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Solids Holding Shape, and Binding
Name: Phil
Status: other
Grade: other
Location: IL
Country: USA
Question:
Since matter is composed of chaotically, rapidly moving
particles, why do solids keep their shape? Why do they not
"evaporate"? A corollary question -
If you clamp two pieces of wood
together, why do they not join into one piece?
Replies:
Phil,
Solids do sublime (the equivalent of evaporation in solids). Think of
dry ice and moth balls. The reason we do not observe most solids sublime
is that the energy that has been transferred in order to get to room
temperature is not high enough to allow most of the particles in the solid
to escape from the bulk object.
Look up a graph called "Distribution of speeds" (it might show up as
distribution of particle speeds or distribution of kinetic energy). The
represents the distribution of particle motion at one particular temperature.
Thus, evaporation (of liquids) means that a certain percentage of particles
are fast enough (have enough kinetic energy) in order to escape the liquid
phase and enter the gaseous phase. Draw a vertical line on the far right of
this graph and call this the escape velocity - particles to the right of this
line will be fast enough to escape (evaporate).
Now imagine that for a solid, this graph is skewed to the left (lower speeds),
there will now be fewer particles which have the speed to escape. Thus, while
it is true that particles are in motion, they do not have enough motion to
break the solid-solid interactions in order to enter the gas phase.
The same reasoning can be used for why wood does not bond. While solid
particles do move around, they move around in place. In other words, they
vibrate. As such, there is no translational motion, just a vibration that
essentially retains the same center of mass for each particle, despite the
motion.
Greg (Roberto Gregorius)
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Update: June 2012
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