I About absorption

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Hello:

Do any one remember a law to calculate an area of a sphere ,some thing like the specific surface area but relative to absorption of radiation ?
 

BvU

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Like ##\pi r^2\ ## ?
 
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Like ##\pi r^2\ ## ?
Yes but with density elements
 

BvU

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In what context ? You seem to have a specific expression for absorption in mind .... what density, and of what ?
 

Vanadium 50

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Hagopbul, will you please write more than one sentence at a time? It is infuriating when it takes many days and may posts before you finally have written the entire question.
 
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I was wondering about radiation absorption in simple radiation pressure equation , a professor on YouTube used the (pi r^2 )
equation .
I start to ask myself can we have other area law , one that includes density ?
Just like specific surface area = 3/(roh*r)
 
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The sphere has the smallest surface area per volume. Maybe you already knew that. But it's not clear what you're asking.
 
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Presumably you already know that the area of a sphere is ##4πr^2##, and that its volume is ##4/3πr^3##.

That's the least surface area per volume for a closed bounded object in ##\mathbb R^3##.

Is surface area to volume ratio what you mean by density in your question about absorption (and not adsorption?) of radiation?
 

jbriggs444

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3/(roh*r)
##\rho## is spelled rho.

"Specific surface area" would be surface area per unit mass. The "specific surface area" of a sphere would be the area of the sphere (##4 \pi r^2##) divided by its mass (##\rho \frac{4}{3} \pi r^3##) yielding a result of ##\frac{3}{\rho r}##

Possibly you are chasing something like absorption per unit mass for spherical pellets of a given density and radius in a uniform omnidirectional light bath. Or, since radiation pressure has been mentioned, possibly we are talking about illumination from a single direction -- in which case we need to divide by four. Both interpretations ignore the problem of self-shading, so perhaps something else entirely is meant.

As has been suggested, we should not have to play guessing games to tease a question out of the questioner.
 
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According to wikipedeia:

Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass,[1] (with units of ##m^2/kg## or ##m^2/g##) or solid or bulk volume[2][3] (units of ##m^2/m^3## or ##m^−1##).​

I was wondering whether the volumetric meaning might have been intended.
 
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jbriggs444 said:
##\rho## is spelled rho. But what ##\rho##r is supposed to denote is anyone's guess.
My guess was that rho meant density and that r meant radius.
"Specific surface area" would be surface area per unit mass. The "specific surface area" of a sphere would be the area of the sphere ##(4 \pi r^2)## divided by its volume (##4/3 \pi r^3##) yielding a result of ##\frac{3}{\rho r}##
I notice that you also guessed that he might have meant per volume rather than per mass by SSA (of a sphere) ##=3/\rho*r##, given that, absent units -- ##gm^3## or ##m^3## -- it could refer to either.
As has been suggested, we should not have to play guessing games to tease a question out of the questioner.
I think I'll have to agree with the grown folks on that. :oldeyes:
 
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