A Possible Theory of Why
So now we are finally ready to put forward a possible theory for why the plunger moves so that the air particle density around the syringe and inside the syringe are the same.
“If the air particle density inside the syringe and the air particle density around the syringe are different, then the rate of particle collisions on the top of the plunger will be different than the rate of particle collisions on the bottom of the plunger. Whichever side has the greater particle density will push harder on the plunger and will push the plunger in that direction. As the plunger moves, the volume on the side with the greater particle density will increase while the volume on the other side will decrease. The plunger will continue to move until the rate of particle collisions is the same on the top and the bottom of the plunger — and that will happen when the air particle density is the same both inside the syringe and around the syringe.”
Adjust the air particle density inside the syringe to see if this is what happens.
Keep in mind that this is still just a theory and it hasn’t been tested yet through predictions (hypotheses) and experiments. But it is a reasonable theory and it makes sense based on what we know. Hopefully, at this point, you have a much better understanding of air, air particles, air particle density, particle motion and collisions, and the forces that air particles exert on walls (movable or otherwise).