Dumb Question [about infrared and ultraviolet light], but still....[Edited title]

Factual Questions
21

>I had heard about the ceramic heaters but didn’t know the source of the heat was IR. Interesting.

Well, they nominally transfer their heat by IR. The source of the heat is electrical resistance. And these things transfer heat by IR and conduction and convection in a combination that depends on their surroundings - if you doubt me, try touching one. Well, don’t, I’m just making the point.

>You’re forgetting infrared LEDs which are used in remote controls all over the place. Their beam is invisible by design.
I’m forgetting no such thing. LED’s don’t have an evacuated glass bulb envelope, so they aren’t relevant either way in re “bulbs”. There are LEDs available at quite a big variety of wavelengths especially in the red and near IR. Whether you can see them depends on a variety of things, but you sometimes see their radiation at wavelengths longer than what some consider the end of the visible spectrum. I mentioned laser diodes, which are are in fact light emitting diodes, albeit ones that lase. The laser diodes at 785 are visible because they are so bright that they can stimulate the eye’s weak response there. Non-laser infrared LEDs, though, could be visible because of their strong IR or because they emit a little at other wavelengths too. Look at an LED through a spectroscope and you’ll see what I mean.

>All heat is IR.
Anne, I don’t take your meaning, I’m sure. Heat is energy in the form of disorganized motion of atoms, molecules, and related things (such as quarks or neutrons in obscure situations).

>Anything that feels warm to you- it’s emitting IR.
>Saying that IR is heat is at least very misleading, if not just plain wrong.

Well, if something feels warm to you, it’s pretty much gotta be emitting IR. I don’t know if Anne means that the IR emission is the reason it feels warm, which is not necessarily so; a jet of hot dry air without CO2 in it will emit almost no IR but can feel warm enough to ignite you. But in principle anything warm will emit at least some tiny bit of IR, as nothing is perfectly reflective or transparent throughout the IR.

Bless your heart, by the way, Anne, for checking the spelling. I was just plain lazy.

Another interesting point - we speak of blackbody emitters, but there are also emitters that have spectral emissivities that vary between 0 and 1 over the IR wavelengths. The mantle in a gas lantern, for example, is thoriated (covered with thorium oxide or thoria) so that its emissivity in the IR is low, and this forces it to go to a higher temperature and emit more visible blackbody radiation in order to shed all the heat it has to. Polymers are often very “colored” in the infrared, and do their emitting mostly in limited wavelength bands.

22

There are infrared sources that emit no visible light. I saw one that was used as a test target for missiles. The infrared lamp was in a housing that was covered with some sort of metallic film.

23

I have a 30W CO2 laser which emits a completely invisible, 10.6µ beam. It’s impressive to see it burn a hole in a piece of wood.

24

I have one of those at home. I use it to remove wrinkles from clothing.

25

He said, Ironically.

26

I had an encounter with one last night. I left a metal spoon in a pot on the stove, then came back to stir it later. Owwww. I said they were dangerous.

27

I get steamed about bad puns.

28

No it isn’t, what you’re describing is internal energy or temperature, heat is “energy transferred from one body or system to another due to a difference in temperature.”

Anne is wrong to state that all heat is IR, though.

29

No, it is not. Napier’s definition is the more correct one, unless you mean to assert than an object isolation contains no heat.

30

Thanks, Q. E. D.

The definitive relationship between heat and temperature is that temperature is the potential function that describes in which direction heat will flow between two objects brought into thermally conductive contact. Is this, naita, what you are getting at?

31

Interesting! What did they say it looked like?

-FrL-

32

Every day on the SDMB brings some new knowledge, even in things I didn’t know I didn’t know. Rumsfeld said there are known unknowns and unknown unknowns. Until today, I would have thought “comparing blackbody curves” would surely be about lovely women. Now I have to look up Stefan-Boltzmann to find some clue of what these folks are discussing. I will ponder sealing wax and onion rings. I’ll get back to you.

33

Wait. Mark Zemansky’s book Temperatures Very Low and Very High says that heat is only the energy that is moving from hotter objects to colder ones, and not the static energy content an object has by virtue of its temperature. Now I’m looking for more definitive sources. Maybe naita is correct on this one.

34

“Discusses the fact that heat only exists at the boundary of a system as energy is being transferred because of a temperature difference between the system and the surroundings. Points out that many texts incorrectly refer to heat as a system component. (MLH)”
from http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordDetails/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=EJ152141&ERICExtSearch_SearchType_0=no&accno=EJ152141
“In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature.”
-Wikipedia
[I am not sure this quite states that heat does not include energy that was or will be transferred in this way, versus that only that energy flowing at the present moment is heat]

“Heat may be defined as energy in transit from a high temperature object to a lower temperature object. An object does not possess “heat”; the appropriate term for the microscopic energy in an object is internal energy. The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is properly called heating.”
from Heat

Looks like naita is right and I’m wrong - at least, the preponderance of references I’m finding seems to say so.

35

This is something I would very much like to know.

There have been other threads on the subject.

What physical/physiological changes would allow humans to see other spectra?
Wife can see UV?
The Visual Light part of the EM Spectrum

Look for “tetrachromat” or “ultraviolet”.

36

To me, this sounds like so much semantic niggling. The Wikipedia entry on heat only serves to amplify this perception:

Um, what? This basically says “An object can’t contain energy, it can only contain energy.” If there’s a good rationale for making this distinction, I’d like to hear it. So far, everything I’ve dug up either doesn’t make sense or boils down to “because we say so.”

37

Maybe the idea is that once the energy is heat, it’s not longer posessed by the object–because its in the process of being transferred away from it.

-FrL-

38

>…an object cannot possess heat, but only energy.

>Um, what? This basically says “An object can’t contain energy, it can only contain energy.”

It only basically says that if heat and energy are the same thing, which distinction is the point of the statement. I take the definitions I am finding to say that energy comes in many forms such as electricity, light, sound, gravitational potential, and so forth, one of which is called “internal energy” and comprises random microscopic motions of all the little particles or elastically fixed masses. And I take them further to say that if internal energy is passing from one body to another because their temperatures are different and they are in thermal contact, then the internal energy moving from one body to the other is called “heat”, but not otherwise.

I don’t know that we are appreciating why heat is better defined this way than as identically the same thing as internal energy. I mean, I follow the distinction, but don’t see why it’s a good idea. But the definition seems like a sound one, even as long as the motivation is obscure.

39

I think you could make that distinction because the amount of that “internal energy” in an object need not have any clear relation to the degree to which that energy gets transmitted away from the object. The latter would depend no only on the amount of energy in the thing, but also on its internal structure, its conductive properties, and so on.

-FrL-

40

As I understand it:

Hot gas has thermal energy. If it loses thermal energy by radiation or conduction to a cooler object, then energy is being lost as heat. But the gas can also lose thermal energy by doing work, e.g. by pushing a piston. In this case there’s no energy transferred as heat, and it may be confusing to say the gas is “losing heat”.