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.

Read our standard health & safety guidance

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

1. Shake the tray in a random motion, on the table.
2. 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.
3. 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.
4. 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 spill over the tray wall: diffusion.

Teaching Notes

• 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.
• Students may ask why the tray needs to be continuously agitated. You do not need to continually 'heat' a real gas. 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.
• Heating a gas causes the particles to move faster with translational random motion. The energy stored kinetically increases.
• Molecules also spin and vibrate. Transferring energy to a gas will increase the energy stored in a variety of ways.

This experiment was safety-tested in January 2005