Problem of the
Day!!!
1/26
If you have a massive molecule and light
molecule with regard to mass and they are
at the same temperature in a closed
container which one would create the most
pressure?
Well actually they both would create the same pressure
because the massive one would have a slower speed
but would hit with more force and the lighter one would
have a faster speed but hit with less force.
The net result would be the massive one would hit less
but with more force and the lighter one would hit more
often but with less force so they both would contribute
the same pressure per unit time.
What happened inside your
can?
A review of the Can Crusher
Experiment
In your minds eye picture an empty aluminum can of soda
that has been sitting on your kitchen table for a long time.
Why doesn’t the can crush? Let’s not use the word
pressure for now.
Is the outside surface area of the can equal to the inside
surface area of the can?
Is the average temperature inside the same or different than
outside the can?
Is the average speed of molecules inside the can the same
or different than outside the can?
Does the inside of the can get hit more often by molecules
than the outside of the can?
When you heat air does it become denser or less dense?
When you heat air does the average speed of the
molecules increase or decrease.
Same question again: Why doesn’t the can crush?
If you heated gas molecules in an empty soda can would
their average speed increase or decrease?
If you heated gas molecules in an empty soda can would
they hit the insides of the can more or less frequently?
If you heated gas molecules in an empty soda can would
there be more molecules or less molecules in the can
after heating?
If you heated gas molecules in an empty soda can would
the number of hits on the inside of the can be equal to,
greater than or less than the number of gas molecule hits
on the outside?
Same question again: Why doesn’t the can crush?
Remember you can not use the word pressure.
If you inverted a heated soda can in water could more gas
molecules enter the can?
If you inverted a heated can in water would the velocity of
the molecules increase or decrease?
Conclusion: Why does the can crush?
Can Crusher Explained
• Gas pressure is really a factor of
number of hits and the force of those
hits.
• There are 2 ways to increase the
number hits, increase the number of
molecules or increase the velocity of
molecules or do both.
So okay let’s put it together to “see” why the heated empty can be
crushed when inverted in the water. As the can was heated gas
molecules began moving faster and faster in the can and a lot of
them actually left the can through the opening.
But the can did not crush because the few inside hitting the inside
at tremendous speed equaled the many outside hitting at a much
slower speed, so the pressure outside was equal to the pressure
inside. Now at some point we remove the can from the heat and
quickly invert it in water.
This does two things, it causes the interior gas molecules to cool
and slow down dramatically and it blocks the opening in the can so
that gas molecules can not get back inside.
Now there are a lot fewer gas molecules inside the can and they are
going at relatively slow speeds. So now the number of hits on the
inside is significantly less than the number of hits outside so much
so that the can is crushed.
Thinking about gas pressure as number hits of molecules
is a useful model. In addition it is useful to think of a gas
as space that has mostly nothing in it.
A “real empty space” that has a few things in it that are
relatively far apart. This explains why it is so
compressible.
The space is maintained by the few particles in it hitting
the walls enough times so that outer hits do not
overwhelm them: if they are overwhelmed the space gets
smaller until the “pressure” due to hits is equalized and
the walls are kept away by the pressure of the gas
molecules hitting them.
What if we pre-cooled the can
before submerging it?
• If you were to pre-cooled the heated can
before inverting it in the beaker of water.
There was less of a dramatic difference in
pressure inside and outside because the
force and number of hits inside and outside
were not that different.
Therefore the can slowly fills with water
without it crushing. If you practice this one
and get the timing right it will almost fill with
water and slowly sink in the beaker. This is
dramatic if you have pushed the unheated
can down into the beaker of water before –
because in that case the water is pushed
out of the beaker with very little water
entering the “empty” can.