Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Crushing an evacuated container

Demonstration

Crushing an evacuated can.

Apparatus and materials

Tin can with bung and tubing

Length of pressure tubing

Empty PET drink bottle, 2 litre

Sharp knife

Vacuum pump

Balloon

Safety screens

Bell jar (OPTIONAL)

Health & Safety and Technical notes


During this demonstration eye protection should be worn.

Take care when clearing away broken glass.

1 The tin can should be rectangular, not cylindrical and preferably 5-litre size or larger. It should have a well-fitting bung in the top with a short length of glass or brass tubing through it.

2 Care should be taken in choosing the can so that its previous contents do not harm the pump.

3 Cut off the curved bottom of the plastic bottle with a sharp knife, ensuring that no sharp points remain. A belljar can be used as an alternative. The outlet of the bottle should have a well-fitting rubber bung with a glass tube through it.


Procedure


Apparatus set-upa Connect the can by pressure tubing to the vacuum pump.

b Pump the air out slowly and the can will collapse. Instead, or as well, you can evacuate a PET bottle or a hollow rubber toy.

c Cut the neck from a balloon and stretch the remainder across the open end of the bottle. It may be held on by its own tension, but you may need to use string or thread.

d Connect the bottle by pressure tubing to the vacuum pump. Pump out the air slowly to show the effect on the rubber sheet – its shape is roughly hemispherical. Eventually the entire bottle may collapse.

You will need eye protection and a safety screen for this part of the experiment.

Vacuum pump

e Close the bell jar with a thick glass sheet (using vacuum grease to seal it) with a partially inflated balloon inside it. Or this can be as effectively done by putting the partially inflated balloon inside a bottle connected to the pump.

f Place a very thin sheet of glass over the bell jar. Pump air out of the bell jar very slowly indeed so that the breaking of the glass can be seen. The glass, which must be very thin, should be sealed to the rim of the bell jar with vacuum grease. A decent trap and filter placed in the bottom of the bell jar should prevent glass chips reaching the pump.

 

Teaching notes


1 This demonstrates that a container needs pressure inside it to balance the pressure of the atmosphere outside. If the air inside the container is evacuated, the container collapses.

2 An alternative approach to crushing the can is as follows. Fill a rectangular can with water to a depth of about 0.5 cm. Put it over a Bunsen burner or gas ring and boil vigorously so that the steam drives out all the air. Turn out the gas flame or remove the can from it, holding it with an oven cloth. Add a tight stopper. Allow the can to cool and it will collapse under atmospheric pressure as the steam inside condenses. The condensation can be speeded up by pouring cold water over the can.

This can be a spectacular demonstration done by a teacher with a large oil drum.

This experiment was safety-checked in July 2007

 

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