Thermodynamics: the study of thermal energy

1. Thermodynamics: the study of thermal energy

2. Heat, Q Heat is NOT temperature. You do NOT measure heat with a thermometer! Heat is the flow of energy. Heat: Transfer of “disordered” energy at the microscopic level as a result of temperature differences Unit: Joules (just like all forms of energy) James Joule first determined the relationship between mechanical energy and heat- (motion transformed into heat)

3. An older unit for measuring heat was the “calorie”. One food Calorie = 4186 Joules I wonder how many joules of energy are in this strawberry shortcake?

4. What is “Cold”? Just as darkness is an absence of light, Cold is a absence of heat. Heat flow is a measurable quantity. As more and more heat flows out of something, it gets colder and colder.

5. When two substances of different temperatures are in contact, heat will flow until they are at the same temperature, “thermal equilibrium”

6. Transfer of Thermal Energy Convection- the bulk flow of fluids, (gases or liquids, NOT solids) “convection currents” Conduction- direct contact Radiation- electromagnetic waves

8. Conductivity A block of wood and a block of metal sit on a table. You touch each. Which one feels colder? Is it really colder? Why does it feel that way?

9. Thermal Conductivity: The ability to conduct heat. Thermal Insulators do not conduct heat readily. Generally, metals have high thermal conductivity and gases have low thermal conductivity.

10. For metals, the thermal conductivity is quite high, and those metals which are the best electrical conductors are also the best thermal conductors. Best conductors Both heat & electrical In order: Silver Copper Gold Aluminum

11. Temperature scales Celsius C = 5/9(F° – 32) Fahrenheit F = 9/5C° + 32 Kelvin K = C° + 273

12. Water (at normal Earth atmospheric pressure) freezes at 0°C = 32°F boils at 100°C = 212°F

13. Is there a HOTTEST possible temperature? Temperatures do not appear to have an upper limit. Is there a COLDEST possible temperature? The lower limit on temperature is called “absolute zero”, which equals 0 Kelvins. However, it is impossible for a substance to be at absolute zero.

14. The Kelvin scale, which has its zero at ABSOLUTE ZERO, was named for William Thomson, “Lord Kelvin”, who found the value for absolute zero using fundamental laws of thermodynamics.

15. What is “Absolute Zero”? For ALL gases, as the temperature drops, the pressure within the gas drops in a direct relationship. Graphing pressure vs temperature for many gases and then EXTRAPOLATING the graphs to a pressure of ZERO (which is impossible) yields the same temperature for every gas: -273 C = 0 Kelvins = Absolute Zero

18. Heat (energy) Transfer HEAT Potential Energy Stored in the vibrations of the molecules Limited by the “degrees of freedom” available to the molecule • Kinetic Energy • - Motion of atoms and molecules • Reflected in the TEMPERATURE of the substance • Faster = higher temperature

19. If the molecules have many “degrees of freedom”, they can store more potential energy, with less change in the kinetic energy of the molecules. Therefore, the temperature will change slowly. If the molecules have few “degrees of freedom”, they can store little potential energy, with more change in the kinetic energy of the molecules. Therefore, the temperature will change more rapidly. HEAT Potential Energy Kinetic Energy

20. Microscopic: Cannot be seen by eyes alone- usually you can’t “measure” the kinetic energy of the molecules Macroscopic: Can be seen with eyes alone- you can measure the temperature! Kinetic-Molecular Theory As a substance gets hotter, its molecules move faster! Faster molecules have higher kinetic energy. A higher kinetic energy is reflected by a higher temperature! You may not be able to SEE molecules moving fast without a microscope, but you can see an increase in temperature on a thermometer.

21. Kinetic-Molecular Theory As a substance gets hotter, its molecules move faster! Faster molecules have higher kinetic energy. A solid: the molecules are tightly packed together and move more slowly. When you add heat… A liquid: the molecules are not packed as tightly together and move around. When you add more heat… A gas: the molecules are not bound together and move very fast. When you add more heat….. A plasma: the atoms themselves are ripped apart to become ions.

22. Heat Transfer Since the atoms in aluminum can store more potential energy than the atoms in gold, as heat flows, the gold atoms will gain more kinetic energy so that the temperature of gold rises much faster than the temperature of the same mass of aluminum.

23. In one episode of the original TV series, “Mission Impossible”, the team utilized that fact that gold heats up so quickly compared to other materials. They drilled a hole in the bottom of a vault and inserted an electric heat rod. The gold in the vault heated up, melted and flowed through the hole in the bottom of the vault before any of the paper money or other coins got too hot. Would that really work?? Hmmm, maybe- but don’t try it!

24. Specific Heat Capacity, “c” • The specific heat capacity, “c”, of a substance is the amount of heat required per kilogram to raise the temperature by one degree. • Different substances have different specific heat capacities. Aluminum has a higher specific heat than gold! More heat must be transferred into aluminum than into gold for the same change in temperature • The higher the heat capacity, the more heat the substance can “hold” or “give off” with minimal temperature change. Unit: J/kg·K (or J/kg·°C)

25. Specific Heat Capacity of Water For example, you put 1 kg piece of metal on a hot plate for two minutes. You also put a container of 1 kg water in on an identical hot plate. Would you rather place your finger on the metal or in the water? The metal will be at a much higher temperature! Both received the same amount of heat energy. But water has a higher specific heat capacity- it can absorb or release more heat energy with little temperature change.

26. Water has one of the highest specific heats of all substances. It can absorb and give off great amounts of heat energy with little temperature change. • It takes a long time to heat water and it takes a long time for water to cool down! • Another example: The filling on a hot apple pie burns our tongues and not the crust even though they are the same temperature because of the water content in the filling. • The filling can give off a lot of heat and STILL be hot.

27. The oceans help maintain a small range of temperature on Earth that is compatible with life by absorbing heat in the day and releasing it at night with little change in the ocean’s temperature.

28. Weather along the Coastline • Coastal regions do not experience a large change in temperature because the water absorbs solar radiation in the day and releases it at night

29. In contrast, in a desert there's a wide daily range of temperature because no water is available to absorb heat in the day and release it at night

30. Table of Specific Heat • The average specific heat capacity of a human body is approximately 3500.

31. Heat transferand temperature change As heat, Q, flows into or out of a substance, its temperature change, DT, will depend on the mass, m, of the substance and its specific heat capacity, “c”. Q = mcDT

32. Example: How much heat, Q, is required to raise the temperature of a 3 kg pan of water from 15°C to boiling temperature? (specific heat of water = 4186 J/kg·K) Q = mcDT What is DT? Final – original = 100° – 15° = 85° Q = 3 (4186) 85 = Q = 1,067,430 J

33. Q = mcDT A 0.2 kg block of metal absorbs 1500 J of heat when its temperature changes from 20 to 35 degrees Celsius. What is the specific heat capacity of the metal? c= Q ÷ (mDT) c= 1500 ÷ (0.2 • 15) c= 500 J/kg·K

34. Q = mcDT What is the change in the temperature of 3.5 kg water if 950,000 J of heat are added? C = 4186 J/kg·K DT = Q ÷ (mc) DT = 950,000 ÷ (3.5 • 4186) DT = 64.9°

36. The Laws of Thermodynamics

37. The Zeroth Law ! If Object 1 is in thermal equilibrium with Object 2 and Object 2 is in thermal equilibrium with Object 3, then Object 1 is in thermal equilibrium with Object 3. Well, duh…

38. The First Law of Thermodynamics The change in the internal energy of a substance is the sum of the heat transferred plus the work done. “internal energy” is associated with the random motion of all of the molecules that make up an object- both the kinetic energy and the potential energy. The more molecules there are, the more internal energy is possible. Which has more internal energy: a hot coal or a frozen lake???

39. 1st Law: The change in the internal energy of a substance is the sum of the heat transferred plus the work done D Internal Energy = Q (in or out) + W (in or out)

40. 1st Law: The change in the internal energy of a substance is the sum of the heat transferred plus the work done The First Law of Thermodynamics is yet another way to state that energy is neither created nor destroyed, but may be transferred or transformed- The Law of Conservation of Energy

41. Heat Engines A “heat engine” takes in heat and with that heat energy performs work. It then gives off a lesser amount of heat at a lower temperature. QH = QC + Wout A heat engine is an example of the first law of thermodynamics: D Internal Energy = Q + W If we rearrange things we get: Q (in) = DInternal Energy + W (out)

42. Heat Engines Even the very best engines are not able to transform all the heat energy into work. Some of it is ALWAYS “wasted”. The “ideal” engine is called a “Carnot Engine”- a perfect, theoretical (but not physically possible) design. Even the Carnot Engine would not have an efficiency of 100%.

43. Examples of Heat Engines Steam Engines produce motion which can be used to do work or generate electricity.

44. What is another well-known “heat engine”??? Think of something most of us use every day that converts heat into motion (with some waste heat).

46. closed closed closed open four-stroke internal combustion engine Internal Combustion Engine Simulator intake stroke: intake valve is open, allowing gas and air to enter the cylinder. The piston moves downward compression stroke: valves are closed as the piston moves upward, creating high pressure in the fuel-air mixture. At the top of the stroke, the spark plug sparks, which ignites the fuel-air mixture. power stroke: the explosion pushes the piston down. The piston’s rod turns the crankshaft, which provides the torque to turn the wheels. exhaust stroke: the piston moves back upward. the exhaust valve opens to allow exhaust gases to leave the cylinder

47. Of course, much of the energy released by the combustion of fuel does not produce the mechanical energy of motion in the car. In what form does much of that released chemical potential energy appear??? HEAT !

48. The 2nd Law of Thermodynamics The Law of Entropy: Natural processes always increase entropy. Entropy: disorder

49. What does “entropy” have to do with heat?? … Go back to the Kinetic-Molecular Theory: The faster atoms are moving, the more “disorder” or ENTROPY they have. So, if you increase the temperature of a substance, you also increase its ENTROPY. Absolute Zero is the temperature at which entropy would reach its minimum value.