Three-dimensional model used to illustrate Boyle's law

Demonstration

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

Read our standard health & safety guidance

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.)

Procedure

1. Increase the voltage to about 6 volts, and mark the position of the piston on the cylinder with the OHP pen.
2. Next, add more paper weights to the piston to show that, as the pressure increases, the volume decreases.
3. 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 3 is to remind the class that Boyle's law applies only to a fixed mass of gas.

This experiment was safety-tested in March 2005