Bell Ringer

1. Bell Ringer Name: 2/23/2009 What do you believe color is? Response

2. Radiation • Radiation, is one more way in which heat transfer occurs • Radiation from heat transfer must not be confused with radioactivity!

3. Radiation • All objects emit radiant energy in a mixture of different wavelengths. • We will study waves and their wavelength in later chapters, but a brief introduction will have to do here.

4. Radiation • Picture a smooth lake or pond. A stone is tossed into the water. After the initial splash waves move away from the center in evenly spaced increments called waves.

5. Radiation • A wavelength is the distance between the tops (crests) of consecutive waves. • If the distance between the crests is reduced the wavelength shortens.

6. Radiation • A few wavelengths that you may be familiar with are: • Infrared • Visible light • Ultraviolet

7. Radiation • The sun is our largest source of radiant energy. • There is nothing but vacuum between the sun and Earth’s atmosphere, so how can you feel the energy from the sun on your skin on a sunny day?

8. Radiation • The further away an object is from a radiant energy source, the less energy it will receive.

9. Radiation • Back to weather • We know the sun releases approximately the same amount of radiant energy from every spot on its surface • How does the amount of radiant energy received on the Earth’s equator, compare to that received at the poles?

10. Radiation • The equator is closer to the sun, than the poles, and will receive more energy per square meter!

11. Radiation • Mr. Everson like the farm and cars, I like astronomy. • Who can provide a good reason why the planets Mercury and Venus are hot, while Mars and Pluto are cold?

12. Radiation • There are two important radiation laws that are used in physics and especially in astronomy. • One law relates the amount of radiant energy released with an object’s temperature • The other law relates an object’s temperature to it’s color

13. Radiation • We won’t be studying the details but it’s good to know that there is mathematics that describe how this is possible. • We will look at some of the simple math in a couple more chapters.

14. Radiation • Remember the demonstration where I held my fingers on the side of the flame without being burnt. This has more to do with radiation than convection.

15. Radiation • The Earth receives a lot of radiant energy from the sun in the infrared wavelengths. • If you sit close to a fireplace, you feel the heat on your eyelid if they are closed. • Put on a pair of sunglasses and the heat is reduced, why?

16. Radiation • The Earth receives a lot of radiant energy from the sun in the infrared wavelengths. • If you sit close to a fireplace, you feel the heat on your eyelid if they are closed. • Put on a pair of sunglasses and the heat is reduced, why? • The sunglasses block some of the infrared wavelengths and allow the visible light wavelengths through! The same with sunlight.

17. Radiation • Why is sun block used for?

18. Radiation • Why is sun block used for? • To block ultraviolet wavelengths from the sun and prevent burning?

19. Absorption • Absorbing and reflecting are opposite processes

20. Absorption • An object or material that is a good absorber of radiant energy, reflects very little. • Can you think of any objects or materials that absorb a lot of radiant energy?

21. Absorption • How about a color of clothing that absorbs energy?

22. Absorption • How about a color of clothing that reflects energy?

23. Absorption • Perfect absorbers are black because they absorb almost all visible light radiation and reflect almost none. • They don’t absorb because they are black. There is a difference. • The pupil of your eye is a good example.

24. Absorption • Why do you suppose photographs taken with a flash sometimes show red pupils?

25. Absorption • Why do you suppose photographs taken with a flash sometimes show red pupils? • Light is reflected off the retina in the back of the eye!

26. Absorption • Why don’t you see white in the closed box?

27. Absorption • Why don’t you see white in the closed box? • The light enters, is reflected many times and loses a little energy with each reflection. Very little comes back out.

28. Emission • Good absorbers are good emitters • Poor absorbers are poor emitters • Objects with mirror-like surfaces reflect most radiant energy they encounter

29. Emission • Tall radio antennas are good absorbers and emitters in the radio wavelengths. • Extending the example, dark objects that absorb a lot of radiant energy must also emit a lot of radiant energy.

30. Emission • As an example: • Imagine two water containers, one white or with a mirror-like surface and one black • Add water of the same temperature into both of them • Place a thermometer in each • Which cools faster?

31. Emission • When absorption rate equals emission rate the temperature remains constant or unchanged.

32. Emission • Back to weather again. • On a sunny day, the Earth’s surface is a good absorber • At night, the Earth’s surface is a good emitter

33. Emission • Is it a good idea to paint a radiator in your home black or silver?

34. Emission • Is it a good idea to paint a radiator in your home black or silver? • Black. Although most of the heat is transmitted via convection to the air, a dull black color would increase the contribution made through radiation.

35. Newton’s Law of Cooling • An object with a temperature different from its surroundings will eventually reach thermal equilibrium. • The rate of object cooling depends upon how much hotter the object is than the surroundings.

36. Newton’s Law of Cooling • Newton studied many things besides gravity. • He also studied how objects cool.

37. Newton’s Law of Cooling • The rate of cooling of an object, whether by conduction, convection or radiation is approximately proportional to the temperature difference between the object and its surroundings. • Rate of cooling ~

38. Newton’s Law of Cooling • An object with a temperature different from its surroundings will eventually reach thermal equilibrium. • The rate of object cooling depends upon how much hotter the object is than the surroundings.

39. Newton’s Law of Cooling • An object with a temperature different from its surroundings will eventually reach thermal equilibrium. • The rate of object cooling depends upon how much hotter the object is than the surroundings.

40. Global WarmingThe Greenhouse Effect • Almost everyone has experienced the temperature difference in an automobile or greenhouse due to window glass.

41. Global WarmingThe Greenhouse Effect • This is a true because of a combination of things, some of which we’ve looked at. • All things radiate energy • The radiation wavelength depends upon temperature

42. Global WarmingThe Greenhouse Effect • This is a true because of a combination of things, and some that are new. • High temperature objects radiate short waves • Low temperature objects radiate long waves • Substances are transparent to some wavelengths

43. Global WarmingThe Greenhouse Effect • Let’s consider the short and long waves • The sun is a high temperature object compared to a fire • High temperature objects radiate short waves • We block Ultraviolet (UV) rays from the sun to prevent burning. • Therefore UV rays are short waves.

44. Global WarmingThe Greenhouse Effect • Let’s consider the short and long waves • Fire is a low temperature object compared to the sun • Low temperature objects radiate long waves • We block Infrared rays from a fire and feel cooler • Therefore Infrared rays are long waves.

45. Global WarmingThe Greenhouse Effect • Now let’s consider transparent and opaque, which are opposites • Glass is transparent and lets visible light through • A piece of metal is opaque and does not let visible light through, it is reflected

46. Global WarmingThe Greenhouse Effect • A substance that is transparent allows waves through. • These waves lose some energy when traveling through the substance • These waves reflect off surfaces that don’t absorb them and lose a little more energy

47. Global WarmingThe Greenhouse Effect • For radiation from the sun reaching Earth this means • The atmosphere is opaque to some waves and these are reflected • Some waves are absorbed by the atmosphere • The atmosphere is transparent to some waves and these are transmitted through the atmosphere

48. Global WarmingThe Greenhouse Effect • The atmosphere lets through short waves, just like glass • These waves lose energy and get a little longer • Some of these waves are reflected and eventually reach the glass or atmosphere again • When they cross the threshold from being transparent to the glass or atmosphere, they are trapped inside because they reflect

49. Global WarmingThe Greenhouse Effect • Before man came along, radiation from the sun balanced the natural terrestrial radiation from Earth’s surface (on average) • We did have extremes called ice ages • But man is releasing energy from fossil fuels and nuclear energy • In addition, man is changing the composition of the atmosphere and changing its transparent and opaque abilities • This is global warming