Hokay folks, I can't resist. I have to weigh in here. Somebody earlier in
this thread suggested an experiment where you cut a ring out of a cylinder
wall, cut the ring, straighten it, freeze it and rejoin the ends. The ring
is smaller, so of course the cylinder gets smaller when it's colder, right?
Wrong.
We all accept that a single chunk of metal gets smaller when colder. How
exactly does it contract? As it gets colder, all the molecules get closer
together, in effect shrinking towards the center. The ends get closer to
the middle, as do the sides. Now, let me suggest a mental experiment that
reasonably reflects the actual circumstances.
Take an engine block, and cut a section out of it that runs from the inside
of one of the cylinders to the outside wall of the block. For simplicity's
sake, pick a spot where there isn't part of the water jacket or an oilway.
Measure it from what was the inner face of the cylinder to the outer face
of the block, freeze it, measure it again and notice that it's a smaller
distance when frozen. The wall of the block between the inside of the
cylinder and the outside of the block got THINNER. So, the inside
dimension of the cylinder INcreases, while the outside dimension of the
block DEcreases.
But wait, you say, the section that you cut also got smaller in every other
dimension, so doesn't that mean that the cylinder gets smaller? No,
because the "pull" created by the walls of the cylinder getting thinner is
going on all around the cylinder, to one extent or another, so the cylinder
still gets larger in diameter, while the outside dimension of the whole
block decreases.
However, all this tension isn't even all the way around, so assuming that
the cylinder is really round when it's warm, it gets distorted into a
less-than-round shape when frozen. Various stresses are set up in the
metal, which cause it to distort all over the place (though it would take
very precise measurements to tell).
THAT'S how the "hole the material is surrounding" can be bigger.
>In a message dated 97-12-01 06:31:45 EST, madamson@compuserve.com writes:
>
>> When materials get cold, ice is an exception, they shrink. The atoms
>> occupy less space due to their decreased activity. Holes in material
>> become larger. A piece of material, say a piston, becomes smaller
>> therefore increased clearances.
>
>Michael:
>
>If material gets smaller when it gets cold - and it does - how can a hole
get
>larger? Since the interior of a hole is lined with material, and the
material
>gets smaller, how can the hole the material is surrounding be bigger?
>
>Ice also shrinks when it gets colder. It's just that in the transition
from
>water to ice, ice occupies more space than the water. Same is true on the
>other end. Steam, at the same pressure, occupies much more space than the
>equivilent quanity of water. If you increase the temperature of steam,
again
>maintaining the same pressure, it also expands. I don't know of any
material
>that gets larger as the temperature decreases, but there may be something
>that does - pure unobtainium, perhaps?
>
>Dan Masters,
>Alcoa, TN
|