Silly heat and color question

[canton_kid]

Ok, this is something I wondered about for other purposes to, but don't really care about for my cases. I still plan to use black :)

If dark colors absorb and hold heat, but bright colors reflect heat, then what type color is best for getting rid of heat from an area like say a computer case? I don't really think it matters that much, and with all the air flow through a case I don't really think aluminum is much better than steel at getting rid of heat in normal situations.

But anyway, if we use black as a color one would think temps might rise as it holds heat, however the paint on the case side is also black so that should make it suck heat off the metal. If using white, it reflects heat so one would think it should be cooler, however the paint on the metal case side is also white which means it should be reflecting the heat back at the metal case instead of absorbing it right.

Now color is realy just a relection of light, and since the paint is stuck to the case it seals out light. Therefore there is no reflection and thus no color, which technecholy means whatever color you use, it is still black on the metal case side. I plan to skip a discusion of transparent paints though like used in sun catchers or candy apple paint jobs for hot rods. Which would normally use a bright base coat anyway.

I do think it might make a slight difference inside the case as black would absorb and hold heat in the internal parts, but I am reffereing to mainly the exterior of the case.

I think we all know a black car gets hotter than a white car (I've had both at the same time parked side by side and they really do) however that is an effect of the sun, not ambient temps. Air temps are far less than the temps in the car. I do not plan to sit my computer in a window in direct sunlight (on purpose).

Any thoughts on this subject?

One more note. Water in a black barrel gets hotter faster than in a white barrel sitting in the sun. That would indicate that although black aborbs heat well, it also transfers the heat to a cooler surface well too! In this case being a metal barrel full of cool water.

[marscheese]

light isnt' what creates the heat...it's only the surface that absorbs it. I guess if what you're asking is, do certain colors of light, when absorbed, create more heat, my answer is no. Aside from an objects color, the material itslef also dictates how much heat is absorbed. Maybe I'm sort of lost on what your question is...bright colors don't reflect heat, bright surfaces don't absorb as much light, changing that energy into heat.

Hope that helps.

[marscheese]

just thought of something. If you're shining a light on a wall that's, let's say, red. If you shine a white light and a green light on that wall, the amount of heat created would be approx the same. But if you shine a red light on the wall, then heat would be negligable.

This would happen because we see the wall as red because it absorbs all colors of light except for red. Thus, a red light shining on the wall would not create as much heat....

Just correcting an incorrect statement in my last post ;)

[Pugsley]

yes... the paint has noting to do with temps. it doesnt matter what color it is... if the heat is comming from inside. now if this thing gets hit with direct sunlight... then color will matter.

[canton_kid]

More of a question on heat than light. I know color is the reflection of light, but in the dark black still seams to heat faster for some reason. There is no color if there is no light right!

Perhaps is has to do with the pigments used to make the color? There has to be a difference as far as light is concernerd to make it reflect red or green. Perhaps it's the pigments that are effected by the heat, some pigments aborb better than others maybe?

Yea, I geuss the question should be more of lose heat than gain heat since the temp is inside. Although it must gain the heat from the case before it can lose it to the air.

I don't really figure it matters at all for the low temps a case has. But was a question that crossed my mind.

Black seams to absorb heat better and maybe hold it longer than say white when building solar boxes of various types. Black heats much faster in direct sun. For water heating I paint copper tubes flat black and the inside of any box they are in black as well. That's in direct sun.

But also Black seams to grab heat from the air better than white. Though I have not used any major means to confirm this, if you put two containers of water next to a wood burner the black one seams to get hotter faster than the white one does. I haven't actually performed any sceintific research to varify this, but personall experience seams to show it correct.

Also water in an open black container seams to evaporate faster than in a white container, which would indicate it was warmer also.

Durring the winter we use wood heat. The air is dry and we have lots of static electricity, so I keep an open can of water near the wood burner. The water does not boil and steam, but being kept warm it evaporates into the air and reduces the static shocks we get after walking across carpet then touching metal :)
Or computers :(

Anyway, I have noticed that when I used two containers at the same time (empty coffee cans) exactly the same other than color and placed them behind the woodburner, the black one seems to get warmer faster and always goes dry faster than the white one does. Light is not a factor in this as both are in a dark location behind the wood burner. Both are the same distance from the wood burner, and I have also reversed which side which is placed. Like I put the black on the right this time, next time I put black on the left. So all is equal, air drafts, airtemps, distances, ect.. Always the black one goes dry first, after several days or so. This is a constant that has been done may times, never has the white gone dry first! Also the temp of the water has been the same when filling the cans. At least once I made sure I rotated which was filled first. Though I normally don't care, after seeing the black was always dry first I did play around some testing the colors. I made sure at least once the white can was filled first and the next time the black can was filled first. Just in case one was getting water from the pipes warmer than the other getting water from the well. Shouldn't matter with that little of water and taking several day to evaporate anyway, but I tried it. Black still dried out first.

The only difference might be the paint itself used on the cans. I am pretty sure I used the same type and brand on both cans. I'm sure if I used a laquer on one can and a plastic coat on the other that would posibably make a diffence, but I didn't do that! And both were a gloss paint.

[marscheese]

if you're just applying direct heat to a material (no light involved), than the only thing that should dictate how hot that materal gets is what the material is made out of.

(throwing some physics out there)

Q=cm(Tf-Ti)
Heat=(specific heat capacity of material)(mass of material)(change in temperature--final minus initial).

So the change in temperature should be--->change in temp=Q\cm

So if you put two objects in a source of heat, that is constant for both, and the matrial that each is made out of is the same, then the mass and specific heat capacity (high heat capacity=conductor, low heat capacity=insulator)...then the change in temperature should remain the same. There's nothing in the equasion that talks about the color of the material, because in the absense of light, color doesn't dictate the absorbtion of heat.

***hope I got all that right, someone correct me if I screwed something up, but I"m pretty sure everything is right.

[smb]

the reason black absorbes heat is because it absorbes all colors of the spectrum.

[marscheese]

and sunlight is *basically* white light, which is why black is hot in the sun...if it's hotter in the sun than with just a white light source, I'm not sure. If that is the case, my guess would be becasue the sun give out a greater amount of light than say a 60W light bulb...and I don't know how many UV rays a white light bulb (or something like it produce)...

[canton_kid]

This all makes sense about the light.
However water still evaporated faster from the black can than it did from the white can with all else being equal as far as I could tell.

Since paint is made of pigments, and the pigments have to be different in order to absorb different shades of light, it could be they absorb heat differently also perhaps.

For instance if using a metal paint made of copper dust, the color is copper in the light, but black with no light. The pigment is still copper dust though and copper absorbs heat differently than alumnium or steel.

So if painting two cans, one with alumnium dust paint and the other with a copper dust paint, they would be different colors in the light but the same color in total darkeness, but the difference in metal dust would still alough them to absorb heat differently regaurdless of lighting.

Some of the old very expensive RED paint for hot rods actually were supposed to contain real Gold Dust.

So it is probably not really the color of the paint itself, but instead the material used in the paint, to produce that color when light is applied that makes the difference.

[marscheese]

Originally posted by canton_kid
So it is probably not really the color of the paint itself, but instead the material used in the paint, to produce that color when light is applied that makes the difference.

I think you hit the nail on the head.

[matt719]

:( *Throws science textbook out the windows*