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?

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

I guess it's because black paint absorbs the heat coming out of the
engine block whilst white paint would stop the heat escaping.

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

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

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.

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).

"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?

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.

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.

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.

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

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.

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

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