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In textbooks, they say "absorb" heat and "give off" heat.

Is there a single word which can perfectly take the place of "give off"?

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Which 'textbooks'? This textbook (warning: biggish PDF) uses radiate to mean "heat given off," with emit used in specific instances. –  Gnawme Nov 8 '11 at 0:33
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@Gnawme: As a few answers mention, radiate has a precise meaning which doesn't always fit; not all heat emission is via radiation. I don't think you'll find much mention of radiated heat in the chapter on convective heat transfer in that book. –  Jefromi Nov 8 '11 at 1:11
    
@Jefromi Yes, but the context of the OP's question, radiate fits best. Several sources on blackbody radiation, for example, state flat out: Heated bodies radiate.. To have been heated is to have absorbed heat; heated bodies then radiate. –  Gnawme Nov 8 '11 at 1:17
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@Gnawme: I'm not sure I see anything in the OP's question to indicate that the dominant mode of heat transfer is radiation. And yes, of course, sources on black body radiation speak of bodies radiating. Similarly, sources on convective heat transfer (which is a very common mode in everyday life) will not talk about radiating heat. (True, in nontechnical contexts, radiate is commonly used figuratively.) But the OP is asking about textbooks, and it's not clear what mode of heat transfer they deal with. –  Jefromi Nov 8 '11 at 1:56
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It's not clear to me whether this question is specifically looking for a one-word replacement for "give off" in the context of heat (clearly radiate, IMHO), or a more general-purpose antonym for "absorb". –  FumbleFingers Nov 8 '11 at 18:12

8 Answers 8

up vote 33 down vote accepted

Emit and radiate are both good candidates,

Emit (v): to throw or give off or out (as light or heat)

Radiate (v): to send out in or as if in rays

while emanate could be used to describe what the heat itself is doing (although it is sometimes used incorrectly to mean emit.

Emanate (v): to come out from a source - a sweet scent emanating from the blossoms

So while emit and radiate mean "give off", emanate means "be given off". A car engine would emit heat, but heat would emanate from the car engine.

Interesting to note: radiate can be used either transitively or intransitively, and can mean either emit or emanaate. What a flexible word!

As FumbleFingers points out:

radiate heat is even more common than emit heat. But I think that's because to some extent the word radiate actually means to emit electromagnetic radiation (which for most purposes means energy, or heat).

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By comparison with emit heat, emanate heat is pretty unusual, to say the least. –  FumbleFingers Nov 7 '11 at 22:59
    
"Emit" is being used quite often when we talk about green house gases. Why don't they just use "Emit" for heat? I suspect there might be some subtle deference between heat and gas regarding the action. –  Terry Li Nov 7 '11 at 23:00
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Emit is the word to go with, if you are talking physics. For example, we talk about absorption spectra (ABSORB) and emission spectra (EMIT) when we are referring to energy transfer in and out of a molecule. –  MετάEd Nov 7 '11 at 23:44
    
@Terry LiYifeng: I think it may be just that when we talk about green house gases we're automatically in linguistic territory dominated by the latter-day usage of emissions. –  FumbleFingers Nov 8 '11 at 0:08
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@FumbleFingers I've edited my answer. I won't ask you to delete yours, but you are more than welcome to make any changes to mine that seem appropriate. –  Jim Jan 12 '12 at 18:28

Physicists say "radiate" in reference to heat given off as thermal radiation (i.e. infrared heat like those heat lamps you see keeping food warm). They also use "convect" and "conduct", but these do not give the sense of direction that "radiate" or "give off" do.

"Emit" comes to mind. I also like "shed".

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"radiate", "conduct" and "convect" are all very different means of heat transfer. Heat "radiates" from a source as invisible but detectable infrared light. Usually that is talked about in terms of astronomical distances, like from the Sun to Earth. "Conduction" is when heat transfers between atoms that are relatively close to each other, usually through solids or from solid to liquid/gas. "Convection" is when heated particles "mix" with unheated particles in a liquid or gaseous medium and distribute heat. –  KeithS Nov 7 '11 at 23:24

If the thing being absorbed is a single unit, then "expel" would be the word to use.

At the moment, though, I can't think of anything outside of a sci-fi/fantasy context. Like in Animorphs #12, The Reaction, Rachel expelled a full-sized crocodile... Although for humor purposes, the word "burp" was used instead.

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Expel is indeed used frequently, thought it has a distinct negative connotation in most cases. Someone may be expelled from some place; oil is expelled from oilseeds... Also, it is used in a sense of involuntary and forceful action of removing. Usage focuses more on what is expelled, than what is causing the expulsion, in contrast to 'emit'. –  Kris Nov 9 '11 at 11:12

A chemist would say the opposite of absorb is desorb (the distinction is made, I guess, because the absorbed thing would be a chemical of some kind and it is more substantive than photons which are emitted or radiated).

http://en.wikipedia.org/wiki/Desorption

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In the context you give, I think "release" is the right word. An example is: "Determine the amount of heat released when one kilogram of steam condenses." To me, heat emission implies radiation.

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I quite like "exude" if the sense is appropriate (i.e. a gradual release).

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While it's not really appropriate to the context being asked about, I like "secrete" and "excrete."

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These have more a flavor of biological sciences to me, but the OP seems to be more in the domain of physics. –  rajah9 Nov 11 '11 at 4:28

Exude literally means discharging / giving off.

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