The study of d.c. transients focuses on the “short term” changes in voltage and current, that occur in some circuits, just after they are switched on or off. After the transient period has passed, the voltage and current settle down to their final values, (including zero). These transient effects are caused by two electrical properties, called capacitance and inductance. In this section we will only consider transients in capacitive circuits. (Transients in inductive circuits will be considered after electromagnetism and inductance have been covered.)
This first section contains a fairly comprehensive, qualitative, description of the principles involved. As well as defining what capacitance is, it also explains how the physical construction of a capacitor, determines its capacitance value. Throughout this section, we will make extensive use of an analogy with an air pump, to help to explain and illustrate the concepts involved. (This analogy does not assume any formal study of the behaviour of gases and will be explained using generic descriptive terms, such as “suction” and “back pressure” etc.)
We will first consider an artificial scenario, of initiating the chemical reactions in a previously dormant, isolated battery. This would create an internal current flow, that quickly decays to zero, as the charge accumulates at the battery terminals. We will then consider how connecting conductive plates to the terminals, enables a similar transient flow of current, in this "external circuit". We will see that after this transient current flow ceases, we will now have transferred more charge, with the same voltage. Finally we will then describe how we can transfer more charge using the same battery, by adjusting the arrangement of the conductive plates. True electron flow will be used in the descriptions throughout this section.
In the next section we will introduce a much simpler analogy that can be used to develop the electrical equations that describe capacitive transients. In that section we will use conventional current flow.
