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Old Jul 21st 2019, 07:15 PM   #11
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Neila9876,
Are you in China.
If yes do you have wechat
My ID there is ****
Maybe we can chat there

(Administrative edit removing ID)

Last edited by topsquark; Jul 22nd 2019 at 11:46 AM. Reason: Administrative edit removing ID
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Old Jul 21st 2019, 07:20 PM   #12
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Neila9876,
Wrong ID
It's ****

(Administrative edit removing ID)

Last edited by topsquark; Jul 22nd 2019 at 11:46 AM.
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Old Jul 22nd 2019, 03:23 AM   #13
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Please don't post contact details of any kind on the forum for anyone to see... it's the first unwritten rule of the internet!

We have a private messaging service if you really want to exchange contact details with someone... but remember... this is the internet.
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Old Jul 22nd 2019, 05:42 AM   #14
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It is easy to get into the thought pattern that you are just chatting to the two or three individuals that are actually posting on the thread.
But just look at the views column of the home page.
Even this fairly simple "hello" thread has had over a hundred viewers.

You can never be sure that all these viewers will be benign,
thus we tend to avoid including too much of a personal nature in our posts.

Note that the "Lounge" only has access to members,
while the other areas are free to view for anyone.

But even in the lounge don't leave yourself entirely open.

If (once you have got to know someone reasonably via their open posts)
you want to share more personal information, use the private messages feature on the forum.
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Old Jul 22nd 2019, 06:30 AM   #15
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Talking neila vs alien

There is a "Beer bar of cosmos" in the Lounge column, the manager of the bar is a Woodpecker from England, the boss is a dragon from USA...haha.
This semi - tramp in cosmos often goes to the bar for beer and chat with that dragon, Woody, benit, Oz, etc,...
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Introducing myself...Philippe from China-neila9876.jpg  

Last edited by neila9876; Jul 22nd 2019 at 05:18 PM.
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Old Jul 23rd 2019, 04:41 AM   #16
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Philippe messaged me privately and asked some questions about the nature of radiation, so I'm replying here.

"What is the emitted flux density (W/m2) and emitted energy (J/m3)"

Short answer: Energy density (J/m3) is a measure of energy content per unit volume and is used to typically describe fields and their energy content, and radiation flux (W/m2) is a measure of the amount of radiation emitted or absorbed, per unit area, on a surface. This is important for relating measurements with theory and is used routinely in solar modelling. For example, it characterises how much radiation is incident on a tilted surface (like a wall or PV panel) or how much energy is absorbed from a particular emitter, like a light bulb or a laser.

Long answer:

Light can be treated as either a particle (photons) or as waves (light waves). Both are valid. Personally, I find it a lot easier to understand basic concepts about light in terms of photons, so I'll go with that, but if you find it confusing, let me know.

Basically, most emission of light occurs due to nuclear or quantum phenomena (such as nuclear fusion, electron de-excitation and pair annihilation). There's other sources too, like Bremsstrahlung, but these are the common light sources for the light we see on Earth.

Photons have an energy content equal to:

$\displaystyle E = hf = \frac{hc}{\lambda}$

where $\displaystyle h$ is Planck's constant, $\displaystyle f$ is the frequency, $\displaystyle c$ is the speed of light in a vacuum and $\displaystyle \lambda$ is the wavelength.

Now consider an isotropic light emitter that emits at a constant rate. It is common to ascribe a power to any light emitter, which is the rate of energy release from the emitter:

$\displaystyle P = \frac{dE}{dt}$

Now consider a light-sensitive sensor that is able to absorb photons. Because the light is emitted evenly over all directions, the only way to capture all of the power is to manufacture a sensor that completely envelopes the emitter (the name for a hypothetical sensor that would absorb all of the radiation of a star, like the sun, is called a Dyson sphere). These kinds of sensor rarely exist (and certainly Dyson spheres don't exist!). Most collectors on Earth are flat-plate collectors with a finite area, A. Because of this, it means that the available radiation that can be captured is a small fraction of the total power of the emitter.

In order to try and calculate how much radiation is captured, there are ways of characterising how much radiation is available.

One measure is the energy density (in units of J/m3) or power intensity (W/m3). This is measure of how much radiation is available in a given volume. For example, the energy density close to an isotropic emitter is much greater than the energy density far from it. However, in my experience, I rarely see energy density and power density being used to describe light emitters and their radiation and instead see them used to describe electromagnetic fields instead.

Another measure is the irradiance, which is the incident power per unit area (W/m2). It is a power per unit surface area. This is a common measure of the amount of radiation you can expect at a collector surface. For an isotropic emitter, the incident irradiance can be determined by considering a sphere with a radius equal to the distance between the isotropic emitter and the receiving surface:

$\displaystyle I = \frac{P}{A_s} = \frac{P}{4 \pi r^2}$

This is useful to determine, for example, the solar irradiance on Earth.

I hope this helps!
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Old Jul 23rd 2019, 06:15 AM   #17
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Thanks Benit13,

I will read again your reply later again.
Browsing the web, looking for radiometry it's so many definitions, so many synonyms, so many different symbols that I'm a bit confused but it will come.
I've supposed that can be applied to blackbodies

I've till now understood the law of Rayleigh - jeans and Wiens displacement law.
After having understood the definitions, units, tomorrow I will start the Planck's law.
I don't know if for this law I've to open a new thread or if I continue on this one... A reply will be welcomed to avoid me to create a mess on this forum...
Thanks a lot and best regards.

Philippe
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Old Jul 23rd 2019, 09:00 AM   #18
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Rambling Threads

It is generally considered a good idea to keep threads short and to the point.

It should be remembered that a number of the viewers will be individuals searching for possible solutions to their problems, that might have been posted previously.
If the threads are nicely organised such searches are greatly facilitated.

However, a brief look through some of the historical threads will quickly show that we don't always follow that guide line particularly closely.

As a new member you will be offered advice rather than criticism if (when) you get it "wrong".
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Old Aug 1st 2019, 11:31 PM   #19
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Cool

我老猪在此敬告他们江门地方黑恶势力钟永康集团/九一四牛顿定律不要再给国家在世界抹黑。
The above Chinese phrase is not for anyone here. It's for the local dark force.
@anyone who is curious in physics/cosmos worldwide:
I am always worrying that I will be isolated from the outside world at any time by the local dark force. And I might suddently die at any time for unknown reason/health problem.
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Introducing myself...Philippe from China-timg1119876.jpg  

Last edited by neila9876; Aug 2nd 2019 at 08:23 PM.
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