Heat and Thermodynamics

1. Heat and Thermodynamics

2. All matter is in constant motion, so the atoms and molecules of all matter have KE. • Gases vibrate the most b/c there are no bonds b/w the molecules • Solids vibrate the least b/c there are the strongest bonds • This KE causes the effect of warmth. Whenever something becomes warmer, the average KE of its atoms or molecules has increased.

3. Check Your Understanding Does a solid desk sitting in a classroom have kinetic energy? • No. The entire desk is not moving so the entirety of the desk does not have KE.

4. Check Your Understanding Do the molecules in a solid desk sitting in a classroom have kinetic energy? • Yes! Even though the desk itself is not moving, the individual molecules are constantly moving. We do not see the desk moving because they are all vibrating back and forth over VERY small distances.

5. Temperature Temperature: the measurement of the average molecular KE of an object. • As molecules gain KE, the temperature increases • As molecules lose KE, the temperature decreases • SI Unit: Kelvin (K) • Ex: 0°C = 273 K = 32°F

6. Temperature is the measure of the AVERAGE KE of the substance, not the total KE. • Ex: there is more KE in a bucketful of warm water than in a cupful of water, but if the temperature of the two samples is the same, the average KE of the two is the same. • There are 3 commonly used temperature scales • Fahrenheit • Celsius • Kelvin

8. A Kelvin is the same size as a Celsius degree • If you increase the temperature by 1°C, you increase it by 1 K • If you increase the temperature by 10°C, you increase it by 10 K Absolute zero: the lowest possible temperature. • At this temperature all motion ceases, so the substance has no kinetic energy. • Scientists have never gotten any substance to reach absolute zero. • SI Unit: Kelvin (k) • Ex: 0 K = -273°C

9. Check Your Understanding Which has a greater average KE, a hot cup of coffee or swimming pool full of ice cold water? • The cup of coffee. As temperature is related to the average KE, the one with the higher temperature has the greater average KE.

10. Check Your Understanding Which has a greater total KE? • The swimming pool of ice cold water. Even though the cup of coffee has a higher average KE, the swimming pool has more mass and therefore a greater overall KE.

11. Heat Heat: The energy transfer from one object to another because of a temperature difference between them. • Heat flows from the higher-temperature substance into the lower-temperature substance. • Heat never flows on its own from a cold substance to a hot substance, just like water will never flow uphill by itself. • Ex: You feel cold holding a piece of ice because heat flows from your hand to the ice cube

12. Thermal Energy: the energy resulting from heat flow. • When heat flows from one object or substance to another it is in contact with, the objects are said to be in thermal contact. • Ex: When you hold a piece of ice, you lose thermal energy while the ice gains thermal energy

13. Thermal Equilibrium Thermal Equilibrium: the state of two or more objects or substances in thermal contact when they have reached a common temperature. • Objects have reached the same temperature • Ex: when a piece of hot metal is added to a cup of cool water, thermal energy flows between them until they reach the same temperature.

14. Internal Energy Internal Energy: the grand total of all energies inside a substance. • A substance contains internal energy, not heat • This energy is due to KE of the moving molecules as well as the PE in the bonds holding the molecules together. • SI Unit: Joule (J) • Ex: An iceberg has a larger internal energy than a small cup of coffee

15. Thermal Expansion • When the temperature of a substance increases this means the average kinetic energy of the substance increases, so the molecules move faster and further apart. • This results in an expansion of the substance. • Almost all forms of matter expand when they are heated and contract when they are cooled

16. Thermometers work off of this concept • As the mercury or alcohol heats up, it expands up the thermometer • When the temperature decreases, the liquid contracts down the tube • Not all objects have the same rate of thermal expansion. • Some objects expand more than others • Liquids tend to expand more than solids

17. Bimetallic strip • A bimetallic strip is two metals welded together, steel and copper • When the strip is heated, it curves • That is b/c the steel side expands at a different rate compared with the copper side

18. Check Your Understanding Why is it advisable to allow telephone wires to sag when stringing between poles in summer? • Because if the lines were pulled tight in the summer, when they contract in the winter, they would snap.

19. Expansion of Water • Water is the exception to thermal expansion • Water at the temperature of melting ice contracts when the temperature is increased, and will continue to contract until it reaches 4°C. • Water is most dense at 4°C • This is why ice floats in water • The ice is less dense than the water

20. Check Your Understanding What would happen if you put a can of soda in the freezer and leave it there overnight? • It will explode. As the water goes away from 4°C, the volume increases. As the pressure builds from the excess volume, the soda can will eventually explode.

21. Heat Transfer • Remember that heat is the transfer of energy • There are 3 types of heat transfer • Conduction • Convection • Radiation • More than one type of heat transfer can occur at the same time, but usually one is more dominate than the rest

22. Conduction Conduction: energy transfer from one object to another when the two are in direct contact. • The materials MUST be touching. • Heat moves from the warmer object to the cooler object. • Ex: a hot piece of metal burning your hand; your hand melting a piece of ice

23. Materials that conduct (move) heat well are called heat conductors. • Objects will change their temperature easily • Ex: metals, glass • Wood would be considered a good insulator. An insulator delays the transfer of heat. • Objects do NOT change their temperature easily • Ex: Styrofoam, wood, air • A poor conductor is a good insulator. • Liquids and gases in general are good insulators. Porous materials having many small air pockets are good insulators as well (the secret behind feathers’, fur’s, and wool’s perceived warmth).

24. Ice HotelIce Hotels and Igloos keep people warm because ice and snow are very good insulators or heat.

25. Check Your Understanding Touch a piece of metal and a piece of wood in you immediate vicinity. Which one feels colder? • Since wood is a poor conductor, the metal feels colder b/c it is a better conductor; heat easily moves out of your warmer hand into the cooler metal.

26. Check Your Understanding Which is really colder? • If the wood and metal are in the same general area, they should have the same temperature (room temperature); thus neither is colder.

27. Check Your Understanding Why can fire walkers walk on red-hot coals without getting injured? • The coals are good insulators, and therefore poor conductors, of heat. Even though the coals are red hot, they give up very little heat in a brief contact with a cooler surface (your foot). Things would be different if they were walking over red-hot metal shards.

28. Convection Convection: heat transfer by movement of the atoms themselves from place to place • In convection, heating occurs by currents in a fluid. • Ex: heat rises from downstairs to upstairs; bubbles in boiling water rises to the top and rolls back to the bottom of the pot

29. Convection occurs in all fluids, liquids and gases alike. • Convection will NEVER occur in a solid! • When the fluid is heated, it expands, becomes less dense, and rises. Cooler fluid then moves to the bottom and the process continues.

30. Check Your Understanding You can hold your fingers beside a flame without harm, but not above the flame. Why? • Heat travels upward by air convection. Since air is a poor conductor, very little heat travels sideways.

31. Radiation • Heat from the sun warms the Earth’s surface after passing through the atmosphere. • As air is a poor conductor, conduction cannot be responsible for this warming. Neither does it pass via convection, as convection begins only after Earth is warmed. • Both forms need molecules to transfer heat.

32. Radiation: heat transfer by electromagnetic waves. • Radiation does NOT require a medium to transfer heat • Ex: The sun warms the earth • Types of EM waves • Radio waves • Microwaves • Infrared waves • Visible light • Ultraviolet waves • X-rays • Gamma rays

33. Absorption of Radiant Energy • Absorption and reflection are opposite processes. • A good absorber of radiant energy reflects very little radiant energy, including light. • Because of this, a good absorber appears dark. • A perfect absorber reflects no radiant energy and appears perfectly black. No visible light is reflected. • Ex: Your eye’s pupil, a Black Hole in space

34. Check Your Understanding If you paint the inside of a box white and cut a hole on one side to see inside, the hole appears black. Why? • Radiant energy that enters an opening has little chance of leaving before it is completely absorbed. That’s why holes appear black.

35. Check Your Understanding Why are most coffee mugs painted white on the inside? • B/c white is good reflector of radiant energy (light). You want the radiant energy to be reflected back into your coffee, keeping it hotter for a longer time.

36. Emission of Radiant Energy • Good absorbers are also good emitters • poor absorbers are poor emitters • poor absorbers are good reflectors • Light colored objects in contact with dark colored objects eventually reach thermal equilibrium. The warmer, darker object must give up (emit) radiant energy to the cooler, lighter object.

37. Check Your Understanding Is it more efficient to paint a heating radiator black or silver? • Black. The radiator painted silver would be a poor emitter and poor absorber or energy. Black would increase the contribution of heat from the radiator.

38. Thermodynamics Thermodynamics: the study of heat and its transformation into mechanical energy. • The foundation of thermodynamics is the law of conservation of energy and the fact that heat flows from a warmer object to a cooler object

39. 1st Law of Thermodynamics The First Law of Thermodynamics: whenever heat is added to a system, it transforms to an equal amount of some other form of energy. • Conservation of energy; energy cannot be created nor destroyed, just change from one form to another • Ex: Food changes from chemical energy to body heat and/or kinetic energy

41. When energy is added to a system (any group of atoms, molecules, particles, or objects), this energy does one or both of two things: • increase the internal energy of the system if it remains in the system • does external work if it leaves the system • Equation: heat = increase in + external work done added internal by the system energy

42. Check Your Understanding If 10 J of energy is added to a system that does no external work, by how much will the internal energy of that system be raised? • 10J If 10J of energy is added to a system that does 4J of external work, by how much will the internal energy of that system be raised? • 10J – 4J = 6J

43. Second Law of Thermodynamics The second law of thermodynamics: heat will never flow by itself from a cold object to a hot object. • In order for heat to flow from a cold object to a hot object, work must be put into it (refrigerators work off of this idea) • Ex: heat flows from your hand to a piece of ice (from hot to cold)

44. Entropy • Natural systems tend to proceed toward a state of greater disorder. Entropy: the measure of the amount of disorder. • As disorder increases, entropy increases. • The second law states that for natural processes, in the long run, entropy always increases. • Ex: liquids evaporating, ice melting, a messy room • You can’t get more energy out of a system then what you put in it.