Back to Our Syringe
So what does this have to do with our syringe? Remember, we started thinking about particle motion, particle collisions, and forces on walls because we wanted to develop a theory that would help us understand the behavior of our syringe systems. We noticed that when the air particle density around a closed syringe was greater than the air particle density inside the syringe, the plunger would move down until the air particle density around the syringe and inside the syringe were the same. And when the air particle density around a closed syringe was less than the air particle density inside the syringe, the plunger would move up until the air particle density around the syringe and inside the syringe were the same.
To truly understand the behavior of our syringe systems, we need to be able to answer the following two questions: (1) How does the plunger move up or down when no one is touching it? and (2) Why does the plunger move so that the air particle density around the syringe and inside the syringe are the same? To answer these two questions, it will help if you can think of a closed syringe as a box and the plunger inside the syringe as a movable wall that can move up and down.
As you can see, as the air particles inside the syringe move around and bounce off the walls, they exert forces on those walls, pushing all four walls outward. However, only one of the walls (the plunger) is free to move.