<P> Kinetic theory provides insight into the macroscopic properties of gases by considering their molecular composition and motion . Starting with the definitions of momentum and kinetic energy, one can use the conservation of momentum and geometric relationships of a cube to relate macroscopic system properties of temperature and pressure to the microscopic property of kinetic energy per molecule . The theory provides averaged values for these two properties . </P> <P> The theory also explains how the gas system responds to change . For example, as a gas is heated from absolute zero, when it is (in theory) perfectly still, its internal energy (temperature) is increased . As a gas is heated, the particles speed up and its temperature rises . This results in greater numbers of collisions with the container per unit time due to the higher particle speeds associated with elevated temperatures . The pressure increases in proportion to the number of collisions per unit time . </P> <P> Brownian motion is the mathematical model used to describe the random movement of particles suspended in a fluid . The gas particle animation, using pink and green particles, illustrates how this behavior results in the spreading out of gases (entropy). These events are also described by particle theory . </P> <P> Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions about how they move, but their motion is different from Brownian motion because Brownian motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule (s) with the particle . The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as would be expected if an individual gas molecule were examined . </P>

What theory is used to explain the behavior of particles in gases