[time-nuts] Thunderbolt stability and ambient temperature
jfor at quik.com
Thu Jun 11 10:42:06 EDT 2009
Your answers seem somewhat 'mechanical'.
Are you a 'bot? Not a joke... REAL question.
> Rex wrote:
>> 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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