The differential equation for the distribution function of charged particles trapped in a strong magnetic field is discussed for the case when the charged particles are constrained in a tube of magnetic lines of force with a small normal cross section (the one-dimensional case). The distribution function is not defined by the six canonical variables, the variables in geometrical space and velocity space, as is done usually when referring to the Boltzmann equation. Instead, it is defined by the space coordinates of the guiding center of a representative particle together with its speed and pitch angle (i.e. five variables, in the three-dimensional case). In some problems, this type of description makes the correspondence between the physical picture and its mathematical description much easier. Several problems relating to trapped radiation are discussed using the differential equation.
