Thermodynamics

1. Thermodynamics Internal Energy Temperature Heat Flow

2. Thermal Energy, Heat, & Temperature WHAT IS THE DIFFERENCE??? • Thermal energy • Total energy of motion of the molecules in an object. • Temperature • Measure of the average kinetic energy of molecules • Measured by thermometers in F°, C° or Kelvin. • Heat • Thermal energy in motion • After heat is transferred from one body to another it is no longer heat but rather thermal energy.

3. Internal Energy • Total energy in all the atoms and molecules of an object • Kinetic and Potential • Molecules can move in 3 ways • Translation • Vibration • Rotation

4. Factors that Affect Internal Energy • Material Composition • Specific Heat • Mass • How many molecules are moving • Temperature • Average Kinetic Energy of molecules • Physical State • Solid, liquid, gas, or plasma

5. Temperature • Temperature is related to the AVERAGE MOLECULAR TRANSLATIONAL KINETIC ENERGY • Translation of molecules causes them to bump into each other • Transfer energy through collisions • “Heat” flow

6. Thermal Energy vs. Temperature • Which has more thermal energy? • Hot iron • Swimming pool with cool water • The temperature of the iron is higher (faster molecules) but…. • The swimming pool has more total energy (more molecules in motion)

7. Thermal Expansion • The higher the temperature, the faster the motion of molecules • The faster they move, the more they want to spread out • Objects expand when heated

8. Thermometers • A glass bulb and capillary tube that contains a liquid (usually mercury or colored alcohol). • When the temperature of the liquid rises, it expands and rises in the tube. • A calibrated scale is provided to read the temperature in F°, C°, or Kelvins.

9. Fahrenheit Scale • The Fahrenheit temperature scale was developed in 1724. Coldest temp his lab with an ice-water-salt mixture was set at 0 degrees. • The temperature of an ice-water (no salt) was set at 30 degrees and the temperature of the human body was set at 96 degrees. • Using this scale, Fahrenheit measured the temperature of boiling water as 212°F on his scale. • He later adjusted the freezing point of water from 30°F to 32°F, to make the interval between the freezing and boiling points of water an even 180 degrees (and making body temperature the familiar 98.6°F).

10. Celsius Scale • Set 0°C to be the freezing point of water • Set 100°C to be the boiling point. • Once called the Centigrade scale • Used by every other country in the world….

11. Kelvin Scale • Same degree spacing as Celsius • “Absolute Zero” is reached when all the molecules completely stop moving • Absolute zero has never been reached…but experimenters have gotten to around 2 or 3 Kelvins. • Absolute zero is -273°C. • There are no negative numbers on the Kelvin scale

12. Temperature Conversions • TF = (9/5 TC) + 32 • Convert 20°C to °F. TF = (9/5 TC) + 32 = 9/5(20) + 32 = 68 °F • TC = 5/9 (TF - 32) • Convert 50°F to °C. TC = 5/9 (TF - 32) = 5/9 (50 - 32) = 10 °C

13. Heat Transfer • Heat energy can be transferred from one object to another in 3 ways: • Conduction • Convection • Radiation

14. Conduction • Results from collisions of vibrating molecules • Objects must be in contact • Heat always flows from hot to cold!

15. Conduction • Conductors • Conduct heat better than others because of loose electrons (metals) • Insulators • Store thermal energy better (don’t let it transfer) • Delay the transfer of heat • It takes more energy to raise the temperature of some things vs. others (higher specific heat capacity). • Which heats/cools faster? • Ocean or sand? • Water or metal pot?

16. Convection • Heat a fluid: Less dense • Less dense: Floats upward • Cooler, more dense fluid sinks • This forms a cycle that repeats

17. Radiation • Transfer of heat energy through space by means of electromagnetic waves (far infrared) • Does not require a medium • Example: • The heat from the fire can be felt from the side, just like heat from the sun can be felt from far away through empty space

18. Factors affecting Radiation • Color and texture of the surface • Good radiators/absorbers: Dark, rough surfaces • Bad radiators/absorbers: Shiny or light, smooth surfaces • Surface temperature • Higher temperature, higher rate of transfer • Surface area • Larger surface area, higher rate of transfer

19. Factors affecting Radiation

20. Thermodynamics • The study of the flow of heat energy • There are 4 fundamental laws • Two you’ve seen before… • Thermal Equilibrium • Conservation of Energy • Entropy • Absolute Zero

21. “Zeroth” Law of Thermodynamics Thermal Equilibrium • Whenever 2 objects of different temperature come in contact, Heat will flow between them until they reach the same temperature

22. First Law of Thermodynamics Conservation of Energy • Heat “lost” by an object equals the heat gained by another or its surroundings • Heat gained by an object equals the heat lost by another object or its surroundings • Energy is not created or destroyed!

23. First Law of Thermodynamics Conservation of Energy • The internal energy of an object is affected when heat flows in or out of a system, and when work is done on the system or bythe system • Change in Internal Energy = Heat added plus the work done on it •  Internal Energy= Heat + Work • Heat leaves or work is done by the system: negative

24. Heat Engines • Use heat energy to do work • Even if we could get rid of friction, engines can never be 100% efficient • Some heat is always rejected (exhaust) • Ei = Ef • Heat in = Work + Heat out • Efficiency: Work out/Heat in QH (input heat) W (Work) QC (rejected heat)

25. Refrigerators • Do the opposite of engines • Use Work to get Heat to flow from a place of lower temperature to higher • Ei = Ef • Work + Heat in= Heat out

26. Second Law of Thermodynamics • Can be stated 3 ways: • Heat always flows from HOT to COLD • Heat engines can never be 100% efficient • Left to itself, the entropy of a system never decreases

27. Second Law of Thermodynamics Entropy • The amount of disorder in a system • Left to itself, everything gets less ordered • Think of your bedroom……. • From useful, ordered energy/work • To less useful, disordered internal energy

28. For a Reversible Process S = 0 For an Irreversible Process S > 0 Reversible and Irreversible Processes Since, for any process, the entropy is either constant or increasing, this means that the entropy of the universe is always increasing.

29. Third Law of Thermodynamics Absolute Zero • It is NOT POSSIBLE for any object to reach the temperature of Absolute Zero Kelvins • Can you tell me why?? • (What are you going to cool it down with?)

30. Absolute Zero 

31. Internal Energy: Total Energy in an object Potential + Kinetic of all molecules Temperature: Average translation kinetic energy per molecule Factors that affect it: Material Mass Temperature Physical State 3 Modes of Heat Transfer Conduction (solids) Convection (fluids) Radiation (no medium required) Factors that affect radiation Color/temperature Surface temperature Surface area Thermal Energy Review

32. Four Laws of Thermodynamics Study of heat flow “Zeroth” Thermal Equilibrium First Conservation of Energy Second Hot to cold, Efficiency, Entropy Third Can’t get Absolute Zero Heat Engines Use heat to do work Refrigerators/AC Use work to move heat Efficiency= Work out/heat in (times by 100 to get %) Entropy Reversible S = 0 Irreversible S > 0 Heat Death of Universe! Thermal Energy Review