Conversion from barn unit to square meter?

Question

I read Wiki:

http://en.wikipedia.org/wiki/Barn_(unit)

on this and calculations under the 'Conversion from squared prefixed SI units' do not look like correct. Is 1fm^2 = 10 mb ? (or 1b = 100m^2 ) . Which one is correct and how do you calculate it? Thanks.

what do you not understand in the table "commonly used prefix versions" ? All are refered to cm^2. http://en.wikipedia.org/wiki/Barn_%28unit%29 . Your link has a missing parenthesis and ends up in a "new page". — anna v, Jul 10 '12 at 13:08
@anna: the link is correct. There is no reference to cm^2 there. — Dukunocil, Jul 10 '12 at 16:19
look at the table in the paragraph "commonly used prefix versions". that is where the units are, on the right . — anna v, Jul 10 '12 at 19:15
Your question says "1b = 100m^2", which is not quite the same thing as "100 fm^2". — Ignacio Vazquez-Abrams, Jul 10 '12 at 23:57

score 2 · Answer 1 · answered Jul 10 '12 at 13:06

2

A barn is $10^{-28}$m$^2$, so it's the area of a square $10^{-14}$m by $10^{-14}$m.

One femtometer is $10^{-15}$m, so the square is 10fm by 10fm, i.e. it's 100fm$^2$.

Conversely, one square femtometer is the area of a square $10^{-15}$m by $10^{-15}$m. If rb is a square root barn ($10^{-14}$m) then the square is 0.1rb by 0.1rb so it's 0.01 barns.

answered Jul 10 '12 at 13:06

John Rennie

355,118

I still do not get it :( – Dukunocil Jul 10 '12 at 16:51
I think I got it :) First part is obvious. Second part: 10fm X 10fm = 100 fm^2 OK. So what is it expressed in barns? I calculate: 100 fm^2 = 10^2 x (10^-15)^2 = 10^2 x 10^-30 = 10^-28 = barn. They should use brackets i.e. 100 [fm]^2 – Dukunocil Jul 10 '12 at 17:02
1

You wouldn't normally use brackets. After all it's very common to talk of a square centimeter and denote it by $cm^2$, or a square kilometer, $km^2$. You wouldn't write these as $[cm]^2$ or $[km]^2$. – John Rennie Jul 10 '12 at 17:13
..then if 100[fm]^2 = 1 barn, therefore 1[fm]^2 = 10^-2 barn = 10 X 10^-3 barn = 10 mb :) – Dukunocil Jul 10 '12 at 17:13
I agree, but not 100%. Units should be in brackets to avoid confusion like this one, particularly when dealing with 'exotic' types of units :) Anyway, thanks for allowing me to see this. – Dukunocil Jul 10 '12 at 17:16
There is a worldwide convention not to write units in brackets, so I really don't understand your reasoning for saying that units should be in brackets. Honestly, I'm still not clear on why you were confused in the first place... but if it makes sense to you now, great! That's all that really matters. – David Z Jul 11 '12 at 01:31

score 0 · Answer 2 · answered Jul 11 '12 at 00:05

An area with a side of 1u has an area of 1 u^2.

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An area with a side of 2u has an area of 4 u^2.

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An area with a side of 10u has an area of 100 u^2.

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An area with a side of 10fm has an area of 100 fm^2 and is called a "barn".

Conversion from barn unit to square meter?

2 Answers2