Yttrium2
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What cools faster
A hot piece of glass that you hold onto
Or
A hot piece of glass that cools in the air?
My guess is that having your hand around the hot glass would insulate it, and conduct.
So which would it be?
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j_sum1
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Your suspicion is probably correct -- depending of course on the temperature of the glass.
Glass held in air will lose heat by the mechanisms of radiation and convection. Radiation heat transfer is proportional to the difference of the
fourth powers of the glas temperature and the surrounding temperature. If the glass temperature is high, then this term will dominate. OTOH, if the
two temperatures are low then heat loss by both radiation and convection will be quite small.
Glass in the hand will lose heat by conduction. This is proportional to the difference in the temperatures of hand and glass. If the two are close
then this could easily be greater than radiative or convective heat transfer. But it will be considerably smaller if the glass is hot. Your hand is
not particuly conductive and so not great at getting the heat away. And also it wil tend to geat up as the glass cools thereby lowering the heat
transfer rate further.
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Sulaiman
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I am sure that hot glass will be cooled quickest by holding in the hand;
1) The radiation heat transfer does not care how far or near the receiving surface is
2) Conduction of heat is almost always greater through solids than gasses
3) The blood will act as a cooling fluid / heat exchange mechanism as it is continuously pumped around the body
but I'd prefer to cool hot glass with air than my hand
CAUTION : Hobby Chemist, not Professional or even Amateur
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Tsjerk
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This is correct, the glass doesn't ''know'' what is surrounding it, it will radiate in exactly the same manner in the two different situations (air
vs. hand).
[Edited on 29-1-2018 by Tsjerk]
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j_sum1
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Quote: Originally posted by Tsjerk  |
This is correct, the glass doesn't ''know'' what is surrounding it, it will radiate in exactly the same manner in the two different situations (air
vs. hand).
[Edited on 29-1-2018 by Tsjerk] |
For radiation, $$ Q=\epsilon \sigma (T_{a}^{4}-T_{b}^{4})$$
epsilon is the emissivity (=1 for black body radiation)
sigma is the stephan-boltzman constant
Ta and Tbare the surface temperatures of the radiating body and the body being radiated to. (One way to think of this is that
body b is radiating back to body a.)
For objects that are properly in contact, radiation is not generally considered to be a heat transfer mechanism. The reason is that within a very
short space of time the two touching surfaces will have the same temperature. If Ta = Tb then radiative heat transfer Q does
not occur.
https://en.wikipedia.org/wiki/Heat_transfer#Radiation
That said, I am going to change my answer -- except for extremely hot glass. The stephan-boltzman constant is pretty small. It will take a rather
high temperature for heat transfer via radiation to exceed heat transfer via conduction. For a good analogy, imagine a steak in the proximity of a
hot cast iron pan. The steak will cook faster if it is touching as opposed to receiving radiative heat only. (And in case anyone is wondering, both
cast iron and polished glass are close to black bodies for radiation purposes. Glass has an emissivity of 0.95 and seasoned cast iron is in the
vicinity of 0.9.)
Convection complicates things somewhat. As a first approximation the rate of transfer is proportional to the temperature difference but it is highly
non-linear and dependent on a very large number of variables. In general though, in a gas, convective heat transfer is less efficient than conductive
heat transfer.
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roXefeller
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What I was taught in my college coursework, was that radiation transfer, at typical temperatures, is only significant when the convection transfer was
natural circulation (as opposed to forced or driven). But the question seems ill defined... fastest to cool to what temperature? Are we cooling to
90F? My hand is usually hotter than room temperature. If we assume fastest to reach 90F and the air in question is 90F, I would say the hand would
cause faster cooling. Heat transfer is heavily dominated by the density of materials and specific heat. Air is very poor in this respect while flesh
is much better.
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Texium
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Thread Moved
1-2-2018 at 07:27 |
LearnedAmateur
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If we’re looking purely at conduction, then here’s something to consider. When you hold hot glass, your hand heats up as a result, which slows the
decrease in temperature as the temperature gradient is smaller. When air cools the hot glass, the heated molecules rise up due to a decrease in
density, where they’re replaced by colder molecules and the process continues. I get this is convection, but when looking directly at the interface
with the glass, the conduction of heat would be faster for air hence it would cool down more quickly, simply because the receiving medium will always
be at a lower temperature, including the fact that air is pretty much always at a lower temperature than the human body (25C on average compared to
37C) unless you were doing experiments in Death Valley or something where air temperatures can reach the mid 50s.
I guess if you wanted an accurate answer, you’d have to factor in all sorts of stuff. Initial temperature of the glass, exact air/skin temperature,
humidity, pressure, the type of glass, that’s just some of the variables that would factor in - you’d get different results trying to figure this
out at the poles compared to the equator, at sea level or on top of Everest.
In chemistry, sometimes the solution is the problem.
It’s been a while, but I’m not dead! Updated 7/1/2020. Shout out to Aga, we got along well.
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happyfooddance
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Yes, the question is not clearly stated so no clear answer could be given. For example, if you are talking about cooling glass from 30°C to 10°,
obviously cool air would win over a warm hand, but 30°C air would do nothing.
If we are talking about high heat a hand will certainly always win, for the plain reason that Sulaiman already stated:
| Quote: Originally posted by Sulaiman |
The blood will act as a cooling fluid / heat exchange mechanism as it is continuously pumped around the body
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