KNOWLEDGE TO GET FROM TODAY S CLASS MEETING Class Meeting #11, Friday, February 12 th

Transcription

1 KNOWLEDGE TO GET FROM TODAY S CLASS MEETING Class Meeting #11, Friday, February 12 th 1) RADIANT ENERGY (Stefan-Boltzmann Law & Wien s Law) Pgs Pgs an in-class exercise 2) Earn a good grade on QUIZ #3

2 EXAM #1 occurs 7 days from today, Friday, February 19 th An EXAM STUDY GUIDE was distributed Mon, Feb 8 th, and is available here in class today, and is also available online in the STUDY-GUIDE folder at the class web site The EXAM will consist of multiple-choice and short answer (quiz-like) questions You WILL NOT need a scantron sheet

3 EXAM #1 will consist of multiple choice questions AND short written-answer questions (like quiz questions) Your QUIZZES and LABS are very good STUDY GUIDES, as are our In-Class exercises You WILL NOT need a scantron sheet for the exam!!

4 HOMEWORK #2 was distributed Monday, Feb 8 th With HW #2, you will be obtaining OBSERVATIONS of the MOON every-other evening for the next two weeks Your set of OBSERVATIONS, and also your answers to QUESTIONS that will be distributed next week, are due to be handed in here in BX102 at the start of class on Friday, February 26 th HOMEWORK #2 is also available online within the class web page HOMEWORK folder You should already have TWO observations

5 NEXT WEEK s Lab will occur back in WH232 You will conduct a lab focused upon PHASES of the MOON Lab available at: QUESTIONS?

6 PICTURE OF THE DAY A FIREBALL (meteor) seen from Arizona and Southern California during Fall frictional heating causes atoms/molecules to emit..

7 THE ELECTROMAGNETIC SPECTRUM All objects are attempting to emit radiant energy at all wavelengths, but hotter objects are more effective at emitting radiant energy at ALL wavelengths,

8 RADIATION (better referred to as RADIANT ENERGY) TODAY WE CONTINUE DISCUSSING: This IS NOT the type of radiation that we worry about in regard to nuclear energy or weapons (that type of radiation involves atomic nuclei released by radioactive materials) Radiant Energy IS the electromagnetic waves (it can also considered as Light Particles) that carry energy from place to place (visible light is one such manifestation of this phenomenon) Radiant energy emission is related to Temperature usually

9 TEMPERATURE = the average kinetic energy of the particles (atoms, molecules) in a system What physical characteristic makes HOT water hotter than COLD water? The speed at which the H 2 O molecules randomly move!! If something is moving faster, is it more likely to break, or be modified by a collision? YES, it is (compared to it moving slower)

10 So, consider an atom (nucleus plus orbiting electrons) Is an electron more likely to be knocked around or off the atom in a cold situation or in a hot situation? HOT, since we know the atoms themselves are moving more rapidly and thus have more kinetic energy (energy of motion) with which to knock each other around [ the ATOM absorbs energy in this process ]

11 Eventually, such a knocked around or off electron will return to its original position within the atom When this electron returns to its original position, there is a release of energy from the atom This released energy is given off in the form of electromagnetic RADIANT ENERGY (= Light of some particular wavelength) [electrons that move to higher energy orbits or become unbound to the nucleus absorb energy; electrons that move to a lower energy orbits or rejoin the nucleus result in emission of energy]

12 So, the hotter the material, 1) the greater the rate of electrons being knocked off or moved to higher orbits and thus B) the greater the number of electrons that will be rejoining the nucleus or dropping to lower energy orbits SO C) the greater the amount of radiant energy being emitted away (we can think of this atom:electron light as VISIBLE light and ULTRAVIOLET light) look at Figure 5.19 in your text

13 At temperatures you and I experience day-to-day here in Las Cruces, it is not electrons getting knocked off or higher but rather molecules bending and stretching that result in the emission of radiant energy; this process is less energetic than the electrons leaving and rejoining the nucleus... we don t see this molecular light with our eyes; this is INFRARED light CO 2 molecule

14 A hot surfaced object generates and emits away a greater amount of radiant energy than does a cold object (per square meter of its surface) This is quantified by the: STEFAN-BOLTZMAN Law Equation: INTENSITY OF EMITTED RADIANT ENERGY = σ x TEMPERATURE 4 = σ T 4 Stefan-Boltzmann constant= 5.67 x 10-8 W m -2 K -4 in units of: Watts of energy per square meter of surface

15 RADIANT ENERGY is classified by its WAVELENGTH ( color ), or alternately by its FREQUENCY Short Wavelength High Frequency High Energy Long Wavelength Low (small) Frequency Low Energy

16 The wavelength ( color ) of light at which an object emits its GREATEST amount of radiant energy is defined by: WIEN S LAW: Wavelength at which the Most Radiant Energy is Emitted (3000 / Temperature in Kelvin) = wavelength in micrometers (one-millionth of a meter) SO The Color at which maximum energy is emitted = [3000 / T (in Kelvin)] (in units of micrometers)

17 Our eyes are tuned to visible wavelengths, from micrometers (1 µm= 10-6 meters ) Some WIEN S LAW EXAMPLES: Earth s average surface: T=288 Kelvin, Wavelength of Max = 3000/288 = 10.4 µm The Sun s Surface: T = 5800 Kelvin Wavelength of Max = 3000/5800 = 0.51 µm Hot Lava: 1500 Kelvin (2240 Fahrenheit) Wavelength of Max = 3000/1500 = 2 µm Look at Figure 5.19 in your text

18 None of the planet s in our Solar System have surface temperatures hot enough to generate any detectable VISIBLE light (radiant energy) SO as we have stated before, we see planets because they REFLECT visible light generated by the Sun BUT all of the stars we see in the night sky are themselves generating the visible light by which we see them (because those stars are HOT!) Which has a hotter surface: a RED STAR or BLUE STAR?

19 Which stars in the image below possess the COLDEST surface temperature(s)? The star trails to the left are formed by opening a camera s shutter and leaving it open for an extended period of time as the star patterns move from east-towest (due to the Earth s rotation) across the night sky. The REDDER stars possess the colder surface temperatures, which is what WIEN S LAW tells us (for objects EMITTING radiant energy)

20 The INTENSITY (watts per square meter) of RADIANT ENERGY that an object generates and emits is based upon that objects TEMPERATURE STEFAN-BOLTZMANN LAW QUANTITY = 5.67 x 10-8 x (TEMP) 4 Watts/m 2 The WAVELENGTH ( color ) at which an object emits the MOST RADIANT ENERGY: WIEN S LAW WAVELENGTH of MOST = (300 0 /TEMPERATURE) in units of micrometers (10-6 meters) Let s work through an exercise to illustrate these LAWS!!!!!

21 EARTH SUN SFC TEMPERATURE ~300 K ~5800 K (~60 o F) (~10,000 o F) Intensity of radiant 459 W/m 2 64,164,532 W/m 2 Energy Emitted (Watts per square meter) (the above numbers calculated using the Stefan-Boltzman Law) Wavelength of ~10 micrometers ~0.5 micrometers maximum radiant (infrared) energy (your eyes and yellow:green emitted mine DO NOT detect this wavelength) (the above numbers calculated using Wien s Law) Let s look at the 10-micrometer infrared wavelength world

22 Which stars in the image below are sending out the GREATEST intensity of Radiant Energy? Intensity = BRIGHTNESS If we assume that all of the stars are the same size, we would expect that the BLUEr stars, which Wien s Law tells us have hotter surfaces than the REDder stars, would also be the stars with a greater overall intensity of EMITTED RADIANT ENERGY, since the Stefan-Boltzman law tells us that the hotter surface emits away the greater intensity of radiant energy. BUT, some of the Red stars are brighter than some blue stars. what might be going on???

23 The Sun emits > 64,000,000 Watts of radiant energy per square meter at its surface but you and I do not FEEL that much energy when we stand here on Earth in the sunlight Why not???? Here on Earth, we are ~1.5 x meters (150 billion meters = 150 million kilometers) from the Sun 1 AU from the Sun 64,000,000 Watts per square meter is equal to having 640, Watt light bulbs packed into an area that is smaller than the screen these words are being projected upon.

24 As the light that the Sun emits travels outward from the Sun, it becomes spread over a greater and greater area (think of an expanding sphere, like the surface of a balloon) This spreading of the light causes its local intensity (Watts per square meter received) to decrease as it moves away from the Sun Since the surface area of the expanding light sphere increases as: 4 times π times the radius squared= 4 π Radius 2, the radiant energy intensity must decrease at the same radius-dependent rate, ( 1 / Radius 2 )

25 As sunlight expands outward from the Sun, its local intensity declines with increasing distance from the Sun (with a 1 / distance 2 dependence), because the sunlight is being spread over a bigger and bigger spherical shape, just like an expanding balloon s material gets thinner-and-thinner as the balloon is inflated larger and larger but sunlight does not POP! Note that the expanding shells of light get dimmer as they move farther-andfarther from the Sun SUN

26 CHANGE in INTENSITY of RECEIVED SUNLIGHT with INCREASING DISTANCE DISTANCE RECEIVED RATIO FROM SUN center SUNLIGHT INTENSITY Sfc (700,000 km) 64,000,000 W/m 2-1,000,000 km 31,830,980 W/m 2 1 2,000,000 km 7,957,747 W/m 2 1/4 4,000,000 km 1,989,436 W/m 2 1/16 8,000,000 km 497,359 W/m 2 1/64 16,000,000 km 124,340 W/m 2 1/256 32,000,000 km 31,085 W/m 2 1/ ,000,000 km (Mercury) 7,771 W/m 2 1/ ,000,000 km (Venus) 2,650 W/m 2 1 / 11, ,000,000 km (Earth) ~1,373 W/m 2 1 / 22,500

27 Since Pluto is on average 40 times farther from the Sun than Earth is: Sunlight intensity at Pluto = Earth / 40 2 (1370 W m -2 / 40 2 ) = 1370/1600 = 0.85 W/m 2 < 1 Watt per m 2!! We will illustrate this one over distance squared concept with a hands-on exercise during class next Monday, Feb 15 th.