heat sink cooling

["Mark Hanson" <mhanson@valcom.com>]

Take 3 equally sized heat sinks , bare alum, one black and one white, put a
50 ohm resistor on each and apply 12 volts. The bare aluminum heat sink will
have the lowest thermocouple reading disproving the "black body radiator"
theory. A black surface only helps when absorbing heat from the sun.

-----Original Message-----
From: Colin Dedman [mailto:cjd121@rsphy1.anu.edu.au]
Sent: Tuesday, May 30, 2000 6:58 PM
To: ev@listproc.sjsu.edu
Subject: Re: FW: 1221B cooling (and zaprabbit stereo)


Mark Hanson wrote:

> I've measured the "Black body radiator" that is repeated incorrectly.
Black
> absorbs heat better but DOES NOT radiate any better, the best is raw
> aluminum or copper, not coated with anything.

Hi,

I didn't see the previous postings on this topic (if there were any) so
maybe I
misunderstand what you are saying here. What I can tell you is that the
radiated heat from a surface is proportional to the 4th power of the
absolute
temperature, and the "emissivity" of the surface. Defining a perfect
black-body
radiator to have an E=1, typical values other surfaces are 0.05 for polished
aluminium or copper, 0.2 for highly oxidised aluminium or copper, 0.9 for
black
paint, and 0.96 for lamp black. Thus it is fair to say in general that black
DOES radiate heat better. In practice though, heat loss through convection,
natural or forced, usually contributes more than radiation unless the
surface
is really hot.