MOLECULAR MOTIONS AND TEMPERATURE 163 



pressure of the gas remains constant unless the volume is 

 changed. This is the experimentally observed phenom- 

 enon of which Boyle's Law describes the general case. 

 Hence the average kinetic energy of the molecules 

 must remain constant and also the average internal 

 energy of its atoms. The molecules, therefore, must 

 rebound, either from mutual collisions or from im- 

 pacts against the walls, with the same average (squared) 

 velocity. 



We are now ready to answer the question of page 

 160 as to the effect of an impact 1 with the walls. Dur- 

 ing it the molecule is first stopped and then started in 

 the opposite direction with an equal velocity. If its 

 mass is m and this velocity is v there is a change of 

 momentum of mv in stopping it and another of equal 

 amount in imparting to it the opposite velocity. The 

 total change hi momentum 

 accompanying each impact 

 is then 2mv. 



An expression may now 

 be obtained for the pressure 

 which these molecular im- 

 pacts exert on the end be of 

 the box of Fig. 13. If we 



multiply the change in momentum per impact (i.e. 2mv) 

 by the number of impacts per second we obtain the total 

 change of momentum per second, that is, the force 

 exerted. Dividing this force by the area be gives force 

 per unit area, that is, the pressure. 



1 Bernoulli! in 1738 was the first to suggest that the molecules 

 of a gas were in constant motion and that their impacts resulted in 

 a pressure. 



