How Charge Is Stored in a Dielectric




Figure:-Capacitance stores the charge in the dielectric between two conductors. (a) Structure. (b) Air-dielectric variable capacitor. Length is 2 in. (c) Schematic symbols for fixed and variable capacitors.


It is possible for dielectric materials such as air or paper to hold an electric charge because free electrons cannot flow through an insulator. However, the charge must be applied by some source. In Fig. (a), the battery can charge the capacitor shown. With the dielectric contacting the two conductors connected to the potential difference V, electrons from the voltage source accumulate on the side of the capacitor connected to the negative terminal of V . The opposite side of the capacitor
connected to the positive terminal of V loses electrons. As a result, the excess of electrons produces a negative charge on one side of the capacitor, and the opposite side has a positive charge. As an example, if 6.25 10 18 electrons are accumulated, the negative charge equals 1 coulomb (C). The charge on only one plate needs to be considered because the number of electrons accumulated on one plate is exactly the same as the number taken from the opposite plate. What the voltage source does is simply redistribute some electrons from one side of the capacitor to the other. This process is called charging the capacitor. Charging continues until the potential difference across the capacitor is equal to the applied voltage. Without any series resistance, the charging is instantaneous. Practically, however, there is always some series resistance. This charging current is transient, nor temporary; it fl owes only until the capacitor is charged to the applied voltage. Then there is no current in the circuit.
The result is a device for storing charge in the dielectric. Storage means that the charge remains even after the voltage source is disconnected. The measure of how much charge can be stored in the capacitance C. More charge stored for a given amount of applied voltage means more capacitance. Components made to provide a specified amount of capacitance are called capacitors, or by their old
name condensers. Electrically, then, capacitance is the ability to store charge. A capacitor consists
simply of two conductors separated by an insulator. For example, Fig.(b) shows a variable capacitor using air for the dielectric between the metal plates. There are many types with different dielectric materials, including paper, mica, and ceramics, but the schematic symbols are shown in Fig.(c) apply to all capacitors.

Electric Field in the Dielectric

Any voltage has a field of electric lines of force between the opposite electric charges. The electric field corresponds to the magnetic lines of force of the magnetic field associated with the electric current. What a capacitor does is concentrate the electric field in the dielectric between the plates. This concentration corresponds to a magnetic field concentrated in the turns of a coil. The only function of the capacitor plates and wire conductors is to connect the voltage source V across the dielectric. Then the electric field is concentrated in the capacitor, instead of being spread out in all directions.

Electrostatic Induction

The capacitor has opposite charges because of electrostatic induction by the electric field. Electrons that accumulate on the negative side of the capacitor provide electric lines of force that repel electrons from the opposite side. When this side loses electrons, it becomes positively charged. The opposite charges are induced by an electric field corresponding to the opposite poles induced in magnetic materials by a magnetic field.