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

Gas pressure: forces required to compress and expand

Class practical

As with a spring, force is required to compress or to expand a gas.

Apparatus and materials

Bicycle pumps or syringes

Health & Safety and Technical notes

Ensure that there are no needles available to fit on the syringes.


Apparatus for demonstrating gas pressurea Drive in the piston of the bicycle pump or syringe with the outlet open. Repeat with a finger placed over the outlet. Ask students for their ideas about what produces the force acting against the applied force. 
b With the piston almost fully inserted in the syringe, place your finger over the outlet. Then try to pull the piston out. Ask students how the directions of the applied force and the force due to the gas have changed.

Teaching notes

1 If you judge your group to be sufficiently responsible to work with syringes, and enough are available, the activity can become a short class experiment. 

2 Note that, with the outlet blocked, the piston tends to return to its original position when you release it. Robert Hooke, the seventeenth century physicist, commented on the 'springiness of air'. 
3 You can use the activity as a reminder of the particulate nature of matter. Particles in a gas are much easier to push closer together or further apart than particles in a liquid or solid. This is because of the relatively large spaces and weak forces between them. Show the contrast between the behaviour of gas and liquid by filling a syringe with water, and then pushing on the piston. (But take care if you want to stay dry!) 
Any convenient solid object will illustrate the difficulty of pushing particles in a solid closer together. (If liquids and solids change shape when you exert a force on them, it is a result of rearrangement of particles rather than change in their spacing, unless the material is subject to high stress.) 
This experiment was safety-checked in January 2006


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