User talk:Bobblewik/units of energy

Hi there - I see that you have added SI units of energy to X-ray astronomy. I am not aware that any astronomers use attojoules rather than electron volts as a unit of energy, and I'm not convinced that the layperson would find the aJ a particularly useful unit either. Putting the equivalent wavelength (nanometres or picometres) instead may be more useful. -- ALoan (Talk) 12:16, 3 Sep 2004 (UTC)

I had heard of the electronvolt but thought it was potential difference rather than energy. Typing '10 electron volts' into google and pressing 'Search' converted it into joules. Thus it revealed that the author was referring to energy. The joule is a very familiar unit of energy to me. I have to admit that I only knew to use the prefix atto because I looked it up, so you are right about the familiarity (of the prefix at least).

If you think that wavelength is suitable for describing X rays, then presumably you also think that frequency is suitable. I am under the impression that frequency is a fashionable unit for electromagnetic radiation in some places. Tips on how to convert keV to frequency would be welcome because I have no idea how to do it. Thanks for the feedback. Bobblewik  (talk) 14:40, 3 Sep 2004 (UTC)

Thanks for the pointer on my talk page (I was watching here too). My comment was really to point out that an astronomer would use eV (or, rarely, nm/pm) not aJ (or frequency). To convert, you need to use $E=h\nu$ and $c=\lambda \nu$, so $E=hc/\lambda$.

Thus, a photon with energy of 1 eV would have a wavelength of around $1.2\times 10^{-6}$ m (1 µm) and a frequency of $2.4\times 10^{14}$ Hz. (I hope my maths and calculator are working today!)

Wavelength is used instead of frequency because it involves fewer powers of 10: a photon of 1 keV would be around $10^{17}$ Hz but $10^{-9}$ m (1 nm) ).

Hope this helps! -- ALoan (Talk) 15:20, 3 Sep 2004 (UTC)

Yes it does help. Thank you. I took a look at the electromagnetic spectrum and that was useful too. I can see now that Hz and J both need prefixes outside the familiar range. As you say, wavelength uses fewer powers of 10 and the prefixes are more familiar. I will have a pause to think about this.

In the meantime, I was going to have a look at the other instances of energy quoted in electronvolts on Wikipedia. You will see that I added 'fJ' (unfamiliar prefix, but the familiar energy unit) to Galileo probe. The application did not appear to be electromagnetic. Perhaps I should either forget it, or write 3.2 x 10^-15 J instead of 3.2 fJ. Your thoughts are welcome. Bobblewik  (talk) 15:45, 3 Sep 2004 (UTC)

I suspect that $3.2\times 10^{-15}$ J is likely to be understood by more people than 3.2 fJ, notwithstanding that the femtojoule would be the SI standard.

(I much prefer to express in <math> than <sup> because TeX-style mark-up is much easier to deal with; I also use &nbsp; between numbers and units to prevent unfortunate linebreaks, but perhaps it is just me.)

For astronomy, the units depend on the context: radio astronomers will use wavelengths as it is handy to talk about cm or m, but they also use frequencies because they are in MHz and kHz. Once you get above GHz, frequencies start to look a bit odd, but wavelengths are still OK down past nm. Particle physicists also use eV (or, rather, eV/c²) as then they can use $E=mc^{2}$ and use natural units where mass and energy are measured in the same equivalent.

(Incidentally, clicking on the section edit button above takes me to the previous section - I don't know why. Very odd. I've also added some extra links to my comments above for the sake of completeness.) -- ALoan (Talk) 16:21, 3 Sep 2004 (UTC)

The preference for 'no break space' is not just you. Somebody else mentioned it to me (they also added to the manual of style - so your preference is somewhat official here). I don't use it, but only because of the additional effort required. I agree that it is a 'good thing'.

I have no idea why the section edit is not behaving as you expect. I don't have section edit buttons so I can't test it.

Just noticed that I *do* have edit buttons. They also take me to the previous section. Strange...

I did not know that there was a <math> option. I will investigate it. Thanks. I only recently found out about the <sup> option. I am doing quite a bit of accidental learning here! I appreciate the explanation of context. To paraphrase the words of the intelligent bomb in the excellent film Dark Star, "I need to think about this some more". Bobblewik  (talk) 16:39, 3 Sep 2004 (UTC)

Back to the left margin. I may have fixed the edit buttons - the header bird articles was missing some "=="s.

I wasn't aware that the Manual of Style preferred non-breaking space: good! You can always copy and paste an "&nbsp;" to save typing. :)

I find out about these things all the time. It took me ages to find out about the definition format ("; xxx : yyy") and piped linkes ("[[like this (blah)|]]") that expand out automatically. ALoan (Talk) 17:06, 3 Sep 2004 (UTC)