Simple approach to the dynamics in superconductors below Tc

We present a set of equations for macroscopic variables capable of describing the dynamics of superconductors for frequencies below 2", where " is the temperature-dependent pair potential. We investigate their relation to microscopic theory. Excitation spectra, response functions, and other quantities are calculated and compared to earlier theoretical results as well as experiments. There still are discrepancies remaining between hydrodynamics and microscopy.