Abstract:
This activity is a
useful one for giving students of any age an introduction to ideal gases. The connection between microscopic
activity and macroscopic observable properties is a tough thing to comprehend,
but with this lesson, students get a hands-on analogy to keep in mind and use
as a platform to develop their understanding. The activity is based around the simple idea of shaking
balls on a large sheet and observing their motion. By drawing the studentsÕ attention to particular aspects of
the ballsÕ motion, the students will be able to see the relationships between
different properties. With some
helpful provocation, more advanced students might be able to work out a crude
form of the ideal gas law.
Materials:
Large sheet (e.g.,
canvas drop cloth)
Tennis balls (at least a
dozen)
Ping pong balls (optional)
Large lightweight board
(optional)
Procedure:
This works best with
groups of ~6-12 students, so for bigger classes, multiple set-ups (or having
students rotate between shaking and observing) will be necessary.
Have the students spread out around the sheet and
hold it in the air between them.
With balls placed in the middle of the sheet, have them shake the sheet
and observe the motions of the balls.
Below are listed 4 basic
activities with related questions for students. These can be mixed and matched, and added to or subtracted
from, to the taste of the teacher.
1) With only the tennis balls on the sheet, have the
students slowly increase the speed of their shaking. Make sure they observe any changes in the motions of the
tennis balls.
a. Do all the balls move at the same speed at all
times?
b. Do the tennis balls move faster or slower (on
average)?
c. Do they bounce higher?
d. Do they collide with each other more frequently?
Shaking
the sheet faster is analogous to heating a container that has gas in it. Heating a container causes the walls of
that container to vibrate and shake faster, just as shaking the sheet causes
the sheet to vibrate faster, and these vibrations cause changes in the
particles (balls) in contact with the container (sheet).
2) Start with only half the tennis balls on the
sheet. While the students shake at
a constant speed, add the rest of the tennis balls.
a. Are the balls moving at a different speed just
because there are more of them, or are they still moving at the same average
speed?
b. Is there more energy in the ÒcontainerÓ now that
there are more particles?
Explain
that in a gas, even if temperature stays the same, by increasing the number of
particles in the same size container, there is more energy in the gas. The particles move at the same speed,
so there is the same energy per particle, but there are more particles, and hence more energy.
3) Place both the tennis balls and ping pong balls
on the sheet. Starting with the
students holding the sheet still, have them slowly start shaking the sheet and
increase the speed of their shaking over time. Have them observe how the two types of balls respond.
a. Which type of ball starts bouncing around sooner
(i.e. at lower ÒtemperatureÓ?)
b. Which type of ball is moving faster and bouncing
higher?
c. Why are the two ÒparticlesÓ reacting differently
to the same stimulus? Are the ping
pong balls at a different ÒtemperatureÓ than the tennis balls?
This
should be used to help students understand that a lighter particle, given the
same amount of energy as a heavier particle, will move faster. It also demonstrates the effects of
inertia in collisions, as lighter particles ricochet away from a collision
faster than heavier ones do (just as the pin goes flying when the bowling ball
hits it, not the other way around).
4) Other possibilities include:
a. Having some students hold a large board above the
sheet and explain the connection between repeated collisions of ball against
board with the force of air pressure that seems to be constant.
b. Having the students use only half of the sheet
and see what effects a smaller ÒcontainerÓ has on the gas (though this was not
very clear to our students when we taught this).
Recommendations:
1) This lab could easily be used as a repeated theme
to come back to throughout a syllabus.
Perhaps doing one of these activities at one point, and another a week
later, etc. would help the studentsÕ retention, and not overburden them with
too many concepts at once.
2) A great partnering lab to this one is having the
students play with a well-designed java script available on the internet. On the following webpage, the ÒGas
PropertiesÓ interactive tool fits perfectly with this lab. Coming up with a few investigative
questions to focus studentsÕ playing with the tool is very productive.
http://www.colorado.edu/physics/phet/web-pages/simulations-base.html
3) The lab works much better if the students can
keep the sheet as taut as possible while doing the shaking. Any significant slack will cause all
the balls to roll to the middle and move together rather than like independent
particles.
4) When investigating light vs. heavy particles,
make sure at least some of the
light balls are towards the outside when starting the shaking. If they are all surrounded by the
tennis balls, they can be hard to see and the tennis balls will smother them
and suppress their motions.
5) Make sure throughout the lab that the students do
not all shake together in rhythm, even though it is likely to be natural human
tendency. This causes all the
balls to move together. Have them
all shake out of phase as much as possible.
6) Some students may intentionally try to have the
balls fly off the sheet into someone else, or some may panic if a ball
inadvertently comes in their direction.
Try to keep the balls on the sheet as much as possible. With a large enough sheet and constant
pressure on the students to make observations, the distraction should be
minimal.
Conclusion:
This lab was developed
as part of a large curriculum on the concept of pressure. The goal was to help students construct
a conceptual model of what is happening in a gas at a microscopic level to
explain the changes in pressure and volume that they observed and learned in
other labs, but we found it to be a generally productive lesson on its own and
extremely adaptable. It was taught
to a group of students who ranged in age from 12 to 17, and the lesson seemed
to work equally well for all ages.
We hope that it works well for you as well.
Special Thanks to John
Vaillancourt (CalTech) and Sarah Hansen for their original work on developing
and teaching this lab.
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