On Sep 15, 2011, at 2:01 PM, Jed Rothwell wrote:
Horace Heffner wrote:
A 0.7°C temperature rise is significant with any thermocouple.
That can't be noise. There is no question there must be a heat
source in the cell.
Yes - it is the 80 kg of cell metal which has stored heat.
Stored heat can only be released monotonically declining. The rate
cannot increase, as far as I know. It is passive. The temperature
can only rise if you increase the insulation or slow down the flow
rate with this system. Or generate heat, of course.
What Catania calls "thermal inertia" can only release heat at a
declining rate.
This is not true. There can be a slow transmission rate in the
flow of heat pulses through matter.
Of course there can be a slow transmission rate or flow of heat! I
didn't say you can't have slow transmission; I said it cannot speed
up on its own without some external or internal change. As far as I
know that is thermodynamically impossible. Can you explain how this
would work, or cite an example of this happening elsewhere? The
flow of heat can only slow down, as the temperature difference
between the two bodies decreases, per Newton's law.
It can never increase the temperature above where it reached when
there was power going into the cell.
Again not true.
Sez who?
Sez me. Who else? 8^) The magic words are open sez a me.
Lemme put it this way: that is my understanding of thermodynamics,
and I have never seen data from a calorimeter that contradicts it.
Calorimeters would not work if this was possible. You could not
tell the difference between power and a situation in which metal
suddenly decides to increase conduction for no apparent reason,
with no change in the lattice.
- Jed
Metal does not change its characteristics.
Jed, it appears you need to read up on "thermal diffusivity".
http://en.wikipedia.org/wiki/Thermal_diffusivity
Your misconceptions demonstrate why high precision thermal FEA
analysis is required to understand dynamic thermal systems. Using
FEA you can see thermal pulses migrate through systems, and get an
intuitive feel for the dynamics.
Here is a site that might be of use to you:
http://ccl.northwestern.edu/papers/ABMVisualizationGuidelines/palette/
examples/Heat%20Difussion/
http://tinyurl.com/3emm6g8
However it only shows dynamic thermal flows through a single plate
made all of the same material. There is no facility for insulating
plate edges etc. Note that you can move the edge temperature sliders
while the simulation is running.
If you visualize chunks of metal as capacitors and the heat
conduction paths between them is resistors, then you can see that if
you apply a variable signal to an RC network that it can propagate
the signal in the form of pulses. It is possible I suppose to build
the thermal equivalent of an RC transmission line for carrying
thermal pulses. I don't know what application such a thing would have
though.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
