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Unformatted text preview: by our atmosphere’s gases e.g.
CO2, water vapour
Two of which could be:
• light reflected directly back from ice/snow/high albedo surface
• Heat reflected back to space from the tops of clouds
• Heat radiated back to space from clouds
• Heat radiated back to space from greenhouse gases
2. A distant star of radius 100 million km has a surface temperature of 3,000K.
Calculate its energy generated per m2 of surface area.
Using the Stefan Boltzmann equation, E = σT4 = 5.67 x 108 x (3000)4 = 4.59 x
106W or equivalent in kW.
If a planet of radius 5,000km without an atmosphere is orbiting at a distance of 1
billion km from the star, what is the average energy intensity that will fall onto the
planet per m2 of its surface area? (4 marks)
Step 1
Total energy output from star = Above answer x star’s surface area
= 4.59 x 106W x 4x pi x (100x109)2 = 5.77x1029 W/m2
Step 2
At our orbit, this amount is spread thinly over a sphere equal to the orbit. Energy
intensity at that orbit is:
= 5.77x1029 W/m2 / 4x pi x r2 where r is the planet’s orbit
= 5.77x...
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 Fall '14

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