1. Heat and Thermodynamics
Chapter 12

2. Temperature Related to average kinetic energy of molecules
Measured on a scale based on some standard
Read with thermometer containing material which expands or contracts with temperature change

3. Temperature Scales
Fahrenheit: used in USA, based on freezing of sea water
Celsius (centigrade): used in most of world, based on freezing and boiling pts of water
Kelvin: SI unit of temperature for all scientific work, based on zero point volume of ideal gas

4. Kelvin Scale
Kelvin temperature also called absolute temperature because 0 K = absolute zero
Kelvin scale has same size gradations as Celsius
Kelvin temp = Celcius temp + 273

5. Temperature Scales

6. Heat
Energy that transfers from one object to another because of temperature difference
Thermal contact -- heat flows due to objects touching
Temperature difference controls direction of heat flow, not total energy

7. Thermal Equilibrium
When no heat flows between objects in thermal contact
Thermometer reaches thermal equilibrium to measure temperature

8. Internal Energy
Total energy of molecules inside substance, potential and kinetic
Objects don’t contain heat, but internal energy
Transfer of heat changes internal energy

9. Heat Quantities
Since heat is energy, it is measured in joules, the SI unit of work and energy
Quantity of heat transferred is measured by its effect -- the temp change
Temp change depends on amount and type of substance as well as quantity of heat

10. Heat Quantities
Common unit for heat measurements is the calorie: amount of heat needed to raise one gram of water one degree Celsius
Kilocalorie (1000 calories) is food Calorie, spelled with capital C
Both are units of energy: 1 cal = J

11. Heat Quantities
Old idea of heat said that substances contain invisible fluid called caloric, released when heat given off
English unit of heat is British thermal unit (Btu), the heat required to raise the temp of 1 lb. of water 1 oF

12. Specific Heat Capacity
Quantity of heat needed to raise temp of unit mass of substance 1 oC
Different substances have different capacities for storing heat
Same amount of heat will cause different temp changes in different stuff
Acts like thermal inertia

13. Specific Heat of Water
Very high; water can store much energy without much temp rise
Also means water cools slowly
Controls climate of coastal areas, moderating temperature changes
Central continental areas have more drastic temp changes

14. Heat Exchange Calculations
Amount of heat transferred between objects is calculated using temp change and specific heat capacities Q = mcDT
Heat lost = heat gained
Calorimeter—insulated container for conducting heat exchange measurements

15. Transmission of Heat Transfer energy from source to other location
Occurs through conduction, convection or radiation

16. Conduction Heat transfer due to direct contact
Either between different materials in thermal contact or different parts of the same material
Materials that conduct heat well are good conductors, usually metals
Collisions between atoms, molecules, and electrons transfer energy

17. Conduction
Metals make good conductors because of many “loose” outer electrons
Materials that don’t conduct well are insulators
Liquids & gases usually good insulators
Insulating materials often have enclosed air spaces to delay heat transfer

18. Convection Heat transfer due to current in a fluid
Warm fluid actually moves, changes position
Warmer areas expand, are less dense and rise
Convection currents create wind, weather patterns, sea breezes

19. Radiation Transfer of energy by electromagnetic waves
Can transfer heat through vacuum of space
Objects emit energy depending on temp -- low temp means long waves, low energy
High temp objects emit high energy short waves

20. Radiation
Infrared waves are absorbed by our skin and produce sensation of heat
Heat lamps emit infrared radiation
Hotter objects emit visible light as well, starting at red (500 oC) moving to white hot (1200 oC) as more colors are added to radiation

21. Absorption of Radiant Energy
Radiation either absorbed or reflected
Good absorbers are poor reflectors and appear dark
Good reflectors appear light, absorb little energy and remain cool

22. Emission of Radiant Energy
Good absorbers are good emitters of energy and vice versa
Dark objects absorb and emit energy well

23. Greenhouse Effect Earth absorbs radiant energy from sun
Earth emits terrestrial radiation into space
If rates of absorption and radiation are equal, temp remains constant
Greenhouse effect is effect of atmospheric gases on balance of absorption-emission

24. Greenhouse Effect
High energy short waves from sun reach earth and are absorbed
Earth emits longer waves (because it’s cooler)
Longer waves are absorbed by carbon dioxide and water vapor in atmosphere
Energy is trapped by atmosphere and earth warms

25. Greenhouse Effect
If carbon dioxide levels increase, global warming can occur
Effect is similar in greenhouse -- glass is transparent to short waves from sun but reflect long waves back into greenhouse

26. Thermodynamics
Study of heat and its transformation into mechanical energy
Basis for industrial revolution and development of factories, railroads, automobiles
Based on conservation of energy and fact that heat flows from hot to cold

27. First Law of Thermodynamics
Heat added to a system equals the increase in internal energy of the system plus any work done by the system
Heat added to a system is transformed to an equal amount of some other form of energy
Conservation of energy extended to include heat

28. Heat Engines Any device that turns heat energy into mechanical work
Steam engine, internal combustion (car) engine, jet engine, etc.
Can never be 100% efficient –see 2nd Law of Thermo

29. Heat Engine Diagram

30. Heat Engines Three step cycle:
absorb internal energy from high temp heat source
convert some of this energy into mechanical work
expel remaining energy into low temp heat sink
Always some heat left over that can’t be converted to work: thermal pollution

31. Second Law of Thermodynamics
Heat will never flow from a cold object to a hot object without the input of work
Absolute zero is unattainable
It is impossible to convert all heat energy into useful work

32. Heat Engine Efficiency
Ideal efficiency of heat engine (Carnot efficiency) = (temp of heat source - temp of heat sink) / temp of heat source
To maximize efficiency, industry uses very high temp heat source and large body of water for heat sink
Friction will reduce efficiency even more

33. Heat Pumps and Refrigerators
Heat engine cycle run in reverse
Work is done on a system to extract energy from a cool region and move it to a warmer region
Uses a gas that can easily be compressed to a liquid (Freon)

34. Absolute Zero Lower limit of temperature
No more energy can be extracted, but some energy remains, the zero point energy
Equals oC, oF, or 0 K

35. Entropy Entropy is a measure of disorder in a system
All natural systems tend to proceed toward greater disorder (increased entropy) -- 2nd law of thermo restated
Heat is disordered energy, more entropy
As fuel is burned, entropy increases

36. Entropy Entropy in system can be decreased only by input of work
As time passes, disorder increases and available energy for work decreases
Entropy controls the direction of time -- always towards increasing entropy in the universe