# How come heat sinks are cold by the touch?

1. Sep 29, 2014

### rogerk8

Hi!

I wonder what makes the often black "painted" aluminum heat sinks for electronics cold by the touch?

To me a shiny heat sink would work better because then all the incoming wavelengths are reflected.

A black heat sink would however absorb all the incoming wavelengths because that is by definition what makes it black.

So if it absorbs all the incoming wavelengths it should get hotter, right?

But touching an unused ordinary black heatsink gives coldness to your hands.

So my empirical studies tells me that I'm wrong.

We may however look at it in another perhaps more proper way.

There is a "lamp" which has both shiny and black wings inside.

These wings have the shiny side on one toroidal direction and the black side on the other.

The whole arrangement is supported by two needles above and below the wings and the support is of almost no friction (due to glass-needle).

Shining on it with a lamp light makes the arrangement rotate along what toroidal direction?

I have learned and seen that it rotates with black side first.

This is because the change of momentum is 2p when you shine on the shiny side and only p when you shine on the black side (which absorbes the incoming photon).

Feels like I'm almost off topic, but this is all I know about black/shiny bodies.

Best regards, Roger
PS
Is my blog really deleted because I can not seem to find it.

2. Sep 29, 2014

### M Quack

If you put your black heat sinks into the direct sunlight on a hot summer day, they would probably heat up quite a bit. Silver, "natural" anodized Aluminum would be a bit better there. Most power electronics, however, tend to be indoors where sunlight is not much of an issue. The black anodization is mostly cosmetic.

Radiation plays only a very very small role in the functioning of heat sinks. Most heat is transferred from the heat sink to the surroundings by convection, i.e. air currents.

Heat sinks are cold to the touch because they have good heat conductivity. They lead heat away from your fingers and spread it over their surface where it will eventually be spread throughout the room by convection.

The device you are referring to is called a light mill or Crookes radiometer. Wikipedia has a very nice explanation of how it works.

3. Sep 29, 2014

### rogerk8

I don't seem to be able to quote you on this new "fantastic" system...

Anyway, I like your explanation very much!

The black anodization being mostly cosmetic and the heat conductivity, that is.

And convection I recognize from electron tubes.

It rotates in the wrong direction!

And Maxwell is more of a god to me than Einstein!

When dp=2p at the silver surfice dp is only p at the black surface and therefore the Crookes radiometer should turn with the silver side trailing!

I refuse to accept these rambling about heat leaking from the hotter (black) side to colder (silver) side around the vanes.

That is just wishful thinking :)

On the other hand, the animated gif clearly shows that it rotates with a trailing black side.

I can't believe it!

And as late as 2009 some people at University of Texas obviously made a light mill where the concave side moved forward due to the convex side receiving more photon energy.

It is interesting to note that photon energy is said to be too small for an ordinary light mill while it suddenly is enough in the Texas case.

Best regards, Roger

4. Sep 29, 2014

### Staff: Mentor

That's a completely separate question from what was in the title and wasn't answered.

A black body is good at both absorbing and radiating and whether it is a net absorber or radiator depends on whether it is warmer or cooler than its surroundings. So if you put a heat sink in the sun, it might do more harm than good to have it painted black. But for a heat sink in a computer case, the black helps it radiate better because the heatsink is warmer than the computer case skin.

5. Sep 29, 2014

### Drakkith

Staff Emeritus

Click "Reply" at the bottom right of a post to reply and quote a single post. Click "Quote" to add the post to your "Quote Que". You can do this with multiple posts, and clicking on "Insert Quotes" at the bottom of the page where you reply will add them to your post. You can also highlight text within a post and quote only that text instead of an entire post.

6. Sep 30, 2014

### rogerk8

Just trying to learn. Clicking this reply button gave me this. Thanks!

@russ_watters:

A black body is good at both absorbing and radiating and whether it is a net absorber or radiator depends on whether it is warmer or cooler than its surroundings. So if you put a heat sink in the sun, it might do more harm than good to have it painted black. But for a heat sink in a computer case, the black helps it radiate better because the heatsink is warmer than the computer case skin.

This sounds a bit too simple. Please elaborate!

Best regards, Roger
PS
I now think I understand the light mill. The black side is getting hotter by the torch light and hotter means more residual air speed (which also means there can't be a true vacuum here) which in turn makes the light mill turn with a trailing black side due to more air atoms in the vicinity hitting it.

7. Sep 30, 2014

### rogerk8

I think someone has to say this, your new system sucks!

Three reasons:

1) Quoting people is hard to understand
2) Editing one's reply seems impossible (your last system allowed at least 10 hours but here I can't even find the button)
3) My blog seem to have been deleted. Very nice indeed!

But do you know, I really don't care partly because there is more to life than physics partly because my blog is now available at wikibooks :D

Why don't you check it out: https://en.wikipedia.org/wiki/Plasma_Fusion_Preface

8. Sep 30, 2014

### Drakkith

Staff Emeritus
I can see how the change to the new system could be confusing at first. Just remember that "reply" will reply to a specific post and "quote" adds a post to a quote que, allowing you to quote multiple posts.

I see the edit button at the lower left of my posts, but I'm a moderator so I'm not sure it's the same for regular members. It may be missing once the time limit for editing a post has expired. I've asked about this here: https://www.physicsforums.com/threads/have-a-problem-read-this-first.771872/page-8#post-4868206

Yes, all blogs were deleted during the move. I believe there were notifications prior to the move informing members of this and other important information.

For any questions, concerns, or comments on the new forum, see this forum: https://www.physicsforums.com/forums/pf-4-0-questions-problems-feedback.236/ [Broken]

Last edited by a moderator: May 7, 2017
9. Sep 30, 2014

### Staff: Mentor

It really is that simple, so I'm not sure I can. Maybe seeing similar statements elsewhere:

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html

10. Oct 1, 2014

### Staff: Mentor

Best of all - just mark the part of the text you want to quote and the local menu will appear. Select Reply and what you marked will be copied into the reply edit field.

11. Oct 1, 2014

### rogerk8

I also wish to beg of your forgiveness regarding my somewhat offensive way of expressing myself.

I will also contemplate the very interesting physics part of this thread.

Now I see the edit "button". I suspect it removes itself within 10 hours(?)

Now I need to get back to work.

Best regards, Roger
PS
I like beer a little too much...

12. Oct 1, 2014

### Staff: Mentor

Even earlier. Yes, edit time is limited.

13. Oct 3, 2014

### rogerk8

I have decided not to read anymore links unless I feel that I really need to.

I have however not read all the "blue-links" but I have read quite a few and I only have one life...

This being said, I also have much latent knowledge that I wish to put together and it's my hope you'd want to guide me.

It doesn't make sense that a black heat sink in a computer would mean that the heat sink radiates more energy than the computer case simply beacuse it is black.

Two formulas:
$$\lambda\propto\frac{1}{T}...[1]$$
$$P_s\propto T^4...[2]$$

where (1) says that if a body has a certain temperature it radiates light of a certain wavelength (the hotter, the shorter).

The other equation tells us how radiated (surface) power is related to temperature.

I hope I don't offend the innovators of these equations just by using them (but I think that the first one is by Wien and the second one is by Stefan).

None of these equations does however tell us that a black body will radiate more power.

A black body will only have a temperature higher than a shiny one and this is because it absorbs all (visible) incoming wavelengths.

This in turn indeed makes it hotter. And according to above this means radiation of shorter wavelength (and thus higher frequency).

Using E=hf then tells us that the black body actually do radiate more (photon) energy!

Best regards, Roger
PS
I can't find out how to change my signature while that link is dead.

Last edited: Oct 3, 2014
14. Oct 3, 2014

### Staff: Mentor

MY PF/Signature

15. Oct 3, 2014

### rogerk8

That path does not excist.

16. Oct 3, 2014

### Staff: Mentor

MY PF is on the bar up there (left to INBOX and ALERTS). When you hover the mouse over MY PF menu opens, and Signature is one of the commands there.

17. Oct 3, 2014

### rogerk8

Yes, I see MY PF and there are lots of choices there but "Signature" does not excist.

Sorry for being so vain.

I suspect that the reason for no "Signature" option is that I have a warning hanging over my head (expires late next year).

18. Oct 3, 2014

### Staff: Mentor

No idea why and I can't delve deeper at the moment. Warning shouldn't matter. In the past signatures were only for gold members, not sure how it is configured ATM.

Edited that for you - removed just the

Code (Text):
My PF blog: [URL="https://www.physicsforums.com/blog.php?b=4612"]Plasma Physics Thoughts[/URL]
part.

19. Oct 3, 2014

### rogerk8

Thank you Borek but my wish was to change it, not remove it :)

Do you perhaps mind changing it to:

My Wikibook: Plasma Fusion Preface

Best regards, Roger

20. Oct 3, 2014

### Staff: Mentor

That isn't what I said. I said: "But for a heat sink in a computer case, the black helps it radiate better because the heatsink is warmer than the computer case skin."

I may have mixed two issues together in a confusing way:

1. The heat sink radiates heat to the computer case (and not the other way around) because it is warmer than the computer case.
2. The heat sink radiates more heat when black than when silver or white becuase it has better "emisssivity":

"The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation."
Not sure where you got that, but it isn't the full Stefan-Boltzman equation: it doesn't include the necessary conversion constant and the required term for emissivity. See the links I provided.

Last edited: Oct 3, 2014