SUMMARY
Before going on to the next chapter, study the below summary to be sure that you
understand the important points of this chapter.

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____THE ELECTROSTATIC FIELD__ - When a charged body is brought close to another charged
body, the bodies either attract or repel one another. (If the charges are alike they
repel; if the charges are opposite they attract). The field that causes this effect is
called the ELECTROSTATIC FIELD. The amount by which two charges attract or repel each
other depends upon the size of the charges and the distance between the charges. The
electrostatic field (force between two charged bodies) may be represented by lines of
force drawn perpendicular to the charged surfaces. If an electron is placed in the field,
it will move toward the positive charge.

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____CAPACITANCE__ - Capacitance is the property of a circuit which OPPOSES any CHANGE in
the circuit VOLTAGE. The effect of capacitance may be seen in any circuit where the
voltage is changing. Capacitance is usually defined as the ability of a circuit to store
electrical energy. This energy is stored in an electrostatic field. The device used in an
electrical circuit to store this charge (energy) is called a CAPACITOR. The basic unit of
measurement of capacitance is the FARAD (F). A one-farad capacitor will store one coulomb
of charge (energy) when a potential of one volt is applied across the capacitor plates.
The farad is an enormously large unit of capacitance. More practical units are the
microfarad (mF) or the picofarad (pF).

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____CAPACITOR__ - A capacitor is a physical device consisting of two pieces of conducting
material separated by an insulating material. This insulating material is referred to as
the DIELECTRIC. Because the dielectric is an insulator, NO current flows through the
capacitor. If the dielectric breaks down and becomes a conductor, the capacitor can no
longer hold a charge and is useless. The ability of a dielectric to hold a charge without
breaking down is referred to as the dielectric strength. The measure of the ability of the
dielectric material to store energy is called the dielectric constant. The dielectric
constant is a relative value based on 1.0 for a vacuum.

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____CAPACITORS IN A DC CIRCUIT__ - When a capacitor is connected to the terminals of a
battery, each plate of the capacitor becomes charged. The plate connected to the positive
terminal loses electrons. Because this plate has a lack of electrons, it assumes a
positive charge. The plate connected to the negative terminal gains electrons. Because the
plate has an excess of electrons, it assumes a negative charge. This process continues
until the charge across the plates equals the applied voltage. At this point current
ceases to flow in the circuit. As long as nothing changes in the circuit, the capacitor
will hold its charge and there will be no current in any part of the circuit. If the leads
of the capacitor are now shorted together, current again flows in the circuit. Current
will continue to flow until the charges on the two plates become equal. At this point,
current ceases to flow. With a dc voltage source, current will flow in the circuit only
long enough to charge (or discharge) the capacitor. Thus, a capacitor does NOT allow dc
current to flow continuously in a circuit.

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____FACTORS AFFECTING CAPACITANCE__ - There are three factors affecting capacitance. One
factor is the area of the plate surfaces. Increasing the area of the plate increases the
capacitance. Another factor is the amount of space between the plates. The closer the
plates, the greater will be the electrostatic field. A greater electrostatic field causes
a greater capacitance. The plate spacing is determined by the thickness of the dielectric.
The third factor affecting capacitance is the dielectric constant. The value of the
dielectric constant is dependent upon the type of dielectric used.

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____WORKING VOLTAGE__ - The working voltage of a capacitor is the maximum voltage that
can be steadily applied to the capacitor without the capacitor breaking down (shorting).
The working voltage depends upon the type of material used as the dielectric (the
dielectric constant) and the thickness of the dielectric.

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____CAPACITOR LOSSES__ - Power losses in a capacitor are caused by dielectric leakage and
dielectric hysteresis. Dielectric leakage loss is caused by the leakage current through
the resistance in the dielectric. Although this resistance is extremely high, a small
amount of current does flow. Dielectric hysteresis may be defined as an effect in a
dielectric material similar to the hysteresis found in a magnetic material.

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____RC TIME CONSTANT__ - The time required to charge a capacitor to 63.2 percent of the
applied voltage, or to discharge the capacitor to 36.8 percent of its charge. The time
constant (t) is equal to the product of the resistance and the capacitance. Expressed as a
formula:

where t is in seconds, R is in ohms, and C is in farads.

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____CAPACITORS IN SERIES__ - The effect of wiring capacitors in series is to increase the
distance between plates. This reduces the total capacitance of the circuit. Total
capacitance for series connected capacitors may be computed by the formula:

If an electrical circuit contains only two series connected capacitors, C_{T}
may be computed using the following formula:

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____CAPACITORS IN PARALLEL__ - The effect of wiring capacitors in parallel is to increase
the plate area of the capacitors. Total capacitance (C_{T}) may be found using the
formula:

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