlight to enter, which warms the ground and air inside the greenhouse.
the ‘house’ is mostly enclosed so the warm air cannot leave, thus keeping the greenhouse warm (a car in the sun does this very effectively!)

27. Solar Power Solar Constant
Describes the Solar Radiation that falls on an area above the atmosphere = 1.37 kW / m². In space, solar radiation is practically constant; on earth it varies with the time of day and year as well as with the latitude and weather. The maximum value on earth is between 0.8 and 1.0 kW / m².
see: solarserver.de