Chapter 15 Temperature, Heat & Thermal Expansion

1. Part III: Heat Chapter 15Temperature, Heat & Thermal Expansion Physics 1 (Garcia) SJSU

2. Temperature Temperature of an object indicates average internal energy (due to molecular motion) of the object. Temperature Scales Physics 1 (Garcia) SJSU

3. Total versus Average The total amount of money in this room is probably around $1000. The average amount of money per person is probably around $20. Temperature of an object depends on the average amount of energy per molecule, not the total. Bucket of warm water can have more internal energy than cup of hot water. 80 °F Physics 1 (Garcia) SJSU 100 °F

4. Internal Energy Internal energy of an object depends on: • Temperature • Mass • Material Iron 1 kg 1000 grams Water Iron 1 kg 1 gram Physics 1 (Garcia) SJSU

5. Money and Happiness Some people need a lot of money to make them happy. Some don’t. Some materials, such as water, need a lot of energy to raise their temperature. Some materials, such as iron, need little energy to raise their temperature. Nicole Richie & Paris Hilton Physics 1 (Garcia) SJSU MAHATMA GANDHI

6. Increasing Internal Energy Can increase internal energy (and temperature) by tapping energy sources. Electric energy heats burner Chemical energy released in fire Physics 1 (Garcia) SJSU

7. Work and Heat May increase internal energy by exerting a force to do mechanical work. Rub hands together for warmth Strike an iron surface with great force and red-hot sparks are created Physics 1 (Garcia) SJSU

8. Demo: Work and Heat Increase internal energy (and thus temperature) by doing mechanical work on an object. Bottle of Mercury Temperature increases by a few degrees SHAKE Physics 1 (Garcia) SJSU

9. Specific Heat Capacity Filling and crust at same temperature yet mouth burned only by the filling. Specific heat capacity is the internal energy required to raise one gram of a material by one degree of temperature. Filling has high specific heat capacity Crust has low specific heat capacity Physics 1 (Garcia) SJSU

10. Demo: Sparklers Iron burns red-hot at a temperature of 5000 ºF Sparks from sparkler don’t burn skin because they have very little energy (small mass and low heat capacity). Walking on red-hot embers is safe for the same reason. Physics 1 (Garcia) SJSU

11. Demo: Boil Water in Paper Cup Because of high heat capacity of water, the large amount of heat added by the flame raises the temperature of the water until boiling. If the cup is filled with sand instead of water then it burns quickly. Cup with sand Cup with water Physics 1 (Garcia) SJSU

12. Thermal Expansion Due to increased molecular motion, most materials expand as temperature increases. Space allows for expansion Sidewalk buckles and cracks due to expansion on a hot summer day Physics 1 (Garcia) SJSU

13. Demo: Slowing Air Molecules Balloon returns to its original state Air molecules slow down and lose kinetic energy Cool balloon using liquid nitrogen Balloon slowly warms up, restoring energy Physics 1 (Garcia) SJSU

14. Demo: Expansion of a Ring Metal ball barely fits past the metal ring. Not surprising that heated ball won’t pass through cold ring. Will cold ball pass through heated (expanded) ring? Physics 1 (Garcia) SJSU

15. Demo: Bi-metallic Strip Different materials have different rates of expansion. STEEL Brass Brass expands more than steel when heated Thermostat COLD HOT Bi-metallic Spiral strip Physics 1 (Garcia) SJSU

16. Demo: Heat, Cool, Break Glass expands when heated. If hot glass is cooled quickly, exterior cools before the interior. Exterior contracts faster than the interior, cracking the glass. GLASS Cracks form COOL (quickly) Physics 1 (Garcia) SJSU HEAT

17. Chapter 16Heat Transfer Physics 1 (Garcia) SJSU

18. Heat Transfer Heat always flows from high temperature objects to low temperature objects. Heat flow stops when temperatures equal. Various ways by which heat may flow. Heat flows from child into air 75º 98º 32º Heat flows from child and air into the ice cream Physics 1 (Garcia) SJSU

19. Conduction Conduction is heat flow by direct contact. Some materials are good thermal conductors, others are insulators. Tile floor feels colder than wood floor 98º 98º 75º 75º Wood is an insulator Tile is a conductor Physics 1 (Garcia) SJSU

20. Demo: Torch the Money Wrap a dollar bill tightly around a copper pipe. Put it into a flame. Physics 1 (Garcia) SJSU

21. Air is a Poor Conductor Because air is such a poor conductor, some pizza ovens don’t have a door. Can safely put your hand in an oven. Metal is good conductor so you need oven mitt to touch it safely (cloth is a poor conductor). Physics 1 (Garcia) SJSU

22. Demo: Boiling Ice Water Water and glass are relatively poor conductors of heat. Can boil water at the top of a test tube with ice at the bottom of the tube. Steel wool prevents ice from floating Physics 1 (Garcia) SJSU

23. Convection Heat transfer in a fluid often occurs mostly by convection. Buoyancy causes warm air to rise, which carries thermal energy directly by its motion. Physics 1 (Garcia) SJSU

24. Demo: Candle Very HOT Rising hot air above a candle carries most of the heat generated by the burning flame. Warm Shadows reveal rising air currents of hot air. Physics 1 (Garcia) SJSU

25. Convection Oven Convection oven has a fan to enhance the circulation of the air, increasing the transfer of heat. Physics 1 (Garcia) SJSU

26. Fiberglass Insulation Air is a poor thermal conductor but easily transfers heat by convection. Fiberglass insulation is mostly air, with the fibers disrupting the convection flow. Physics 1 (Garcia) SJSU

27. Radiation Light has many different wavelengths, most of which are not visible to the eye. All light carries energy, thus transfers heat. Heat Lamp Physics 1 (Garcia) SJSU

28. Emission of Radiant Energy 75º All objects radiate light; higher the temperature the higher the frequency. At room temperature the radiated light is at frequencies too low for our eyes to see. Special cameras are sensitive to this infrared radiation. 98º Attics in this house were kept warm for growing marijuana. 70 Physics 1 (Garcia) SJSU

29. Reflection of Radiant Energy White objects reflect light, black objects don’t. Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out. White tubes look black inside. Physics 1 (Garcia) SJSU

30. Controlling Heat Transfer Thermos bottle eliminates conduction and convection by having double-walled sides with vacuum. Silvered interior walls minimize heat transfer by radiation. Physics 1 (Garcia) SJSU

31. Greenhouse Effect Glass is transparent to sunlight (short-wavelength). Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat. Physics 1 (Garcia) SJSU

32. Greenhouse Carbon Dioxide Over past 1000 years temperatures nearly constant until CO2 emissions increased starting with the industrial revolution. Physics 1 (Garcia) SJSU Industrial revolution begins

33. Chapter 17Changes of Phase Physics 1 (Garcia) SJSU

34. Phases of Matter Four Phases of Matter: • Solid • Liquid • Gas • Plasma Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas). Water Plasma Ice Steam Physics 1 (Garcia) SJSU

35. Evaporation Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid. A random molecule at the surface acquires enough energy to escape the attraction force among the molecules (which holds the liquid together). GAS LIQUID Physics 1 (Garcia) SJSU

36. Evaporative Cooling Because only the most energetic molecules can escape the surface, evaporation removes internal energy from the liquid, that is, evaporation cools. Brr WET CLOTH HEAT WET BODY & TOWEL WET TONGUE HEAT HEAT Wet towel cools head Wetness cools person Wet tongue cools dog Physics 1 (Garcia) SJSU

37. Condensation Condensation is the reverse of evaporation, a change of phase from gas to liquid that takes place at the surface of a liquid. A random molecule from the gas strikes the surface and sticks instead of bouncing back into the gas. Condensation heats. GAS LIQUID Physics 1 (Garcia) SJSU

38. Hot and Humid A 90 degree day in a dry climate, like San Jose, is more comfortable than a 90 degree day in a humid place like New Orleans. In a dry climate you’re cooled by evaporation, in a wet climate you’re heated by condensation. Heat index is the apparent temperature a person feels for a given humidity. Physics 1 (Garcia) SJSU

39. Demo: Wet/Dry Bulb Thermometer Pair of thermometers; one is kept dry while the other’s bulb is wrapped in wet cloth. Difference of their temperatures gives relative humidity. Wet bulb Dry bulb Large temperature difference indicates high or low humidity? Low humidity; evaporative cooling is significant. Physics 1 (Garcia) SJSU

40. Fog & Clouds Warm air rises. As it rises, it expands. As it expands, it cools. As it cools, vapor molecules condense into water droplets. This forms a cloud (or fog if warm, moist air cools near the ground). As vapor expands, it cools and tiny, visible, water droplets (liquid) condense. Cool Water vapor (gas) is invisible Warm breath feels cool when it expands Warm Physics 1 (Garcia) SJSU

41. Boiling When the temperature of a liquid is high enough that evaporation occurs everywhere, not just the surface, then the liquid boils. The temperature required depends on the pressure; lower the pressure, the lower the boiling temperature (boiling point). Tiny bubbles grow due to evaporation at their surface Physics 1 (Garcia) SJSU

42. Liquid Nitrogen Liquid nitrogen boils at atmospheric pressure and room temperature. Boiling point is -320 ºF and freezes at -346 ºF. Physics 1 (Garcia) SJSU

43. Demo: Slowing Air Molecules Balloon returns to its original state Air molecules slow down and lose kinetic energy Cool balloon using liquid nitrogen Balloon slowly warms up, restoring energy Physics 1 (Garcia) SJSU

44. Demo: Low Pressure Boiling Water boils at room temperature if the pressure is low. Cooking at high altitudes is difficult due to this effect; coffee brewed in the mountains always tastes lukewarm. Physics 1 (Garcia) SJSU

45. Melting Melting is the change of phase from solid to liquid. Melting is a cooling process; the solid must absorb heat to melt. Physics 1 (Garcia) SJSU

46. Sublimation Sublimation is change of phase from solid to gas without passing through liquid phase. Solid carbon dioxide (dry ice) sublimates at a chilly -109 °F. Put dry ice into warm water to create dense fog of tiny water droplets. Physics 1 (Garcia) SJSU

47. Demo: Carbon Dioxide Carbon dioxide, released when dry ice sublimates, is heavier than air. • Burning candle • Extinguished under CO2 layer Bubbles float on layer of dry ice. (c) Scoop out some CO2 in a cup (d) Pour it on candle to extinguish Physics 1 (Garcia) SJSU

48. Freezing Freezing is the opposite of melting, that is, the change of phase from liquid to solid. Heat must be removed from a liquid in order to freeze it into a solid. Lava (liquid) freezes into rock (solid), heating the seawater. Seawater (liquid) boils into vapor (gas), cooling the lava. Physics 1 (Garcia) SJSU

49. Demo: Freeze Solid Materials become brittle when frozen solid. Organic materials appear solid but cells contain large amounts of liquid water. Physics 1 (Garcia) SJSU

50. Energy & Changes of Phase Physics 1 (Garcia) SJSU