  The case of ac transport at in-phase alternating applied magnetic fields for
a superconducting rectangular strip with finite thickness has been
investigated. The applied magnetic field is considered perpendicular to the
current flow. We present numerical calculations assuming the critical state
model of the current distribution and ac loss for various values of aspect
ratio, transport current and applied field amplitude. A rich phenomenology is
obtained due to the metastable nature of the critical state. We perform a
detailed comparison with the analytical limits and we discuss their
applicability for the actual geometry of superconducting conductors. We also
define a loss factor which allow a more detailed analysis of the ac behavior
than the ac loss. Finally, we compare the calculations with experiments,
showing a significant qualitative and quantitative agreement without any
fitting parameter.
