  We discuss $s$-wave scattering in an atomic binary collision with two coupled
channels, tunable by an external magnetic field, one channel open and the other
closed for the incident energies considered. The analysis is performed with a
stylized model of square-well potentials. This simplification allows for a
pedagogically thorough discussion of the different scattering resonances that
appear in coupled channels. One of the them, the potential resonances at
vanishing energy, occur as a bound state of the coupled system emerges, in
turned tuned at a very precise value of the external field. The other
resonances, described by Feshbach theory, occur when the incident energy is
near a bound state of the closed channel, as if it were decoupled from the open
channel. These resonances exist for values of the external field above a
particular threshold value. Besides the potential intrinsic value of this study
in a quantum mechanics course, as the analysis can be performed with minor
numerical calculations, it is also an aid for the understanding of current
research advances in the exciting field of ultracold gases.
