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A simple unified discussion of branch imbalance and gap relaxation in superconductors is presented. Both phenomena are treated within the framework of the ordinary Boltzmann equation, supplemented by the BSC gap equation. It is shown that the physics of the process commonly referred to as quasiparticle branch imbalance relaxation may be understood simply if one introduces a two-fluid model for the charge in the superconductor, and regards the process as one in which charge associated with the normal component is converted into charge associated with the superfluid. The exact solutions of the Boltzmann equation, which are valid near Tsub(c) are derived, and the effects of anisotropy are allowed for. The comparison between relaxation rates measured in the superfluid and those obtained from normal state measurements and calculations is discussed. A set of two-fluid hydrodynamic equations based on the two fluid model for the charge is derived and it is found that the current of charge associated with the normal component is not in general equal to the usual normal current. On the basis of these equations expressions are derived for the characteristic quasiparticle diffusion length near phase slip centres and for the frequency of the recently observed collective mode. The result is compared with those of both microscopic and phenomenological calculations. (Atomindex citation 10:478390)