Capacitor Basics

A capacitor is a two-terminal device that consists of two conducting bodies separated by a nonconducting material. The material is known as an insulator or a dielectric. The construction of the physical device is suggested by the circuit symbol. A capacitor, physically consists of two conducting surfaces on which charge may be stored, separated by a thin insulating layer which has a very large resistance. If we assume that this resistance is sufficiently large, that it may be considered infinite, then equal and opposite charges placed on the capacitor “plates” can never be combined by any path within the element.

They must be transported between the conducting bodies via external circuitry connected to the terminals of the capacitor. The conducting bodies are flat, rectangular conductors that are separated by the dielectric material.

i = \tfrac{dq}{dt}

Capacitors and Current

The displacement current flowing internally between the capacitor plates is exactly to the conduction current flowing in the capacitor leads; Kirchhoff’s current law is therefore satisfied if we include both conduction and displacement currents. The relationship is linear and the constant of proportionality is obviously the capacitance C:

i = i = C\tfrac{dv}{dt}

Capacitors and Charge

Charges removed from one plate always appear on the other so that the total charge remains zero. We should also observe that charges leaving one terminal enter the other. This fact is consistent with the requirement that current entering one terminal must exit the other in a two-terminal device. A constant voltage across a capacitor requires zero current passing through it. The movement of this charge causes the upper terminal to become more positive than the lower one by an amount

dv = \tfrac{1}{C}i dt.