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Earths Energy Balance The Sun, our atmosphere and our planet

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2 Sources of Energy Which of the following energy sources derive their energy ultimately from sunlight? (choose all that apply) (1)Solar Cells (2)Windmills (3)Hydroelectric Dam (4)Coal (5)Gasoline

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Energy Flow To solve a crime you follow the money; to understand many environmental problems, follow the energy. Energy changes form but the total amount is always conserved. Energy changes form but the total amount is always conserved. Energy eventually degrades into heat. Energy eventually degrades into heat. A place reaches a higher temperature when energy spends more time therebut just as much energy is always emitted as is absorbed. A place reaches a higher temperature when energy spends more time therebut just as much energy is always emitted as is absorbed.

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Energy Balance Diagram

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5 The Brightness of Sunlight Light is composed of photons. They that spread out over a larger and larger area as they travel through space. The brightness (the amount of light collected by your eye, for example) declines as the distance increases.

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6 The Inverse Square Law The light is spread out over the area of each successive sphere, so the same number of photons cross through a larger and larger surface area, which grows as the square of the distance. Brightness 1 / d 2 distanceBrightness 1 m36 photons/m 2 2 m9 photons/m 2 3 m4 photons/m 2 36 photons

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7 Calculating the Suns Luminosity On a sphere completely surrounding the Sun with a radius 1 AU (1.5 x 10 11 m), each square meter would receive 1300 Watts. The Suns luminosity is therefore: L = B × 4 d 2 = (1300 watts/m 2 ) × 4 ( 1.5 x 10 11 m ) 2 ~ 1.3 x 10 3 W/m 2 × 2.6 × 10 23 m 2 ~ 4 x 10 26 Watts

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8 Human Energy Consumption Humans worldwide use ~4 x 10 20 Joules of energy per year. (U.S. 25% of this.) Humans worldwide use ~4 x 10 20 Joules of energy per year. (U.S. 25% of this.) In just 1 second the Sun generates enough energy to supply humans for: In just 1 second the Sun generates enough energy to supply humans for: time = 4 x 10 26 Joules/4 x 10 20 Joules/year time = 4 x 10 26 Joules/4 x 10 20 Joules/year = 10 6 years = 10 6 years The amount of sunlight striking the Earth is ~ 5 x 10 24 Joules per year. The amount of sunlight striking the Earth is ~ 5 x 10 24 Joules per year.

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9 Whats Keeping the Core Hot? Above about 5,000,000K protons (hydrogen nuclei) slam into each other and FUSE, turning matter into energy The net result is: 4 H 1 He + 2 neutrinos + ENERGY Hydrogen Fusion

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10 Energy and Mass Einsteins famous equation E = m c 2 relates how energy and mass can be converted into each othermass is really just a form of energy. The Sun must convert 4 million tons of matter into energy every second to produce its luminosity!

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11 Core Core Energy must be generated here to keep the Sun hot. Energy must be generated here to keep the Sun hot. Radiation Zone Radiation Zone Energy is transported by photons Energy is transported by photons Convection Zone Convection Zone Energy is transported by upwelling gas Energy is transported by upwelling gas The Sun from the Inside Out

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12 Solar Photons Energy in the form of photons travels out from the hot core of the Sun. Energy in the form of photons travels out from the hot core of the Sun. It may take a photon a million years to random walk from the center to the surface. It may take a photon a million years to random walk from the center to the surface.

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13 Solar Granulation The surface is boiling (convecting) because of the heat coming from below.

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Earth Radiation Budget Experiment What comes out must go in

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Solar Energy Potential

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