IB Physics HL B.2.2 Emissivity Question Bank

Practise IB Physics HL B.2.2 by analysing emissivity, net radiation and planetary energy balance with Stefan–Boltzmann models.

Exam points

Calculate net radiation from an emitting body inside a warmer or cooler enclosure using emissivity and T⁴.
Derive planetary equilibrium temperature by combining emissivity, albedo and absorbed solar intensity.

[Maximum number: 3]

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

Figure for Question B.2.2—Emissivity question 1 — IB Physics HL

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}

Determine the average intensity re-radiated by the atmosphere towards the surface. Assume that the emissivity of the surface is 0.90 .

Emitted intensity $=\ll 5.67 \times 10^{-8} \times 0.90 \times 288^{4}=>351 \mathrm{~W} \mathrm{~m}^{-2}$

Intensity leaving Earth $=238 \mathrm{~W} \mathrm{~m}^{-2}$

Re-radiated intensity $=\ll 351-238=>113 \mathrm{~W} \mathrm{~m}^{-2}$

Marking guidance:

Ignore units as they are not required for the answer.

The steps in the calculation must be checked.

Accept outgoing intensity $=240 \mathrm{~W} \mathrm{~m}^{-2}$ for MP2.

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B.2.2—Emissivity question 1