Pressure exerted by a stream of balls
This demonstration models gas molecules hitting a wall.
Apparatus and materials
Toy shop marbles, steel balls or lead shot
Spring balance, domestic or lever-arm balance
Tray, a deep one, or lots of books
Health & Safety and Technical notes
Either a domestic balance or a lever-arm balance can be used for this experiment. Turn over the scale pan so that the balls bounce off without collecting on the pan. Place a deep tray under the balance (or set up a barrage of books round the balance) to catch the balls.
Pour, by hand, a stream of balls, from a height of 30 cm or so, onto the inverted scale pan on the balance.
As the balls hit the pan and bounce off they exert small impulsive forces. The comparatively great mass of the pan and balance smear out these forces into a steady push shown by the balance. Gas pressure results from impact forces like these spread over a surface.
P = F/A, where P = pressure, F = force, and A = area over which force acts.
This experiment was safety-checked in March 2005