Changing the volume available for "molecules" in the Molecular Motion Demonstrator (MMD) has an effect on how often the molecules hit the walls of their container, illustrating Boyle's law.



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Possible Incorporated Topics:

  • thermodynamics
  • molecular motion
  • gases


Boyle's Law says that the product of the pressure (P) and the volume (V) of a gas is a constant at a given temperature (T). In symbols...

PV = constant (for a given T)

Practically speaking, this means that if the volume of the gas decreases, the pressure must increase, and vice versa.

If the MMD is partitioned and several "molecules" are placed on one side of the partition, the molecules will move around randomly and collide with the walls of the container at some average rate. If the partition is moved to decrease the amount of space available to the molecules, the molecules will collide with the walls more frequently. Because the molecules are colliding more frequently with the walls, they exert more force (and therefore more pressure) on the walls. To summarize, a gas will have a higher pressure if it occupies a small space than if it occupies a large space.



Figure 1: Boyle's Law in action




  • 12 blue beads
  • the molecular motion demonstrator (MMD)
  • a metal barrier



  • Set up the MMD on a flat surface.
  • Place 12 blue beads in, adjust the speed to a medium setting, and keep this setting for the duration of the experiment.
  • Without the metal border, let the MMD run for a little while, observing the rate at which the beads strike the walls.
  • Now insert the metal barrier about 3 cm from one edge to decrease the amount of space available to the beads.
  • To insert the barrier without changing the speed of the MMD, disconnect the power supply before placing the barrier.
  • Now connect the power supply again. As before, observe the collisions for awhile, then decrease the space available to the beads again. Each time the volume available to the beads (molecules) decreases, the number of collisions with the walls (and thus the pressure of the "gas") increases.