(b) 1 picofarad = 1/1,000,000,000,000 farad = 0.000,000,000,001 farad =
1 x 10-12 farads.
(2) One more fact about capacitors: they should never be used in circuits
that place a voltage across the capacitors which exceeds the maximum
safe rated value of voltage. To do so may destroy the dielectric and
cause a breakdown of the circuit.
16.
OPERATION OF CIRCUIT ELEMENTS
a. A knowledge of the individual properties and characteristics of the three
circuit elements, resistance, inductance, and capacitance, is necessary to
understand how these circuit elements operate in an actual circuit.
Figure 24
shows a circuit containing all three circuit elements, so arranged that if switch
S1 is closed, dc will be applied to the circuit, and if switch S2 is closed, ac
will be applied to the circuit. The ground symbol shown on the diagram indicates
that all points in the circuit so marked with this symbol are connected to a metal
chassis, or base, on which the circuit is constructed; thus, all points bearing the
ground symbol are actually connected together (via the metal in the chassis). This
chassis ground symbol is used quite frequently in circuit diagrams to indicate that
a part or a circuit element is connected to the chassis.
The symbol does not
necessarily mean that the part is actually connected to an earth ground, although
it is sometimes used in this way in transmitter and receiver circuits.
b. Examination of the circuit in figure 24 shows that there are three
possible paths through which current may flow.
The first is through resistor R1
and back through ground (or the chassis) to whichever power source is in use; the
second is through capacitor C and resistor R2 and back through ground; the third is
through inductor L and resistor R3 and back through ground.
Note that all three
paths are connected in parallel.
The path through the dc voltmeter (V) passes a
negligible amount of current and therefore is not considered to be a current path.
c. The first step in the study of this circuit is to turn switch S1 on.
Direct current will flow through resistor R1, the first path; the amount of current
which flows through this path will depend on its resistance.
Practically no
current will flow in the second path since the dielectric of capacitor C acts as an
insulator, and thus the capacitor will not pass dc. Although no current is flowing
in the second path, voltmeter V, which is connected across all three paths in
parallel, indicates that there is a voltage present across R2 and C. Also, if the
across it, since there is no current flowing in this path and, consequently, there
is no voltage drop across R2. This example shows that it is possible for a voltage
to be present in a circuit, even though the circuit is open (that is, there is no
flow of current). Current will flow in the third path, since the only opposition
of current flow in this branch of the circuit is the dc resistance of the coil
windings of inductor L and the resistance of resistor R3.
The amount of current
flow will be determined by the total resistance in this path; that is, the sum of
the dc resistance of L and the resistance of R3.
d. The second step in the study of this circuit is to turn switch S1 off and
turn switch S2 on. Low-frequency ac is now applied to the circuit. When
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