The Crushing Can Demonstration: Physics of Atmospheric Pressure
What Is Atmospheric Pressure?
Tiny particles of air, although invisible, have weight and take up space. The pressure exerted on objects—including ourselves—by these air molecules is known as air pressure. There are a lot of particles of air above us, and although we are largely unaware of this pressure pushing down, it is equivalent to a weight of 11,000 pounds. Why does this weight not crush us and other objects on earth? Humans and other objects are not crushed because of the air contained within them that pushes outward at a force equal to the pressure pushing down and in.
How to Crush an Aluminum Can Using the Principles of Air Pressure
- aluminum soda can
- pot with water covering the bottom
- enough cold water to half fill the bowl
- If the can opening is not completely immersed in the water, air will leak into the can and the demonstration will not work.
- If you are having trouble immersing the can under the water, flatten the sides slightly, making it easier to hold the can with the tongs.
- Practice turning the can upside down in the water with the tongs before you heat it.
- Always have a responsible adult present when doing this demonstration.
- Tie back long hair.
- Roll back long sleeves, if applicable.
- Rinse an empty soda can to remove any sticky residue.
- Make sure the bottom of the pot is covered with water.
- Put it on a stove element and turn on the element to high.
- Allow the water to heat until it bubbles slightly.
- Using the tongs, grasp the can and place the open end into the water allowing it to sit on the pot.
- Heat the can for about 45 seconds, keeping the tongs ready in case the can should tip.
- Quickly remove the can using the tongs and dunk into the bowl of cold water, open end into the water, and observe the result.
Crushing The Can DemonstrationClick thumbnail to view full-size
Applications Of This Effect
- The principle of air pressure used in the crushing can was applied successfully by Thomas Newcomen who developed the first successful steam engine.
- Syringes take in fluid by changes in air pressure.
- Changes in the human diaphragm change the air pressure in the chest cavity making inhalation and exhalation possible.
- Drinking through straws is made possible by air pressure changes.
The Physics Behind the Crushing Can
- The boiling water turns to steam, which is of much higher volume than liquid water and enters the tipped can displacing most of the air in the can.
- The steam in the can creates pressure equal to the outside air pressure so at this point the can is not affected.
- Dunking the can into the cold water causes the steam to condense.
- The water, a liquid again, is of much lower volume and therefore the pressure exerted against the inside wall of the can is greatly reduced.
- The effect happens very quickly as the outside air pressure can now exert a force on the outer sides of the can great enough to crush the can before water can re-enter the can to equalize the pressure.
What to Do With the Crushing Can Demonstration
This activity is fun and dramatic enough to capture the attention of all age groups of children. It would be an excellent demonstration of the wonders of air pressure in any science curriculum. For older students, physics teachers could modify the data collected looking at temperature, time and pressure variations. The depth of explanation could also be increased in the high school curriculum.
Gas laws in the chemistry curriculum could also benefit from this demonstration as it also can be explained by Charles Law, Boyle's Law, and Gay-Lussac's Law.
As a simple homeschooling demonstration or as a science lab in school, the Crushing Can Demonstration is simple and can be completed with easily obtainable materials.
NASA. Kids Earth: NASA Government. It's A Breeze: How Air Pressure Affects You. January 22, 2003.
Walker, Nick. That's The Way The Winds Blow. Air Pressure and Wind. 2005.