Radiation Fundamentals 7

[curiousmechanical]

Hello Everyone,

Recently, I was trying to explain the fundamentals of thermal radiation to a non-engineer and I discovered that I myself have a poor understanding of it. I understand that "thermal radiation is continuously emitted by all matter whose temperature is above absolute temperature." However, I do not understand who or what is causing these electromagnetic waves to be emitted. According to my textbook [HEAT TRANSFER: A Practical Approach, 2nd ed.], "accelerated charges or changing electric currents give rises to electric and magnetic fields. These rapidly moving fields are called electromagnetic waves or electromagnetic radiation, and they represent the energy emitted by matter as a result of the changes in the electronic configurations of atoms or molecules." In layman's terms, what does this really mean? When atoms heat up, they bounce around faster and faster and produce more and more electromagnetic waves? Why does bouncing around create electromagnetic waves anyway?

I suppose that I was too busy trying to solve partial differential equations to think of these kinds of questions in college. There was no time to think or ponder. This seems to be the trend as I try to go back and relearn subjects. I gave all the right answers on the tests, but I think I missed some of the more important fundamentals. Those were not on the tests.

Thank you all in advance!

 

[Flareman]

curiousmechanical
You're not alone. This is that aspect of physics where wave form theory concides with particle theory. If we had such a good handle on this to explain it properly in layman's terms we'd be on a forum for quantum physicists not engineers.
Try thinking of radiant heat just as energy getting thrown out of the molecule as the atoms bounce around more and more rapidly. That energy hits something and that something warms up as a result. Just as it would if you hit it continuously with a hammer instead.
The energy loss at the source could be analogous to the noise (energy) you would hear if the atoms were all brass balls.
Hey! You said layman's terms.

David

 

[Compositepro]

There is no "one right answer". There are various models or theories that work. You can use waves or photons or probably quantum electrodynamics. Radiant heat transfer principles are a topic of "classical" Physics where wave theory works to explain things. Quantum mechanics uses photons and is more useful for some things. Other than that I can't really answer you question very well. I would say that oscillating charges do radiate, and that atoms contain oscillating charges called electrons and protons.

 

[25362]

Previous answers are correct. Microscopically, thermal radiation is associated with thermal motion of atoms and molecules, so it is not surprising that it increases with temperature.

In 1900 the German physicist Max Planck described the thermal radiation arising from the vibration of molecules, and established that energy emitted by a vibrating molecule is quantized, meaning it can have only certain discrete values. His revolutionary work won Planck the 1918 Nobel Prize.

 

[racookpe1978]

A second part of the adiation problem is the relative magnitude of the radiation problem - for EACH different heat transfer problem addressed. Many,many times in heat trabsfer, their simply is not enough radiation effects to matter because surface-to-surface conductive transfer simply overwhelms the radiative heat transfer.

A third part of the radiation problem is the (usually) two ABSOLUTE temperatures involved. NOT the two RELATIVE temperatures involved between the two materials.

A fourth part is is the surface textures and colors and physical surface area involved between the (two) heat exchanging surfaces. NOT the materials of the surface themselves, nor their amount of "touching" surfaces as is the case for "normal" heat transfer.

So, to a non-radiation transfer person, even to most engineers!, explain it as follows: EVERY surface at ANY temperature emits infrared radiation - what we "feel" WITHOUT touching a surface as the "heat" coming from that surface (the stove top burner, the fire, the refrigerator shelf, the ice cube tray) of that material to our hand.

Our hand, in turn, is ALSO radiating energy from our hand to that surface (the stove, the fire, the oven door, the refrigerator shelf, the ice cube tray). How much radiation is emitted is proportinal to the type of surface (reflective and smooth and shiny, rough and black and textured like a sandpaper-covered basketball) and the absolute temperature of the surface.

Absolute temperature is calculated from true interstellaar space ZERO.ZERO So, a black basketball at room temperature is at 25 degrees Celcius, and is at 273 + 25 degrees = 298 degrees Kelvin. It will radiate according to the fourth power of the temperature in degrees K. So a stove at 212 Farenheit, 100 degrees C, is at 373 degrees Kelvin. Far hotter than 298 degrees K, and even more when 298^4 and 373^4 are compared.

Like light waves, emitted when the stove starts to glow orange then red hot if nothing is on the burner - snd like light waves emitted from a light bulb when it is heated by electricity - every object when heated emits more infrared radiation and more heat by radiation as it gets hotter. But heat transfer by touching (conduction) and convection (movement of cold liquids) is more efficient than by radiation - put a cold pot with cold water in it on the oven burner, and the pot cools the burner much, much more quickly. With no pot on the burner, the burner just kept getting hotter until the rate of radiation can equal the rate of radiation. (Which may not happen until the burner melts!)

But, the air between you and the stove is an insulator, so only radiation heat can be transferred if there is no pot.

In a vacuum or thremos bottle with hot coffee inside, only the radiation from inside can transfer heat energy out: So, the vaccum helps insulate the suspended (not touching!) silvery bottle. Little radiation because of the silver surfaces? Little heat transfer = Your coffee stays hot longer. Put a cold drink in the same thermos, the inside gets colder than the outside. the radiation of energy is by the same infrared energy photons, but this time many more photons go from outside surface to the inside surface, and the previously cold drink gets slightly hotter.

the IR transfer is by photons (little wavelets of energy packets - as technically described above in terms of "real" physics) going from one surface to another. In space, the "background" of "nothing" is also available to radiate energy into from a surface or mass like the earth itself or a satellite or manned vehicle. The gold foil wrapped around the lunar lander was to reflect infrared energy to help maintain the lander inside from getting too hot in the direct sunlight, and too cold in the shadows of the moon when the sun was hidden.

 

[vpl]

racookpe1978

Star for you! That's a really good explanation.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[IRstuff]

The OP's question relates to how the process actually works. This would be mostly explained in

http://en.wikipedia.org/wiki/Planck's_law,

which hints at the actual process, which, based on Planck's derivation of the blackbody equation, must be photonic in nature.

So, we know that blackbodies generate photons in the spectral distribution described by the blackbody equation. We know that energy can be accumulated in both the atomic bonds and electron energy levels. We know that excited bonds or electrons eventually "decay" to a lower energy level by giving up photons, or gain energy by absorbing photons. And, in most cases, there is a finite lifetime for the excited state, before it decays to a lower energy.

Thus, for a blackbody's surface atoms, which are at the boundary between something hot and something cold, energy must flow through these atoms from the hotter interior to the colder exterior ambient. Some of the energy is conducted away through momentary contact with air molecules, which is the basis for convection cooling. Another portion is, because of the finite lifetime of the excited states, releasing photons everywhich way. The ones that head outward contain more energy than the ones emitted toward the blackbody from the cooler ambient, so there is a net deficit, and the surface is cooled.

Since the interior of the blackody is hotter, there is a net surplus coming from the interior that attempts to raise the temperature of the surface. If the energy outflow at the surface were zero, as in the case of a perfect blackbody cavity, the surface would be able to sustain the temperature of the interior. If the ambient is cooler, then there would be a net loss of energy, which accounts for the temperature gradient from the interior to the exterior surface.

TTFN

FAQ731-376

 

[racookpe1978]

Thank you Miss L.

(But knock me firmly upside the hardhat next time I write without a more thorough spell check ...)

 

[25362]

Speaking of the fundamentals of thermal radiation I took the liberty of reproducing the following excerpts taken from the internet:

Thermal emission, which depends only on the temperature of the emitting object, includes blackbody radiation, free-free emission in an ionized gas, and spectral line emission.

Thermal emission is perhaps the most basic form of emission for EM radiation. Any object or particle that has a temperature above absolute zero emits thermal radiation.
The temperature of the object causes the atoms and molecules within the object to move around.
When molecules bump into each other, they change direction.

A change in direction is equivalent to acceleration. When charged particles accelerate, they emit electromagnetic radiation. So each time a molecule changes direction, it emits radiation across the spectrum, just not equally. As a result, the amount of motion within an object is directly related to its temperature.

Scientists call this blackbody radiation. A blackbody is a hypothetical object that completely absorbs all of the radiation that hits it, and reflects nothing. The object reaches an equilibrium temperature and re-radiates energy in a characteristic pattern (or spectrum).

The spectrum peaks at a wavelength that depends only on the object's temperature.

Objects that are cooler than about 1000 K emit more infrared than visible light, such as the Earth or brown dwarfs (dim, cool objects too massive to be planets but not massive enough to be stars). Hotter objects, like stars, emit mostly optical light.

Very hot objects emit mostly ultraviolet radiation, such as white dwarfs (dying stars that have burned up all of the hydrogen in their cores). The major difference in the type of energy emitted by these objects is their temperature.

The Sun and other stars are, for all intents and purposes, considered blackbody radiators. By looking at the frequency or "color" of the radiation they emit, scientists can learn about the temperature of these bodies. For example, cooler stars appear red and hotter stars appear bluish-white.

"Free-free" emission or "bremsstrahlung" is another form of thermal emission comes from gas which has been ionized. Atoms in the gas become ionized when their electrons become stripped or dislodged. This results in charged particles moving around in an ionized gas or "plasma", which is a fourth state of matter, after solid, liquid, and gas. As this happens, the electrons are accelerated by the charged particles, and the gas cloud emits radiation continuously. ".

Spectral line emission involves the transition of electrons in atoms from a higher energy level to lower energy level. When this happens, a photon is emitted with the same energy as the energy difference between the two levels. The emission of this photon at a certain discrete energy shows up as a discrete "line" or wavelength in the electromagnetic spectrum.

 

[curiousmechanical]

To All,

Outstanding! Thank you all for your excellent feedback. I would especially like to thank those of you who took the time to leave such extremely detailed explanations (stars all around). I greatly appreciate you taking the time to respond to my post.

If anyone wishes to continue this discussion I do have a follow-up question.

I noticed that thermal radiation (radiation emitted by bodies due to their temperature) defines a portion of the electromagnetic spectrum that includes the entire visible and infrared radiation as well as a portion of the ultraviolet radiation.

Does this mean that if a body was subjected to strictly visible light it would heat up? Or, is it just the infrared and/or ultraviolet portion that causes objects to heat up? Or, does all types of radiation cause objects to heat up?

Thanks again!

 

[IRstuff]

Possibly, no, possibly.

The absorption on radiation is dependent on the object's absorptivity, which may not necessarily be a constant for all wavelengths. Thus, one can imagine, and actually build, a material that transmits visible, and reflects infrared, or the inverse.

TTFN

FAQ731-376

 

[Compositepro]

If an object absorbs energy of any wavelength then its energy content obviously increases. This usually involves an increase in temperature but may also result in a new molecular bond or even a change in the nuclear structure of an atom if the energy is sufficient.

Infrared and visible light are low energy radiation that usually only results in heating when absorbed. Ultraviolet and higher energy radiation referred to as ionizing radiation because the energy is enough to knock electrons out of atoms. This cause sunburns.

 

[curiousmechanical]

IRStuff and Compositepro,

Thank you for both of your valuable replies!