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Q: White paint reflects heat, black absorbs but black car engine block runs cooler ( Answered ,   13 Comments )
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 Subject: White paint reflects heat, black absorbs but black car engine block runs cooler Category: Science Asked by: fillw-ga List Price: \$4.00 Posted: 25 Mar 2006 11:46 PST Expires: 24 Apr 2006 12:46 PDT Question ID: 711843
 ```In the hot farmlands, a house is painted white because it reflects the heat. Black absorbes heat (as I understand). Why then when a motor mechanic paints an engine block black it runs cooler than if he paints it white?```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler Answered By: hedgie-ga on 26 Mar 2006 06:56 PST Rated:
 ```fillw-ga good observation: Black engine should run cooler than white. When people say 'black absorbs more heat than white' they making an implicit assumption, assumption which is valid, for example on a sunny winter day. To put it very simply: Black paint is like open door. On a cold day it will let the energy from the sun be absorbed easily (by a cold object). White paint is like (partly) closed door for the heat. Because the engine is hotter then environment, open door will allow the heat to leave - and engine will run cooler. To put it into a mote technical language: Black paint promotes equlibration, white will slow it down. In a special case (of hot sun, cold box or house) the general rule is reduced to : cold box will absorb more heat when painted black. school lab: http://www.pbs.org/saf/1404/teaching/1404_1.pdf. http://www.angelfire.com/ms2/hatchett/ It is a FAQ: http://www.newton.dep.anl.gov/askasci/phy00/phy00405.htm Hedgie```

 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: probonopublico-ga on 25 Mar 2006 12:29 PST
 ```I guess it's because black paint absorbs the heat coming out of the engine block whilst white paint would stop the heat escaping.```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: kemlo-ga on 25 Mar 2006 13:46 PST
 ```I cant see how painting the block a different colour would make any differnce. this is probably an urban myth. more to do with concealing oil leaks Kemlo```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs coo From: bipolarmoment-ga on 25 Mar 2006 13:50 PST
 ```It's easy to think of it the same way except reverse the source of energy (heat). Black paint absorbs energy and re-radiates it. In the case of your house it absorbs from the sun and radiates it into the house. With the engine it's doing the exact same thing but by radiating the heat into the environment. The effect is called blackbody radiation, here is a practical discussion I found: http://www.newton.dep.anl.gov/askasci/eng99/eng99320.htm```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs coo From: qed100-ga on 25 Mar 2006 15:12 PST
 ```Let's ask this: Do you know for an empirical fact that a black painted engine block runs cooler than otherwise? Or is it possibly just something that's been passed from one mechanic to the next without criticism? It's necessary to get this out of the way before attempting to explain it with established knowledge. Explanations of non-existent phenomena (understand, I'm not claiming that it doesn't really happen) can become excrutiatingly convoluted.```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: larryg999-ga on 25 Mar 2006 17:57 PST
 ```It is an empirical fact that a black-anodized aluminum heat sink radiates heat better than a chrome-plated heat sink or an un-plated aluminum heat sink. Physics theory predicts (as I recall) that the heat radiated per unit of surface area (from a sphere), for a black-body, is 2x the heat radiated from a 100% reflective surface. Note: This statement applies to radiated energy only (i.e. object in a vacuum in space; no other radiant sources). Heat conduction is an entirely separate topic. Also, absorption of heat from all other nearby objects must be calculated separately (all objects radiate heat, except those at temperature = absolute zero).```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs coo From: qed100-ga on 26 Mar 2006 07:18 PST
 ```"It is an empirical fact that a black-anodized aluminum heat sink radiates heat better than a chrome-plated heat sink or an un-plated aluminum heat sink." Sure; I agree. What interests me is in what way the OP knows that this is true. Does the individual notice that, when driving, one engine's thermometer reliably reads higher or lower than another's?```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: larryg999-ga on 27 Mar 2006 08:23 PST
 ```Physics theory (corrected): Heat (electromagnetic energy) radiation rate, per unit of surface area, is proportional to the fourth power of the surface temperature. Heat radiation rate, per unit of surface area, is directly proportional to the ?emissivity? characteristic of the surface. Emissivity = 1.0 (approx.) for a dull black surface; emissivity = 0.3 (approx.) for a smooth, shiny copper surface. Hence the rate at which heat is radiated from an engine block with a dull black surface might be roughly estimated as 3x the rate at which heat is radiated from an engine block with a shiny metal surface.```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs coo From: politicaloyster-ga on 29 Mar 2006 07:27 PST
 ```I thought I should just comment and remind everyone that although black absorbs heat the most readily among all other colours, it also loses heat most readily. It is, if you will, the best conductor of heat in terms of colour.```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: richardhassinger-ga on 02 Apr 2006 18:22 PDT
 ```This makes little sense. What if both the white and black painted engine blocks were in the dark? How would the color matter? They both generate and release heat, how would the black one be any more effective at releasing that energy.```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: hedgie-ga on 02 Apr 2006 21:12 PDT
 ```richardhassinger-ga on 02 Apr 2006 18:22 PDT said: "This makes little sense.." Thanks for asking sensible questions. The ambient light is not important. In the darkness there is no outside light and it still works. Like this: a) Heat is transfered in three ways: radiation, conduction, convection http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html Here we will deal with radiation only, and that is controlled by Absorption and Emission (coeficients of the surface) http://www.efunda.com/formulae/heat_transfer/radiation/overview_rad.cfm For a dark surface, both coeficients are high, and for a white or mirrored surface, they are both low. So, imagine N spheres in a vaccum, with temperatures T1, T2, ... TN placed in a mirrored box (to eliminate outside light and other radiation). The hot spheres (high Ti) will radiate more than the cold ones. The hot and black ones (high emissivity) will radiate the most of all. search teems: Wien's law, Stefan's law see also commentlarryg999-ga So, even if there is no outside light, the box will fill with (infrared ..)radiation and, in the long run, all spheres will have the same temperature. That's is called thermal equilibrium. http://www.madsci.org/posts/archives/dec96/835000890.Ph.r.html White spheres will take longer to reach it (closed door) but will reach it too. In the 'naive picture' (seeprobonopublico-ga on 25 Mar 2006 12:29 PST) "black likes heat and will attract it," or "hold on to it" . That picture contradicts the second law of thermodynamics: In the erroneous understanding of the effect of color, the black object would hoard heat and also attract more, and therefore get hotter and hotter. That's not possible. From the above reasoning, we can see that the fact that emmission and absorbtion coeficents have same values (high for black, low for white) is not a coincidence. It is a deep consequence of the Second Law -- a special case of the Osanger reciprocity principle. Is it more clear now? Hedgie```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: probonopublico-ga on 02 Apr 2006 22:12 PDT
 ```Sorry, Hedgie, but I NEVER said on 25 Mar 2006 12:29 PST or at any other time: "black likes heat and will attract it," or "hold on to it". Please remove those quotation marks IMMEDIATELY! I immediately recognised it as 'a special case of the Osanger reciprocity principle' but, as I have no principles, the one finger that I use on the keyboard refused to do the necessary tapping. That said, a GREAT question, GREAT comments and a GREAT answer.```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: bob_the_builder-ga on 05 Apr 2006 08:15 PDT
 ```Well... your question is very difficult indeed. The answer is as follows: It is well known that a black-coloured aluminum heat sink radiates warmth better than a chrome-plated heat sink or an not plated aluminum heat sink. Physics theory predicts that the heat radiated per unit of surface area for a black-body, is 2 times the heat radiated from a 100% reflective surface. Your welcome, Bob T Builder```
 Subject: Re: White paint reflects heat, black absorbs but black car engine block runs cooler From: hedgie-ga on 05 Apr 2006 18:01 PDT
 ```Re: From: probonopublico-ga on 02 Apr 2006 22:12 PDT Please remove those quotation marks IMMEDIATELY! Consider the quotes removed. I was paraphrasing a possible interpretation, not quoting. Sorry about the quotes. I wish software would allow us to edit postings. Hedgie```