12 Feb, 2013
PRESENTATION: A small amount of water is put into a soft drinks can and the temperature is brought to boiling point. As the preasure is low when the temperature is decreased, the atmospheric pressure crushes the can.
- Collapsing can, Paul Hewitt,Phys. Teach. 51, 8 (2013)
- An Extension of the Imploding Can Demonstration, L. M. Gratton and S. Oss, Phys. Teach. 44, 269 (2006)
- Collapsing soda cans and efficiency, James McGahan,Phys. Teach. 28, 550 (1990)
INTRODUCTION: Air is a gas with weight, which means that it exerts pressure. It was Evangelista Torricelli (1608-1647) who came up with a method to measure the pressure of the atmosphere by inventing the mercury barometer in 1643. The pressure of the atmosphere at any point is numerically equal to the weight of a column of air for a cross-section unit of area that extends upwards from that point to the top limit of the atmosphere. It is possible to deform a tin-can by using the imbalance of pressures on the inside and outside.
OBJECTIVE: The main principle is to make observations and conjectures about why the soft drinks can is crushed. This will be closely linked to the idea of pressure difference and related concepts.
MATERIALS: empty drinks can, tongs, container holding cold water, heater or Bunsen burner.
SETUP: A small amount of water is put into the can – about 1/8 of its capacity. The can is then placed on the Bunsen burner or heater and left until the water inside begins to boil. At that moment, using the tongs, the can is picked up and put into the water in such as way that its open mouth is the first part in contact with the water. The can is seen to be crushed.
EXPLANATION: When the water inside the can begins to boil, the steam expells the air that was originally inside into the atmosphere. When the can is turned over and put into the cold water, the steam condenses back into a liquid state. The water molecules in a liquid state are much more compact than those in a gaseous state, which means the water takes up much less space and this produces a decrease in the pressure in the can to much below that of the atmosphere. That is why the container implodes: to balance atmospheric pressure with the pressure inside the can.
CONCEPTS: material states, changes of state, atmospheric pressure, steam pressure, boiling point, thermodynamic equilibrium, ideal gases, ideal gas law, properties of gases, properties of liquids.
- Tipler P.A. Física. Barcelona: Reverté, 2010.
- De Juana J.M., Física General, Pearson, 2009.
- Serway R.A y J.W.Jewett. Física, Thomson-Paraninfo, 2010.
STUDENTS 2012-2013: Antonio Sixto, David Zúñiga, Javier Valladares, Pablo Pedreira
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