maths/astronomy question

[KrazyHorse]

Originally posted by Ramo

It's the same thing. E_n isn't unitless - it's expressed in Joules, eV's, whatever. So you divide 5 J (for example) by 1 eV, and you get a unitless quantity.

A rather large unitless quantity, however...

[KrazyHorse]

Originally posted by Ben Kenobi
I don't know, someone help me on this.

Are the CGS units of nm and eV equivalent?

Why the **** would they be? One's a length and one's an energy. There are no unitless fundamental constants in CGS to get you from one to the other.

Even the "God-given" system makes energy = 1/distance, and that's as close as you'll get in any reasonable units system.

For everything non electrodynamical in nature the only difference between CGS and SI is unitless constants.

[KrazyHorse]

And ffs you don't need a table to go from eV to J

Energy (in Joules) = Energy (in eV) * 1.6 * 10^-19

That 1.6*10^-19 is the fundamental unit of charge expressed in coulombs ( known as e, about number 5 on the most important physical constants to remember)

1) c
2) hbar
3) G
4) k
5) e
6) permeability of free space
7) fine structure constant
8) Avogadro's number
9) stefan-boltzmann constant
10) R

[KrazyHorse]

Originally posted by Ben Kenobi
The top line is n = infinity, which is equivalent to the ionisation energy of the hydrogen atom, and the energy required to strip the electron completely away from the atom.

The bottom line is n= 0, which is the rest state of the hydrogen electron

No it isn't. The spectrum of n, the primary quantum number, runs from n = 1 to n = infinity. The ground state of hydrogen is the n=1, l=m=0 state.

[Ben Kenobi]

That 1.6*10^-19 is the fundamental unit of charge expressed in coulombs ( known as e, about number 5 on the most important physical constants to remember)

You might remember that number, but I don't. That's why we have tables in the first place, so we can look these things up.

Why the **** would they be? One's a length and one's an energy. There are no unitless fundamental constants in CGS to get you from one to the other.

That's not what I was asking. I just needed to know whether Joules is to metres as nm are to eV. I don't use cgs, but many astronomers do because it makes for simpler calculations.

[KrazyHorse]

No, many astronomers use it because they're dense mother****ers.

[KrazyHorse]

It doesn't actually make for simpler calculations, Ben. There's no reason it would unless you were using the Gaussian brand of CGS and spending a lot of time doing classical electrodynamics. Astronomers are just dumb and refuse to conform.

[Ben Kenobi]

Ah, I was thinking of ergs - Joules.

I don't disagree with you, KH.

[Ben Kenobi]

Well, then why have a unit like eV for energy? Why not use Joules?

Is it because the partical physicists are dense, or is it because it makes no sense to use Joule for the energies they deal with?

[KrazyHorse]

Hell, I'll agree to use parsecs if they would just bite the bullet and go to SI like they should.

[Ben Kenobi]

Parsecs are hella easier than metres.

All the astronomers I know love to make you frusterated with the units, just because.

[KrazyHorse]

Originally posted by Ben Kenobi
Well, then why have a unit like eV for energy? Why not use Joules?

Is it because the partical physicists are dense, or is it because it makes no sense to use Joule for the energies they deal with?

a) I hate using eV too, but at least that number is obvious from e

b) Ben, do you mind explaining to me what happens on the scale of ergs that astronomers are interested in? Depending on what they're doing they need stuff on the scale of eV (12 orders of magnitude off), GeV (4 orders of magnitude off) or stuff on the scale of billions of joules (many many many orders of magnitude off)

[KrazyHorse]

Originally posted by Ben Kenobi
Parsecs are hella easier than metres.

I meant for astronomical scales. At least that unit makes sense.

[Ben Kenobi]

b) Ben, do you mind explaining to me what happens on the scale of ergs that astronomers are interested in? Depending on what they're doing they need stuff on the scale of eV (12 orders of magnitude off), GeV (4 orders of magnitude off) or stuff on the scale of billions of joules (many many many orders of magnitude off)

It's all about units of luminosity, as far as I can tell.

[Urban Ranger]

Originally posted by Ben Kenobi
Well, then why have a unit like eV for energy?

Because it's easier to use in some instances - like this one.