IB Physics SL B.2.6 Mean Solar Intensity

Practise deriving the global mean solar intensity from planetary geometry and combining S/4 with albedo and emissivity to calculate absorbed intensity or equilibrium temperature.

Exam points

derive S/4 by dividing intercepted power SπR² over the planetary surface area 4πR²
calculate mean absorbed intensity as (1 - α)S/4 rather than using the subsolar value
equate absorbed mean intensity to εσT⁴ when solving an equilibrium temperature

[Maximum number: 2]

The diagram shows a simplified energy-balance model for the Earth surface–atmosphere system.

Figure for Question B.2.6—Mean solar intensity question 1 — IB Physics SL

The following data are given:

\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

Show that the average global intensity of radiation absorbed by the surface is about $240 \mathrm{Wm}^{-2}$ .

Average incoming intensity $=\frac{S}{4}$ « $=340 \mathrm{~W} \mathrm{~m}^{-2}$ »

Absorbed intensity $=(1-0.30) \times 340$ or $238 \mathrm{~W} \mathrm{~m}^{-2}$

The steps in the calculation must be
shown.

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B.2.6—Mean solar intensity question 1