🚀 go-pugleaf

In article <10DEC199219275429@trentu.ca> lchardon@trentu.ca (PETIT PAPA NOEL.)
writes:

>        In my thermodynamics class, we were all happily playing with liquid 
>nitrogen, freezing squash balls, making erasers explode (BTW, why do 
>plastic erasers explode when in liquid nitrogen?) and all that kind of 
>things that you do when you have 10 litres of nitrogen to waste, when a 
>prof dropped by and told us to try to immerge some piece of metal in the 
>nitrogen. Sure enough, somebody pulled out a penny and dropped it in a 
>full styrofoam cup. What happens is the following:
>The nitrogen will boil for some time, like it boils when you drop anything 
>in it, but then suddenly it will boil really hard, about 3 times more 
>violently than it did before, for about half a second, and then it will 
>just gently bubble away (When the metal is in thermal equilibrium with it, 
>i guess...).
>
>        WHY ???????????????
>        WHY JUST METALS ???

As Michael Kagalenko has beat me to the punch in answering the first question,
that the so-called "Leydenfrost effect" is exemplified by this demonstration, I
will answer the second question. The effect you have described is seen in
metals because they are thermal conductors. The entire surface of a conducting
object is approximately at the same temperature. Thus the critical temperature
for the breakdown of the vopor barrier is reached simultaneously over the
entire surface, resulting in a large sudden increase in the heat transfer rate
from the metal to the liquid nitrogen. For a thermally insulating material this
breakdown occurs at different times for tifferent places on the surface, and
the heat transfer within the object limits the rate of transfer as well.

I might opine that your thermodynamics professor is a piker. Here in western
Canada I do that demonstration with a machined one-kilogram copper cylinder.
The effect is much more dramatic. Your prof could, at the very least, spring
for a silver dollar! :-))

Finally, I will reveal a demonstration similar to one you did which I
originated, and which I have never explained satisfactorilly. If you take a
Pink Pearl eraser and immerse it for one minute in liquid nitrogen, and then
remove it and place it on an insulating stand in the ambient open air, it will
*subsequently* explode. This demonstration is not highly reproducible, and it
will not work at all with other brands of erasers in my limited experience, but
it works with more than one size of the correct brand. If any kid out there
with access to a liquid nitrogen source would like a nice science fair project
I'd be glad to collaborate. Optimization of time and conditions, examination of
probable effects, etc., are still to be done. Somehow I felt that this project
would be inappropriate for me to undertake myself, and I think no graduate
student would find it attractive.

Merry fig day,

Leigh