Evesham pressure box
Classroom Activity
for 11-14
What the Activity is for
Using one volume of air for two pistons.
This activity shows how one volume of air can exert different forces on the two pistons. You can use it to discuss the microscopic model of pressure, relating the forces acting on the pistons to particle bombardment.
What to Prepare
- an Evesham pressure box (schools who taught Nuffield Physics are likely to have one of these)
- six identical 1 kg masses
- video clip (see below)
What Happens During this Activity
Show the apparatus, focusing on the construction of the box.
View the video clip (see below)
Make sure you point out that both pistons have the same volume of air in contact with them, but that one is four times the area of the other. Ask which piston will rise first if you inflate the bag. Inflate the bag (the large one rises first).
Check the pupils' answers against what happened. Now place a 1 kg mass on the smaller piston.
Ask how much mass needs to be added to the larger piston so that next time you inflate the bag the two pistons rise together (4 kg). Try it out.
Finish by reinforcing the point that the force exerted by the pistons depends on the area in contact with the air in the inflated bag. It's the same pressure everywhere in the bag, so the same number of collisions on each area of the bag. More area in contact with the bag produces more collisions, so leading to a larger force.
