Earth’s Energy BalanceEarth’s Energy Balance

The Sun, our atmosphere The Sun, our atmosphere and our planetand our planet

22

Sources of EnergySources of EnergyWhich of the following energy sources Which of the following energy sources

derive their energy ultimately from derive their energy ultimately from sunlight? (choose all that apply)sunlight? (choose all that apply)

(1)(1) Solar CellsSolar Cells

(2)(2) WindmillsWindmills

(3)(3) Hydroelectric DamHydroelectric Dam

(4)(4) CoalCoal

(5)(5) GasolineGasoline

Energy FlowEnergy FlowTo solve a crime you “follow the money”; to To solve a crime you “follow the money”; to

understand many environmental problems, understand many environmental problems, “follow the energy.”“follow the energy.”

Energy changes form but the total amount Energy changes form but the total amount is always conserved.is always conserved.

Energy eventually “degrades” into heat.Energy eventually “degrades” into heat. A place reaches a higher temperature A place reaches a higher temperature

when energy spends more time there—but when energy spends more time there—but just as much energy is always emitted as just as much energy is always emitted as is absorbed. is absorbed.

Energy Balance DiagramEnergy Balance Diagram

55

The The Brightness Brightness of Sunlightof 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.

66The Inverse Square The Inverse Square LawLaw

The light is spread out The light is spread out over the area of each over the area of each successive sphere, so successive sphere, so the same number of the same number of photons cross through a photons cross through a larger and larger surface larger and larger surface area, which grows as the area, which grows as the square of the distance.square of the distance.

Brightness Brightness 1 / 1 / dd22

distance Brightness

1 m 36 photons/m2

2 m 9 photons/m2

3 m 4 photons/m2

36 photons

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Calculating the Calculating the Sun’s LuminositySun’s Luminosity

On a sphere completely surrounding the Sun with a radius 1 AU (1.5 x 1011 m), each square meter would receive 1300 Watts. The Sun’s luminosity is therefore: L = B × 4 d2

= (1300 watts/m2 ) × 4 (1.5 x 1011 m)2

~ 1.3 x 103 W/m2 × 2.6 × 1023 m2 ~ 4 x 1026 Watts

88

Human Energy ConsumptionHuman Energy Consumption Humans worldwide use ~4 x 10Humans worldwide use ~4 x 102020 Joules Joules

of energy per year. (U.S. 25% of this.)of energy per year. (U.S. 25% of this.) In just 1 second the Sun generates In just 1 second the Sun generates

enough energy to supply humans for:enough energy to supply humans for: time = 4 x 10time = 4 x 102626 Joules/4 x 10 Joules/4 x 102020 Joules/year Joules/year

= 10= 106 6 yearsyears

The amount of sunlight striking the The amount of sunlight striking the Earth is ~ 5 x 10Earth is ~ 5 x 102424 Joules per year. Joules per year.

99

What’s Keeping What’s Keeping the Core Hot?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

1010

Energy and MassEnergy and Mass

Einstein’s famous equation E = m c2 relates how energy and mass can be converted into each other—mass is really just a form of energy.

The Sun must convert 4 million tons of matter into energy every second to produce its luminosity!

1111

CoreCore Energy must be Energy must be

generated here to keep generated here to keep the Sun hot.the Sun hot.

Radiation ZoneRadiation Zone Energy is transported by Energy is transported by

photonsphotons

Convection ZoneConvection Zone Energy is transported by Energy is transported by

upwelling gasupwelling gas

The Sun from the Inside OutThe Sun from the Inside Out

1212

Solar Solar PhotonsPhotons

Energy in the form of photons travels out from the Energy in the form of photons travels out from the hot core of the Sun.hot core of the Sun.

It may take a photon a million years to “random It may take a photon a million years to “random walk” from the center to the surface.walk” from the center to the surface.

1313

Solar GranulationSolar Granulation

The surface is “boiling” (convecting) because of the heat coming from below.

Earth Radiation Budget ExperimentEarth Radiation Budget ExperimentWhat comes out must go inWhat comes out must go in

Solar Energy PotentialSolar Energy Potential