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Emissivity


Emissivity13-08-2019 17:55
olyz
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(44)
Emissivity is the ratio of the radiant energy emitted by a surface to that of a black body at the same temperature.

So if the radiant energy of a body was all at a certain frequency, and the radiant energy of a block body at that temperature were the same, the emissivity of that body would be 1 although it was only radiating at a certain frequency.

I started a new thread to clarify emissivity in light of the confusion I see in other threads.
13-08-2019 18:58
IBdaMann
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(4328)
olyz wrote: Emissivity is the ratio of the radiant energy emitted by a surface to that of a black body at the same temperature.

Not exactly.

Radiance = Absolute_Temperature^4 * Emissivity * Boltzmann

Ergo, Emissivity is the following ratio:

Emissivity = Radiance / [ Absolute_Temperature^4 * Boltzmann ]

Emissivity is simply a coefficient representing thermal efficiency, applied to Radiance (not applied to the quantity of radiant energy), which is a percentage between zero and one.

Your basic idea is correct, you just used incorrect wording from a dishonest article promoting scientific illiteracy, e.g. it mixes "black bodies" and "grey bodies" interchangeably, it incorrectly refers to atmospheric particulates as "a body," it convolutes the topic by unnecessarily adding "albedo," it dishonestly claims to KNOW earth's albedo and emissivity, it absurdly claims that earth's radiance only comes from two places 1) the surface and 2) the top of the atmosphere, it uses the bogus invented term "planetary energy balance" and other stupid crap.

The dissertation on Stefan-Boltzmann is intended to confuse, not to inform, because the obvious objective to is to force the target audience to just accept the provided conclusion which is found in the opening sentences, that atmospheric gases are "warming" the earth's surface, i.e. Global Warming is REAL, that no definitions should be required but that pages of confused, extremely boring and pointless math should be sufficient for the target audience to simply tap out.

I read through the entire article and I was not fooled. Were you?

olyz wrote: So if the radiant energy of a body was all at a certain frequency, and the radiant energy of a block body at that temperature were the same, the emissivity of that body would be 1 although it was only radiating at a certain frequency.

This is a contradiction. A body cannot emit thermal energy per Planck's, per Kirchoff's, per Wein's and per Stefan-Boltzmann with emissivity = 1 and yet only emit at one wavelength.

.


Global Warming: The preferred religion of the scientifically illiterate.

Ceist - I couldn't agree with you more. But when money and religion are involved, and there are people who value them above all else, then the lies begin. - trafn

You are completely misunderstanding their use of the word "accumulation"! - Climate Scientist.

The Stefan-Boltzman equation doesn't come up with the correct temperature if greenhouse gases are not considered - Hank

:*sigh* Not the "raw data" crap. - Leafsdude

IB STILL hasn't explained what Planck's Law means. Just more hand waving that it applies to everything and more asserting that the greenhouse effect 'violates' it.- Ceist
13-08-2019 19:44
olyz
☆☆☆☆☆
(44)
IBdaMann
There are bodies other than black bodies.

Their emissivity is defined as:

"the ratio of the radiant energy (radiance) emitted by a surface to that emitted by a blackbody at the same temperature."
https://www.merriam-webster.com/dictionary/emissivity

If it is not a black body, it will not have a black body radiation spectrum ala Planck and Stefan-Boltzmann.

Two bodies at the same temperature can have the same radiance and completely different radiation spectrums. All that matters is the integration of the radiation spectrum over frequency, or wavelength.

WHOOPS You are right.

Just found: All-real bodies have an emissivity less than 1.
Edited on 13-08-2019 19:59
13-08-2019 20:20
IBdaMann
★★★★★
(4328)
olyz wrote:There are bodies other than black bodies.

Nope. All bodies in nature are black bodies ... when discussing thermal activity. You have been reading too much Global Warming propaganda.

olyz wrote: If it is not a black body, it will not have a black body radiation spectrum ala Planck and Stefan-Boltzmann.

Only someone who has no clue what he's talking about would tack "Stefan-Boltzmann" onto that list. Stefan-Boltzmann has no radiation spectrum. Stefan-Boltzmann calculates a RADIANCE value, ... a single, solitary value.

Helpful Hint: Just stick with Planck's graph of an ideal black body.

olyz wrote:Two bodies at the same temperature can have the same radiance and completely different radiation spectrums.

Can two bodies at the same temperature have the same radiance but with different emissivities?

olyz wrote: All that matters is the integration of the radiation spectrum over frequency, or wavelength.

Now you're blowing chunks big time. Stefan-Boltzmann is the integration of Planck's over all wavelengths ... ergo it is not a spectrum ... it is just a single calculated value.

olyz wrote: WHOOPS You are right.

It's no accident.

olyz wrote:Just found: All-real bodies have an emissivity less than 1.

... and greater than zero.

.


Global Warming: The preferred religion of the scientifically illiterate.

Ceist - I couldn't agree with you more. But when money and religion are involved, and there are people who value them above all else, then the lies begin. - trafn

You are completely misunderstanding their use of the word "accumulation"! - Climate Scientist.

The Stefan-Boltzman equation doesn't come up with the correct temperature if greenhouse gases are not considered - Hank

:*sigh* Not the "raw data" crap. - Leafsdude

IB STILL hasn't explained what Planck's Law means. Just more hand waving that it applies to everything and more asserting that the greenhouse effect 'violates' it.- Ceist




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