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

Three-dimensional model used to illustrate Boyle's law


Apparatus and materials

Electric motor, fractional horsepower

L.T. variable voltage supply (capable of 8 A at 12 V)

Kinetic theory model kit (transparent cylinder with small steel balls)

OHP pen (soluble ink)

Retort stand, boss, and clamp

Health & Safety and Technical notes

Apparatus set-up to illustrate Boyle's Law

See the warning on the apparatus page on the use of a Electric motor, fractional horsepower.

Fix the rubber base over the lower end of the tube, which is held as shown in the diagram. Adjust the height of the tube until the rubber base is a millimetre or two above the vibrating rod in its mean position. 
Connect the DC terminals of the variable voltage supply in parallel to the field and armature terminals of the motor. About three dozen small, phosphor bronze balls are needed. Put the paper disc, with wire attached, in the tube as a piston. The wire goes through the hole in brass cap which should be put over the top of the tube. 
Add cardboard weights on top of the disc to increase the 'pressure' on the 'gas'. 
(More modern versions of this apparatus are available, of course.)



Example of fixed mass of gas

a Increase the voltage to about 6 volts, and mark the position of the piston on the cylinder with the OHP pen. 

b Next, add more paper weights to the piston to show that, as the pressure increases, the volume decreases. 
c Now turn off the motor and double the number of balls in the cylinder. Then switch on the motor again. The paper piston will now be seen to settle higher up the tube. Additional paper weights must be added to return it to its original position.

Teaching notes

The point of step c is to remind the class that Boyle's law applies only to a fixed mass of gas. 

This experiment was safety-checked in March 2005