Collapsing Soda Can


 
Investigating the Force of Pressure: P = F/A

 
 
 
You'll need a small burner, an empty soda can, tongs, and a pan of cold water (some ice in it would be good). 

Put about 5 ml of water in the soda can (just enough to cover the bottom of the can). Heat the can over the hot plate or burner until water is boiling vigorously. 

After it is boiling, in a single motion, using the tongs, remove the soda can from the burner and invert it in the bowl of water, submerging the opening. What do you observe? 

 


Watch closely because the can will implode very quickly.

Simple Pressure Questions

1. What was in the can initially (after soda is gone) along with the water?

2. What changes take place in the can when the water is boiled?

3. What happens to the air in the can as water vapor is formed?

4.(a) Describe what happened in your own words. (b) Draw the changes
that took place in the can before and after the implosion using arrows to
indicate the pressure.

 5.(a) What force was acting on the outside of the can? (b) How might one
calculate that force? (see rationale for hint)

 6. Does the calculation change if you are at different elevations (compare
Flagstaff, AZ at 7000 feet (2.14 km) with Anchorage, AK at sea level)?
(note: pressure is approxemately halved for every 5 km of altitude up to 20 km)

 7. (a) What would happen if we took an empty pop can and took it down
to 50 feet below sea level? (b) What if you took a full can down to that depth?
 


Rationale

Initially the can is filled with water and air. By boiling the water it changes
states, from a liquid to a vapor. The water vapor forces the air out of the can.
When the can is inverted and submerged the vapor is rapidly cooled, causing
the water vapor to change states again, condensing back to liquid water.
The water vapor occupied all of the space in the can and was in equilibrium
with the atmospheric pressure while it was being heated. Once it is cooled,
however, the liquid water occupies only a small volume of the can while air
should occupy the rest of the volume. But by submerging the opening of the
can, air is prevented from rapidly flowing back into the can causing a decrease
in internal pressure. We see the atmospheric pressure pushing on the can
causing it to collapse. Using Fa = paA (where F = force, A = area, and pa = 10N/m2 = 1atm at sea level) one can calculate the force on the can. Pressure is approximately halved for every 5km of altitude up to 20km, therefore the calculation for force does change for Flagstaff.



 

References:

This experiment can be found many places in written form including:

1) D. Rae Carpenter, Jr. and Richard B Minnix (1993). The Dick and Rae
Physics Demonstration Notebook, Published by Dick and Rae Inc.
Lexington, VA 24450-0304.

2) Wilson, J. College Physics. Engelwood Cliffs, NJ: Prentice Hall; 1994.

3) This and other science demonstrations from Nebraska Educators 
Really Doing Science (N.E.R.D.S)
 


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