IB Physics SL B.2.2 Emissivity Question Bank

Practise IB Physics SL B.2.2 by relating emissivity to emitted intensity, black-body spectra and planetary energy balance.

Exam points

Relate emissivity to emitted intensity and spectrum while comparing surfaces at the same temperature.
Use emissivity and Stefan–Boltzmann energy balance to estimate surface or atmospheric radiation.

[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 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}

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

All question bank results loaded

B.2.2—Emissivity question 1