Stupid question of the day....

"TokaMundo" schreef in bericht news: snipped-for-privacy@4ax.com...

I was here, now i'm there. You're not here so are yot there. Wait a moment I will go there

Now I'm here which was there a moment ago and first here. Strange a moment ago was there here and here there.

Of course you did. People are always mis-typing "magnetic" when they mean "gravitational." The conclusion was still wrong, either way.

Bob M.

Well I *almost* agree with you. To get a severe gradient, you do need to run a lot of current. But it still does *NOT* matter what is in contact with the outside surface. The internal gradient is a function of the heat generated per unit mass and the thermal conductivity of the material. Period. Nothing else.

The external medium will determine the exact temperature of the outer surface, and by virtue of the gradient for the specific material/power, the centerline temperature. But the shape and relative height of the gradient is irrespective of the external surface (as long as the thermal conductivity and heat produced are assumed constant).

For good thermal conductors such as copper or Al, the gradient is tiny unless you're working with extremely high powers. Poorer thermal conductors like iron or ceramic-like allows, the gradient can be higher.

daestrom

No, it is *not* for cylinders with only a central source. You jump to the wrong conclusion and stop reading without applying 'the little grey cells'.

It is for when the heat source is throughout the medium, just like a copper wire. Heat is generated in each unit cross-section of the wire. The heat in the central area must be conducted through the surrounding material. The surrounding material also generates heat which must also be conducted away. The further out from the center you get, the more and more heat must be conducted outward through the surrounding material. Hence a larger temperature gradient as you move from the center.

Maybe if you read through the whole explanation instead of just snipping it off, you'd learn something new. Maybe you should open up your mind a bit. Maybe you should study heat-transfer in a heat producing medium for a few years.

daestrom

To put it another way, if there is *any* heat generated in the central core of the wire, the only way it can be conducted to the surrounding material is if it is at a higher temperature than the surrounding material.

If the central core is *exactly* the same temperature as the surrounding material, then *ZERO* heat would be transferred by conduction away from the central core. And if heat is generated 'throughout the medium' (which it is), then the heat in the central core will raise the temperature of the central core. Now its temperature is higher than the surroundings and heat begins to conduct away from the central core.

Often called the 'zeroth law of thermodynamics', no net heat is transferred without a temperature difference.

daestrom

Sure it does. If you run a copper wire in air, and dump in enough current to produce a decent radial gradient, it will vaporize. You'd have to water cool it (boiling water is ideal) to sustain the power levels necessary for a non-trivial gradient.

Copper conducts heat about 12,000 times as well as air, and there's a lot more air available than copper in most situations. So, very roughly, a 1mm copper wire surrounded by a 10 mm air gap, with enough current flowing to create a 1 deg C internal gradient, will have a surface temp of 120,000 C.

Not once it's gaseous.

Thermal conductivity is itself a function of temperature, so the gradient does depend mildly on the absolute temperature of the whole rig. Especially after the copper melts.

John

On Fri, 05 Aug 2005 18:26:03 GMT, "Bob Myers" Gave us:

Even after I admit I was wrong, you are still an asshole. Fuck you.... Bob. Like I said, I likely knew about Io long before you did. ALL about it.

Which leads into some interesting stuff about thermal conductivity integrals and Richard Garwin.

RG sounds like he was quite a guy, involved in all sorts of stuff including development of the FFT. He's worth googling.

John

On Sat, 06 Aug 2005 15:23:30 -0500, John Fields Gave us:

You're full of shit, boy. If you had read the thread, you would have noted where I said that I have had Nasa's Laser Disc on the subject for over 15 years.

Did your lame ass even know what decent video was back then? Did you get into laser discs? I doubt it. You're a twit. Nothing more.

You're an idiot.

You still like mouthing off when you're more than an arm's length away. You're still also full of shit, Stinky.

Nah... The thermal conductivity of a film coefficient for air is not the same as the thermal conductivity of air. The thermal conductivity is only relavent in a very thin layer against the surface (much less than 10 mm). Moving outward, the viscosity and velocity of the air become dominant. Given the film coefficient of air against a vertical surface of about 25 W/(m^2-K), I make it out to only be about 8,000 C. ;-) Having forced air convection (or a good 'stiff' wind) can improve the film coefficient to almost 200 W/(m^2-K) (down to 1,000 C ;-). Water cooling can be as high as

But larger wires, and those of Al can develop such a gradient more easily. And true, boiling heat transfer can be several orders of magnitude better, but one then has to worry about exceeding the critical heat flux (also known as 'departure from nucleat boiling', 'boiling transition', or 'dryout'). Whether the water is circulating or not, and how far the bulk water temperature is from saturation also become important (i.e. becomes a real engineering nightmare).

The industry has a long history of success using pressurized hydrogen. Most large generators and their connections to step-up transformers are cooled this way. Much better cooling than plain air, allowing much higher current densities. And with the same material properties, stronger temperature gradients.

Except all of the H2-cooled gen-xfmr leads that I've seen use hollow conductors with H2 forced through the center as well as surrounding the outside. Similarly, the water-cooled conductors that I've seen are those found in generators and the water flows down the center of the hollow conductor. Not much of a temperature profile when the cross-section is mostly cooling water ;-)

True, but one usually designs to avoid melting, much less boiling.

Fact is, in 60hz applications, the usual design restrictions regarding skin-effect overshadow any problems with centerline temperature concerns. Perhaps engineers working with high-current DC applications are more concerned with the temperature gradient issues. But I suspect it is still small for good thermal conductors like copper.

I jumped into this fray when 'TokaMundo' said, "In a wire,....would show the wire at the same temp from center to outer surface". I think we agree this is wrong. And I agree that the temperature gradient is not severe for conductors made of Cu or Al under normal circumstance such as air cooling. But *some* gradient *must* exist, otherwise the centerline temperature must increase (due to heat generated and not conducted away) until a gradient begins to conduct heat away as fast as it's created by the electric current.

Wonder how bad it is for graphite rods used in electric furnaces? Of course graphite has a much higher melting temperature so it can withstand a strong gradient. But graphite, with its lower thermal conductivity and higher resistivity, probably develops a very strong gradient. Coupled with the temperature coefficient of resistivity, it might make for an interesting current distribution. Even for DC applications.

True. But below the melting point, it isn't hard to approximate the variance with a low-order polynomial using temperature alone as the independent variable. I would think this would make it relatively easy to incorporate into the integration. Haven't tried it though, so who knows???

daestrom

On Sun, 07 Aug 2005 13:07:00 -0500, John Fields Gave us:

Yeah right. You were a technician... oh... strike that... an assembler... oh wait... that's right... you swept the floors in the lab. Sound familiar, f*****ad?

So your plastic garbage can melts at 212F, or you let the wire touch the can?

When I was in the shipyard, we load-tested 2 and 4 MW units by dipping three 'blades' of steel into a tank filled with tidal water standing on the pier. Adjusted the load by controlling the depth of the 'blades'. Nice load but had to run a 'fill' line to keep the tank topped off ;-)

But this discussion forced me to 'sharpen the pencil' and do the actual calculus. Turns out it is a pretty small effect in most practical conductors (but not 'Toka's zero). But those graphite ones you mentioned in another thread could be interesting.

Yeah, especially when solid 'phase-changes' take place. Some steels undergo some interesting crystaline changes from 'body-centered cubic' to 'face-centered' at high temperatures and actually absorb a far amount of heat doing it with no appreciable temperature rise. So I can imagine such 'shifts' in thermal conductivity can occur.

daestrom

On Sun, 07 Aug 2005 13:07:00 -0500, John Fields Gave us:

Bullshit. It isn't a threat, veiled or otherwise, now or ever, so even your "again" bullshit is just that.... bullshit.

The shit the other asshole pulled were direct threats. Big difference, Johnny come stupid.

Like I said, there is a world of difference between the shit he pulled and what you have quoted me posting.

I was telling you that you wouldn't be such a mouthy f*ck if you were sitting right next to me. Which constitutes essentially me telling you that your a no more than a mouthy little wuss. He was making direct, blatant threats. There are at least two orders of magnitude of difference between the two.

On Sun, 07 Aug 2005 22:15:39 GMT, "daestrom" Gave us:

I believe I said "near zero", Daystruck. I also believe that my analogies were and are closer to the truth than the curve you declared.

Someone out there must like them for them to be a product.

On Sun, 07 Aug 2005 22:15:39 GMT, "daestrom" Gave us:

That's why welding works the way it does, and soldering works the way it does.

On Sun, 07 Aug 2005 17:29:17 -0500, John Fields Gave us:

Get it through your head, you retarded f*ck. I don't need your assessments.