Developing a model of pressure
What the activity is for
This activity provides a vital set of experiences and an introduction to pressure as a large scale measurement of the state of the particles in a fluid. You can also make the link that the force acting on the walls of the fluid container comes from the collisions of the particles with those walls. The higher the pressure, the more collisions a second, so the larger the force acting.
What to prepare
- a wooden tray with 20 marbles per group, with a wooden divider (best)
- a kinetic theory demonstration (very much second best)
- a set of air filled syringes, sealed at one end
What happens during this activity
Shake the tray of marbles. Listen for two kinds of collisions – the snick of marbles against each other and the clack of marbles against the side. Point out that this can be a good model for a gas. You might ask in what ways it is a good model, depending on where the pupils are up to with their understanding of particles.
Once you have established that the marbles represent the particles in the gas, you can ask which of two collisions – snick or clack – will support forces on the walls. This is a very engaging model of the origins of pressure, so we urge you to use it.
You can take it further by asking what will happen to the forces acting on the walls if you squeeze all the particles closer together. Pupils can feel this by squeezing the air in the sealed syringe. Take a moment to imagine the particles at work there – can you hear them? No? OK, so your imagination will have to work overtime – this is physics, and so you need a good imagination.
Then return to the model and insert the wooden divider. Hear the increase in the number of collisions a second as you squeeze the particles into a smaller and smaller space. See if you can get pupils to relate this to the experience of compressing the air in the syringe. Try also to get them to relate the noises they hear – so the number of collisions a second – to the forces they exerted with their fingers. You might point out that the density of the gas has increased – but go on to suggest that physicists cannot relate the force acting on the sides of the container to the density alone.
Ask pupils to shake the trays harder and ask what has changed about the gas now? (Temperature). What has changed about the number of collision a second? (More). Then you can introduce pressure as the measure that summarises the effects of density (how many particles there are in a particular volume) and temperature (how fast these particles are whizzing around). If you know the pressure, then you can predict the force, because the pressure is a measure of the number of collisions per second.