Series RC circuits revisited

The measured and predicted properties of a series RC circuit containing a battery, a constant resistance R. and a time-dependent capacitance C are reported and compared with those of a conventional RC circuit with constant R and C and a time-dependent input voltage. The apparatus needed for these experiments is available in most undergraduate physics departments and the experiments are straightforward. Despite the simplicity of these circuits, some of the experimental results are unexpected. We describe several mathematical techniques that can be used to provide a detailed understanding of these results. (C) 2004 American Association of Physics Teachers.