#### hagopbul

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

Homework Helper
Like $\pi r^2\$ ?

#### hagopbul

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

#### BvU

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

Staff Emeritus
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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#### hagopbul

equation .
I start to ask myself can we have other area law , one that includes density ?
Just like specific surface area = 3/(roh*r)

#### sysprog

The sphere has the smallest surface area per volume. Maybe you already knew that. But it's not clear what you're asking.

#### BvU

Homework Helper
Just like specific surface area = 3/(roh*r)
Never heard of it
In spite of my answer in #2, not me. But then: WHO ?
one that includes density

#### sysprog

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 $\mathbb R^3$.

#### jbriggs444

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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#### sysprog

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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#### sysprog

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.

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