# Kinetic theory: two-dimensional model

##### Class practical

Students use marbles in a tray to explore models.

#### Apparatus and materials

For each pair of students

Metal tray lined with cork mat

Marbles, coloured, 20 - 24, about 1 cm diameter

#### Health & Safety and Technical notes

Beware of marbles on the floor.

The metal tray should have near-vertical sides and a thin cork base. Each tray should contain 20 to 24 coloured marbles.

The marbles need to be of random colours so the students can concentrate on a particular one if they wish. The thin cork base reduces the noise and helps students distinguish between collisions between marbles and those with the walls.

#### Procedure

Shake the tray in a random motion, on the table.

To model a solid: Tilt the tray so the marbles are at the bottom and the marbles are able to vibrate but not change places.

To model a liquid: Vibrate the tray more violently so the marbles occupy approximately the same space but are free to move around. These energetic marbles are modelling particles that do not spend long enough near any one particular particle to get locked into a crystal array.

To model a gas: Vibrate the tray more violently, keeping it flat on the table. This will cause particles to spread out more. Some particles may acquire enough energy to spill over the tray wall: diffusion.

#### Teaching notes

1 Students should be able to hear the difference between the glass-glass collisions of 'atoms' colliding with each other and the glass-metal edges of the tray of the 'atoms' colliding with the walls, modelling pressure of a gas.

2 Students may ask why the tray needs to be continuously agitated. A real gas does not need a continual supply of energy to the walls of the container. The model can only tell so much of the real story: the walls of all containers, on a molecular scale, are themselves in constant agitation.

3 Heating a gas causes the particles to move faster, increasing their individual kinetic energies of translational random motion.

4 Molecules also acquire other forms of energy than translational energy, such as spin energy and vibrational energy.

This experiment was safety-checked in January 2005