Go Back   Physics Help Forum > Physics Forums > Physics

Physics Physics Forum - General Physics Discussion and Physics News

Like Tree2Likes
Reply
 
LinkBack Thread Tools Display Modes
Old Jul 23rd 2019, 08:39 PM   #11
Junior Member
 
Join Date: Jul 2019
Posts: 26
Dear All,

To close the subject of definitions, symbols and units.
Are the radiometric quantities as described in
https://light-measurement.com/basic-...ic-quantities/
Are they valid and applicable to the radiations emitted by a Black body
Thanks in advance and regards

Philippe
philippe is offline   Reply With Quote
Old Jul 23rd 2019, 10:35 PM   #12
Junior Member
 
Join Date: Jul 2019
Posts: 26
Dear All,

To be clear, from I read about radiometry
They are as quantities, units and symbols:
- Radiant energy E (J)
- Radiant flux phi (W)
- Radiant emittance M (Wm*-2)
- Radiant irradiance
- Radiant intensity I (Wsr*-1)
- Radiance L (Wsr*-1m*-2)

My intention is to use that for Blackbodies but replacing Radiant by Emitted and delete irradiance, as far a black body don't reflect any radiation. This will be valid for the overall spectral emission of a black body

I'm right or wrong?

Regards

Philippe
philippe is offline   Reply With Quote
Old Jul 23rd 2019, 11:05 PM   #13
Junior Member
 
Join Date: Jul 2019
Posts: 26
Dear All,

Really sorry to bother you.

Related to my previous message I forget to tell
My intention as well is to replace the term
Radiant emittance by Emitted flux density.

All radiometric quantities symbols have the subscript e
In my case for black body spectral emissions (ex M subscript e) may I use the same?

Sorry to bother you so much

Regards

Philippe
philippe is offline   Reply With Quote
Old Jul 24th 2019, 02:22 AM   #14
Senior Member
 
Join Date: Oct 2017
Location: Glasgow
Posts: 426
First: there's no need to apologise for bothering anybody. It's a forum, so people will reply if they want to. Like most internet forums, you won't be guaranteed a reply.

Second: You've said you want to replace "Radiant emittance by Emitted flux density". What is the purpose of this? Like I said in my previous post, emitters have an emitting power (W). The photons emitted are spread out over a volume as a function of time. You can consider a volume in space that has radiation passing through it and try to derive an expression equal to the average energy content per unit volume (J/m3) at some point in time, but I don't see why that's useful to anyone. Usually people care about radiation absorbed on some kind of absorbing surface.

What is the original physics problem you are trying to solve?

Last edited by benit13; Jul 24th 2019 at 02:28 AM.
benit13 is offline   Reply With Quote
Old Jul 24th 2019, 04:05 AM   #15
Junior Member
 
Join Date: Jul 2019
Posts: 26
Dear Benit13 and All,

The original physics I try to solve is the black body radiation emission. Why?

Because since few weeks ago I'm interested and self learning about stars evolution.

Stars evolution, are defined by the Hertzsprung-Russel diagram giving Them of stars vs Luminosity or stars spectral type vs absolute magnitude.
As stars evolution are charecterisized by change of temperature and luminosity that's mean color change.

As I read that a star can be considered as a black body.
I looking about that trying to understand laws of Rayleigh-Jeans, Wien, Stefan-Boltzman and Planck.

So started by Rayleigh-Jeans law, reason why I joined this Forum, the first and only one.

Because they are different way to express these laws

And because from articles I can read on the net the same parameter have several different symbols and units.

To reply to your question why from what I read, I don't fine anything specific related to black body so I looked through Radiometry and found what I posted and that you gently replied.
Radiant, as I explained in my previous message is replaced by Emitted
Black body absorbing all radiations without reflecting or transmitting any electromagnetic energy but just emitting a electromagnetic radiation due to his thermal equilibrium reach theoretically from X Ray to radio wave.
You have high energy gamma ray burst out sun is seen yellow (spectral type G2V).
More hot s star is blue she idy, more colder she is more red she is

Here is the full story of my questions posted.

Best regards
philippe is offline   Reply With Quote
Old Jul 24th 2019, 04:18 AM   #16
Junior Member
 
Join Date: Jul 2019
Posts: 26
Wonder why my messages are posted twice.
I'm using my cell phone
philippe is offline   Reply With Quote
Old Jul 24th 2019, 10:12 AM   #17
Senior Member
 
Join Date: Oct 2017
Location: Glasgow
Posts: 426
Originally Posted by philippe View Post
Dear Benit13 and All,

The original physics I try to solve is the black body radiation emission. Why?

Because since few weeks ago I'm interested and self learning about stars evolution.

Stars evolution, are defined by the Hertzsprung-Russel diagram giving Them of stars vs Luminosity or stars spectral type vs absolute magnitude.
As stars evolution are charecterisized by change of temperature and luminosity that's mean color change.
Awesome! I have a PhD in Stellar Evolution (specifically massive star evolution), so I can definitely help you out

As I read that a star can be considered as a black body.
I looking about that trying to understand laws of Rayleigh-Jeans, Wien, Stefan-Boltzman and Planck.

So started by Rayleigh-Jeans law, reason why I joined this Forum, the first and only one.
Perfect. A good place to start.

Because they are different way to express these laws
They are not different ways to express the same law, they are different laws.

The only *similar* laws are Planck's law and the Rayleigh-Jeans law. Planck's law is a corrected version of the Rayleigh-Jeans law, which doesn't work for very high frequencies (see the ultraviolet catastrophe). Planck's law is the one you should care more about because it's used everywhere.

The Stefan-Boltzmann law is absolutely crucial to relate temperature with emitted radiation. It's vital in physics. You'll probably want to remember this one from memory. It's everywhere. Hot things radiate and things that absorb radiation get hot. If anything like that appears, the Stefan-Boltzmann law appears.

Wien's law tells you the position of the peak of Planck's blackbody curve for a given temperature and is a very, very nice law to have in the tool-belt and very important in astronomy. I didn't use it much when looking at theory though.

And because from articles I can read on the net the same parameter have several different symbols and units.
I don't think they are the same parameters... just different names for different things.

Personally, I wouldn't bother doing a survey of all of the different parameters. I'd recommend instead that you focus on the laws/equations themselves to get a feel for the processes they describe. That's the good stuff that's actually interesting and you'll make much better progress.

Whichever route you take, we'll be probably be on hand to help out.

To reply to your question why from what I read, I don't fine anything specific related to black body so I looked through Radiometry and found what I posted and that you gently replied.
Radiant, as I explained in my previous message is replaced by Emitted
Black body absorbing all radiations without reflecting or transmitting any electromagnetic energy but just emitting a electromagnetic radiation due to his thermal equilibrium reach theoretically from X Ray to radio wave.
You have high energy gamma ray burst out sun is seen yellow (spectral type G2V).
More hot s star is blue she idy, more colder she is more red she is

Here is the full story of my questions posted.

Best regards
There's a lot of theory that cares about the transmission/propagation, reflection, absorption, re-emission and scattering of radiation. All of those different processes have their own theories, laws, parameters and measurements, most of which are agnostic to the type of emitter.

This is why I recommend focussing on the processes themselves and the laws that describe them... it will help stop you getting too bogged down.

Gamma-ray bursts are something I don't know too much about. However, I do know that they come from extreme events related to stars (e.g. core collapse supernovae or neutron star mergers). They are certainly not blackbody radiation from stars.

Massive stars (O and B types) tend to emit in the bluer part of the HR diagram, but can be quite varied in their colour. Super-massive stars emit over many different wavelengths in the visual range (red to blue) because they are typically undergoing rapid phases of core and shell burning, which means that the surface temperature is widely dependent on which phase they are in (typically shell helium burning or core helium or carbon burning). X-rays tend to come from more extreme environments, like black holes and neutron stars, but you'll get the tail end of the blackbody curve from massive stars for sure.

Also... something to watch out for... the term "giant stars" refers to large stars (in terms of size)... these can be main sequence stars that are undergoing shell hydrogen burning. They are very different to "massive stars", which have a lot of mass. This distinction catches a lot of people out. Initial mass for a star is *the* key metric for a star's evolution (along with it's initial metallicity). Size is unimportant in stellar evolution; it's all about mass and metallicity.
topsquark likes this.

Last edited by benit13; Jul 24th 2019 at 10:17 AM.
benit13 is offline   Reply With Quote
Old Jul 24th 2019, 07:43 PM   #18
Junior Member
 
Join Date: Jul 2019
Posts: 26
Dear All,

After having the Rayleigh-Jeans law and the UV catastrophe.
I'm now moving to Planck's law.

My intention is to open a new thread. Most of probably it will be called "Planck law"

The is to is:
- to understand this law
- integrate it on the overall spectrum to find back the Stephan-Boltzman law
- equal it's derivative to 0 to retrieve the Wien displacement law
- to search for some star's spectrum and to apply these laws

Best regards

Philippe
philippe is offline   Reply With Quote
Reply

  Physics Help Forum > Physics Forums > Physics

Tags
law, rayleighjeans



Thread Tools
Display Modes



Facebook Twitter Google+ RSS Feed