[time-nuts] Thunderbolt stability and ambient temperature
magnus at rubidium.dyndns.org
Thu Jun 11 07:29:24 UTC 2009
> Now, back to the subject of heat, I have a strange observation that I
> posted on the web a few years ago. A few people thought they had seen
> the same thing, but most thought what I noticed was not real. I posted
> because, if it was true, it seemed unexpected and I had never heard
> anything that could explain it.
> I was welding or heat treating steel. Imagine a steel bar about 1 inch
> (2.54 cm) in diameter and a foot to 18 " (30-40 cm) long. The bar is
> clamped in a vise and with a torch one end is quickly brought up to red
> heat. The other end is still cool enough that with my bare hand I can
> hold the bar by the cool end and carry it into the next room. I carry it
> there to cool it in the sink. A stream of cold water turned on, I
> quickly cool the hot end in the water. My observation, from doing this
> several times, is that the cold water quickly absorbes heat from the red
> end, but also seems to chase a lot of the heat quickly up toward the
> cold end, making the bar rapidly uncomfortable to hold. So that's my
> observation. I think the sudden cooling of the very hot end has somehow
> chased a glob of heat toward the cool end. If true, I have no
> explanation. I don't think it is related to steam; it seems to me to be
> something happening inside the bar.
The propagation speed for the heat comes into play. It takes time for
the heat from the hot end to reach the cold end.
As we are fairly off track here, let me relay a similar story. My mother
has been working with food all her professional life. A christmas
tradition here in Sweden is to have big lumps of ham from which you
carve slices. However, the damn thing needs to be cooked. If you do it
in the oven it dries out, if you only boil it you do not get that crisp
surface people want. You can do a bit of both. However, one year she
thought about cooking it in the microwave oven. She has no formal
training in thermodynamics and didn't really involve me in the thought
process, but she figured that if she ran the microwave for half an hour,
after wrapping the ham in microwave-grade plastic, just to avoid it to
dry out, and then just let it sit on the bench, then it would hit those
70 degrees in the core after a while anyway. Sure thing, it did. Worked
like a charm. Perfectly cooked, juicy. What happends is that it takes
time for the heat-wave to reach the core, so even if she stopped
providing more heat the heat-wave was still in progress and just could
not be stopped.
> Most people thought it was coincidence of heat propagating up the bar
> just at that time, or steam. Could be, but I still think it is real. The
> cold end of the bar was slowly getting warmer as I carried it, but after
> the sudden cooling of the hot end, the cold end seemed to get hot fast.
Yes... it would have got hot regardless of cooling or not at the hot end.
Also, the heat-wave wavefront isn't a flat surface...
> I meant to try an experiment with two bars and dual thermocouples, but I
> never got around to it. The main problem is getting things close enough
> to compare without questioning the heated states. My plan would have
> been: attach two themocouples to the cold end of two identical bars.
> Heat the two other ends rapidly to red heat (that is the very hard part
> to get right and balanced) and then just cool one bar rapidly while
> recording both temp profiles of the cold ends. If I figure out how to
> do the heating quick and balanced, I may still try the experiment.
That would be a neat exercise to demonstrate things...
> So I started with a bit of complaining about the rambling of the thread,
> and now I've rambled it in a whole nother direction. Sorry, I guess.
I guess it is an interesting side topic, the food aside.
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