Solid, Liquid, Gas

States of Matter

States of matter Solid – closely, regularly spaced. Liquid – closely, irregularly spaced. Particles can swap relative positions – ‘flow.’ Gas – wide spaces, high speeds, random motion

Brownian motion The (seemingly) random motion of particles such as pollen in water or smoke in air.

Animation of Brownian motion

Gases exert pressure… through collisions between the container walls. The faster the particles move, the greater the pressure will be. Greater speed means more change in momentum and hence a bigger force and pressure.

Kinetic theory of gases This enables us to derive some simple equations about the behaviour of gases. We need to make some assumptions first.

Assumptions Gas particles are infinitely small. Collisions between particle and container are elastic (no loss of KE.) Particles are in constant and random motion and their motion depends only on the temperature of the gas. Molecules do not exert forces on one another. Container volume >> than total volume of particles.

Kinetic theory of gas – pressure equation p = 1/3ρ<c2> Where ρ is the density <c> is the root mean square speed

Example Air of density 1 kgm-3 is placed in a container at pressure of 1 atm. Calculate the root mean square velocity.

Distribution of speeds All the particles are not at the same speed but they follow a distribution.

Maxwell-Boltzmann distribution Obeys ‘statistical mechanics’