[time-nuts] Thunderbolt stability and ambient temperature

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Jun 11 22:36:07 EDT 2009


Perhaps the answer is somewhat more prosaic.

Radiation and convective losses from the hot end of the bar are significant.
In particular the radiative loss is (as a first approximation)
proportional to the difference of the 4th powers of the bar temperature
and ambient temperature.
When one modifies the model to include radiative losses near the hot end
that are in effect switched off by cooling then some overshoot can occur
at the hand held end of the bar.


Bruce

Bruce Griffiths wrote:
> John
>
> That doesn't appear to reproduce what was claimed to have been observed
> at all.
> The input is more like a step function that switches from hot to cold.
> This allows the simulated bar to reach a steady state temperature
> distribution before decaying smoothly to a lower temperature.
>
> Bruce
>
> J. Forster wrote:
>   
>> The effect that was described was absolutely NOT a result of thermal
>> conductivity being a function of temperature.
>>
>> It was a dynamic effect... a transient condition. The result of applying a
>> short heat pulse to a long Time Constant, distributed system.
>>
>> Do the simulation I suggested hours ago.
>>
>> -John
>>
>> ==============
>>
>>
>>   
>>     
>>>> Tom
>>>>
>>>> The thermal conductivity isnt constant with temperature.
>>>> It also varies between different crystalline forms of the same material.
>>>> This can be seen in more comprehensive tables of thermal conductivity.
>>>> In particular at cryogenic temperatures the thermal conductivity can
>>>> change dramatically (eg in superconductors)
>>>>
>>>> Bruce
>>>>       
>>>>         
>>> Excellent. Not constant; and perhaps not even linear?
>>>
>>> If you run across a thermal conductivity table for steel
>>> from say 0 to 1000 C let us know. From that graph we
>>> should be able to calculate what Rex felt when he put the
>>> red hot (1500 F?) end of the 1 inch bar into cold water.
>>>
>>> Better yet, if some metal or material has an even more
>>> pronounced thermal conductivity function it would make
>>> a great party trick.
>>>
>>> /tvb
>>>
>>>
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>>>     
>>>       
>>
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>>     
>
>
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