# faq5 - FREQUENTLY ASKED QUESTIONS February 6 2012 Content...

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FREQUENTLY ASKED QUESTIONS February 6, 2012 Content Questions What is σ ? What is the difference between σ and S ? The quantity σ is known as the “cross-section”: it’s a measure of the proba- bility that a photon will be absorbed by an atom. (The cross-section quantity is actually used more generally for any kind of interaction, but here we’re considering the specific case of absorption of photons.) It can be considered a kind of “effective area”, in analogy with a physical object with a par- ticular cross-sectional area: the bigger the area, the more likely it is that incident particles will be blocked; similarly, the higher the cross-section, the more likely it is that an incident photon will be absorbed by an atom. The cross-section has dimensions of area (m 2 in SI units). S is just the area transverse to the photon direction of the little volume element we considered in the derivation of the equation of radiative transfer. In the first slide with the velocity distribution, was that for a given volume? The Maxwell-Boltzmann distribution is a function of temperature, so it will describe the distribution of velocities for any volume at a given temperature, so long as the volume is in thermodynamic equilibrium at a given T . The y - axis of that plot shows the number of molecules as a function of velocity, such

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