Power loss in a capacitor may be
attributed to dielectric Hysteresis and dielectric leakage. Dielectric
Hysteresis may be defined as an effect in a dielectric material similar
to the Hysteresis found in a magnetic material. It is the result of
changes in orientation of electron orbits in the dielectric because of
the rapid reversals of the polarity of the line voltage. The amount of
power loss due to dielectric Hysteresis depends upon the type of
dielectric used. A vacuum dielectric has the smallest power loss.
Dielectric leakage occurs in a capacitor as the result of LEAKAGE CURRENT through the dielectric. Normally it is assumed that the dielectric will effectively prevent the flow of current through the capacitor. Although the resistance of the dielectric is extremely high, a minute amount of current does flow. Ordinarily this current is so small that for all practical purposes it is ignored. However, if the leakage through the dielectric is abnormally high, there will be a rapid loss of charge and an overheating of the capacitor.
The power loss of a capacitor is determined by loss in the dielectric. If the loss is negligible and the capacitor returns the total charge to the circuit, it is considered to be a perfect capacitor with a power loss of zero.
CHARGING AND DISCHARGING A CAPACITOR CHARGING
In order to better understand the action of a capacitor in conjunction with other components, the charge and discharge actions of a purely capacitive circuit are analyzed first. For ease of explanation the capacitor and voltage source shown in figure (6) are assumed to be perfect (no internal resistance), although this is impossible in practice.
In figure (6-A), an uncharged capacitor is shown connected to a four-position switch. With the switch in position 1 the circuit is open and no voltage is applied to the capacitor. Initially each plate of the capacitor is a neutral body and until a difference of potential is impressed across the capacitor, no electrostatic field can exist between the plates.
Figure (6). - Charging a capacitor.
To CHARGE the capacitor, the switch must
be thrown to position 2, which places the capacitor across the terminals
of the battery. Under the assumed perfect conditions, the capacitor
would reach full charge instantaneously. However, the charging action is
spread out over a period of time in the following discussion so that a
step-by-step analysis can be made.
At the instant the switch is thrown to position 2 (fig. (6-B)), a displacement of electrons occurs simultaneously in all parts of the circuit. This electron displacement is directed away from the negative terminal and toward the positive terminal of the source (the battery). A brief surge of current will flow as the capacitor charges.
If it were possible to analyze the motion of the individual electrons in this surge of charging current, the following action would be observed. See figure (7).
Figure (7). - Electron motion during charge.
At the instant the switch is closed, the positive terminal of the battery extracts an electron from the bottom conductor. The negative terminal of the battery forces an electron into the top conductor. At this same instant an electron is forced into the top plate of the capacitor and another is pulled from the bottom plate. Thus, in every part of the circuit a clockwise DISPLACEMENT of electrons occurs simultaneously.
As electrons accumulate on the top plate
of the capacitor and others depart from the bottom plate, a difference
of potential develops across the capacitor. Each electron forced onto
the top plate makes that plate more negative, while each electron
removed from the bottom causes the bottom plate to become more positive.
Notice that the polarity of the voltage which builds up across the
capacitor is such as to oppose the source voltage.
In studying the charging process of a capacitor, you must be aware that NO current flows THROUGH the capacitor. The material between the plates of the capacitor must be an insulator. However, to an observer stationed at the source or along one of the circuit conductors, the action has all the appearances of a true flow of current, even though the insulating material between the plates of the capacitor prevents the current from having a complete path. The current which appears to flow through a capacitor is called DISPLACEMENT CURRENT.
When a capacitor is fully charged and the source voltage is equaled by the counter electromotive force (cemf) across the capacitor, the electrostatic field between the plates of the capacitor is maximum. (Look again at figure (4).) Since the electrostatic field is maximum the energy stored in the dielectric is also maximum.
If the switch is now opened as shown in figure (8-A),the electrons on the upper plate are isolated. The electrons on the top plate are attracted to the charged bottom plate. Because the dielectric is an insulator, the electrons can not cross the dielectric to the bottom plate. The charges on both plates will be effectively trapped by the electrostatic field and the capacitor will remain charged indefinitely. You should note at this point that the insulating dielectric material in a practical capacitor is not perfect and small leakage current will flow through the dielectric. This current will eventually dissipate the charge. However, a high quality capacitor may hold its charge for a month or more.
Figure (8). - Discharging a capacitor.
To review briefly, when a capacitor is connected across a voltage source, a surge of charging current flows. This charging current develops a cemf across the capacitor which opposes the applied voltage. When the capacitor is fully charged, the cemf is equal to the applied voltage and charging current ceases. At full charge, the electrostatic field between the plates is at maximum intensity and the energy stored in the dielectric is maximum. If the charged capacitor is disconnected from the source, the charge will be retained for some period of time. The length of time the charge is retained depends on the amount of leakage current present. Since electrical energy is stored in the capacitor, a charged capacitor can act as a source emf.