In the previous lectures we have seen how the presence of an electric field results in the movement of charge within a conductor.  When the relationship between the potential difference across the ends of the conductor and the resulting current is a linear one, then we say that the conductor is a resistor.  In the circuits that we have studied thus far, a power source pushed  current through one or more loops in the circuit.  Other than the power source, the circuit has consisted solely of resistors. Our approach to finding the current in the circuit has been to reduce the resistors to one equivalent resistance, and then effectively to apply Kirchhoff’s voltage law to solve for the resulting current. In this formalism it is implicitly assumed that the current remains constant not only in direction, but also in magnitude.  In this lecture we shall discuss circuits which contain a new circuit element, called a capacitor.  In such circuits the current magnitude will not be constant with time, but will rather vary in a very specific mathematical manner. This lecture will introduce us to the capacitor and to the qualitative description  of circuits containing a capacitor.  The next lecture will introduce us to the quantitative description of such circuits.