# Kinetic theory model for a gas

##### Class practical

Here phosphor bronze balls model a small volume of a gas. Click here to view a Windows Media Player movie of this experiment.

#### Apparatus and materials

Three-dimensional kinetic model kit

Power supply, low-voltage, variable

Retort stand, boss, and clamp

#### Health & Safety and Technical notes

See apparatus page for Electric motor, fractional horsepower.

photo courtesy of J Kinchin

The model kit may include its own motor.

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 d.c. terminals of the variable voltage supply in parallel to the field and armature terminals of the motor. Add the small phosphor bronze ball bearings until they cover about two-thirds of the base. Put the cap over the top of the tube to prevent the balls escaping and to cut down the noise.

More modern versions of this apparatus are available, from scientific suppliers.

#### Procedure

a Turn the voltage up so the vibrator is set in motion, and you have simulated kinetic theory motion.

b Gradually increase the motion to show that the kinetic energy of the particles increases.

c To aid visibility for a class, place a translucent screen behind the model. Put a lamp behind the screen to form a silhouette.

#### Teaching notes

1 Students should notice fewer particles in the higher parts of the tube.

2 You can put a cardboard disc in the tube to act as a lid. The disc will fall to the bottom when the vibrator is switched off. When the vibrator is switched on again, the disc rises to a position where its weight is just balanced by the force of the particles' bombardment, the pressure of the 'gas'. More discs can be added on top of the first one, so increasing the pressure of the 'gas'. It is easy to model gas law behaviour with this model.

• The higher the temperature (and kinetic energy of particles), the larger the volume, at fixed pressure.
• The higher the pressure, (weight of the disc), the smaller the volume at a fixed temperature.
• The higher the temperature, the greater the pressure (number of collisions with the wall) for a fixed volume.

3 Notice too the density of particles at various heights which models the atmosphere.

This experiment was safety-checked in February 2006