[time-nuts] Thunderbolt stability and ambient temperature
bruce.griffiths at xtra.co.nz
Thu Jun 11 05:57:31 UTC 2009
> Hal Murray wrote:
>> phk at phk.freebsd.dk said:
>>>> Can I get reflections without some inductance?
>>>> Is there any inductance in a system of alternating
>>>> layers of insulation/storage?
>>> I think you are overstretching the badly chosen nomenclatures
>>> parallels to electricity.
>> It was actually a (somewhat?) serious question on several grounds.
>> Can I get reflections from a lumped circuit model of a transmission
>> line made out of just Rs and Cs? If so, I can probably do the same
>> in the thermal world.
>> Can I get reflections in a thermal context? Bruce's URLs say yes,
>> but my math is rusty enough that I can't quickly understand what's
>> going on.
>> If a thermal problem can generate reflections, does that mean it also
>> has something corresponding to inductance? If so, what is it?
>> It's possible that the key idea is time-delay. In the electrical
>> world, a delay is a transmission line which has both C and L. I'm
>> not sure what the one-dimensional equivalent in the thermal world is.
>> What's the speed-of-light equivalent in the thermal world?
> Why were you somewhat serious about this?
> If you want to extropolate heat into electromagnestic waves, what
> would be the analog of frequency? There are a few parallels in the two
> realms by analogy but that doesn't mean they map in all aspects.
> Sometimes, to help learning ohms law, the analogy of water is used
> with pressure = voltage, flow = current, resistance = narrow pipes. It
> sort of makes the concepts easier to grasp, but when you get to AC and
> wave reflections I think one has to struggle to make the water analogy
> useful. For heat, I think the water analog might be more useful than
> trying to map the EM waves to heat.
> The reflection idea did remind me of something that occurred to me, a
> gallows-humor joke from years back. I'm sure most of you remember
> hearing about the 1989 San Francisco earthquake. The earthquake
> epicenter was between Santa Cruz and San Jose, about 40 miles south of
> San Francisco, but a lot of the serious damage and fires occurred in
> San Francisco near the tip of the penninsula at the bay shore. There
> was a lot of discussion about this localized damage so far away, and
> how that could happen. San Francisco is at the tip of a peninsula that
> forms the Bay. I immediately thought that the problem was obvious. The
> penninsula was excited at its bottom end and was left improperly
> terminated at San Francisco. I couldn't tell this joke for two
> reasons, one: it was in bad taste, but two: I only knew a few people
> who would get it -- the mismatch/termination joke.
> 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.
> 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.
> 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.
> 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
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your experience with the hot bar is quite common.
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