Pressure is generated by particles colliding with the walls of the container. A particle that bounces elastically off a wall reverses the perpendicular component of its velocity, and by Newton’s third law transfers to the wall an impulse equal to the change in its own momentum. Summed over billions of collisions per second, these impulses give a constant average force, and the force per unit area is the pressure.
The microscopic picture immediately explains what pressure depends on:
- the faster the particles are (high temperature), the greater the impulse transferred at each collision and the more frequent the collisions;
- the more numerous the particles are (high density), the more collisions occur in the same time.
In both cases the pressure increases. This is the microscopic translation of a familiar macroscopic fact: a hot or compressed gas pushes harder on the walls.
Link with temperature
Since temperature measures the average kinetic energy of the molecules, saying “hotter gas” is equivalent to saying “faster molecules”. This is the thread that, in the next note, allows us to turn the picture of collisions into the equation of state .
Collegamenti
Argomenti: Teoria cinetica dei gas Concetti: Pressione · Legge dei gas perfetti Competenze: Interpretazione micro-macro
Esercizi collegati: Problem — Temperature that doubles the pressure · Problem — Ranking of pressures by temperature · Problem — Pressure of an ideal gas