Moving molecules

It might seem obvious that a gas will expand to fill its container, but this simulation aims to show why this happens at a molecular level. The 'molecules' in this container are set up to bounce off each other and the walls elastically (so there is no change in total kinetic energy) and don't interact unless they collide (they don't attract each other or have any long range repulsion). The molecules have a range of sizes and start with random position, speed and direction.

When the container grows or shrinks, the molecules will adjust to fill the new area available because they are continuously moving.

Learning outcomes

• Appreciate why randomly moving molecules will adjust to fill their container.
• Explain why this happens using thermodynamic principles.

Instructions

The 400 molecules will start moving as soon as the page loads.

You can adjust the width and height of their container using the sliders towards the top of the page.

Try quickly increasing the size of the box and watch what happens to the molecules as they fill the new space.

If the molecules are pushed close together (by making the box very small) they may stick together, but should break apart again when they are left to equilibrate in a larger container.

Questions

1. Does the disorder in the system increase or decrease when the molecules spread out in a larger box?
2. Which law of thermodynamics explains why gases expand to fill their container?
3. Does the internal energy of a gas increase or decrease when it is compressed?
4. What type of system does this simulation represent?
5. When a gas is compressed quickly, are the heat and work changes positive, negative or zero?
6. What happens to any energy that is transfered?
7. Does the simulation reflect this? If not how could it be changed so it does?