1. Earth’s Energy Balance
The Sun, our atmosphere and our planet

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

3. 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 eventually “degrades” into heat.
A place reaches a higher temperature when energy spends more time there—but just as much energy is always emitted as is absorbed.

4. Energy Balance Diagram

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.

6. The Inverse Square Law Brightness  1 / d2
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 / d2
36 photons
distance
Brightness
1 m
36 photons/m2
2 m
9 photons/m2
3 m
4 photons/m2

7. Calculating the Sun’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

8. Human Energy Consumption
Humans worldwide use ~4 x 1020 Joules of energy per year. (U.S. 25% of this.)
In just 1 second the Sun generates enough energy to supply humans for:
time = 4 x 1026 Joules/4 x 1020 Joules/year
= 106 years
The amount of sunlight striking the Earth is ~ 5 x 1024 Joules per year.

9. What’s 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

10. Energy 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!

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

12. Solar Photons
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.

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

14. Earth Radiation Budget Experiment
What comes out must go in

15. Solar Energy Potential