that the current lags, or trails the voltage.
In other words, the
current through the coil changes somewhat after the time the voltage
across the coil changes.
(6) If the ac voltage is varied in frequency from low to high, it will be
found that XL increases in proportion to the increase in frequency.
15.
OPERATION OF CAPACITORS IN AC CIRCUITS
The opposition to a change of current through an inductor results from a
change in that current. In other words, an inductor tends to oppose any change in
current flow.
A capacitor develops a counter EMF too, not as the result of a
current change, but as the result of a voltage change; that is, a capacitor tends
to oppose any change in the applied voltage. Therein lies the basic difference in
the operation of these two components in ac circuits.
a. Voltage Across a Capacitor.
As illustrated earlier, a capacitor
consists essentially of two conducting plates separated by an insulating
dielectric. Because of the absence of an electrical connection between the plates,
current does not flow through a capacitor. Nevertheless, we know that a capacitor
appears to pass a radio signal. How does this take place? Its secret of operation
lies in the polarity change that takes place across a capacitor.
(1) Assume that an ac generator is placed in series with a capacitor as
shown in figure 23.
Assume further that the generator output voltage
has the waveform shown in figure 17.
The generator output voltage
varies from 0 at 0 rotation to maximum positive at 90, to 0 at 180,
to maximum negative at 270, and again to 0 at 360.
(2) At 0, the distribution of voltage across the capacitor is uniform (no
polarity).
As the voltage changes through one complete cycle, the
polarity changes across the capacitor.
Electrons are drawn from one
plate of the capacitor by the positive charge developed by the
generator.
These electrons are forced around the circuit by the
generator and deposited on the opposite plate of the capacitor. Then,
as the generator output voltage swings to the opposite polarity, the
reverse action occurs.
The changing polarity across the capacitor in
figure 23 is related to the angular degree of generator rotation for
one complete cycle of the generator output waveform.
(3) Electron flow is equivalent to current flow. Therefore, current starts
to flow around the circuit at the instant that the voltage starts to
change across the capacitor.
However, the capacitor will not assume
maximum charge immediately; it takes a little time.
It is therefore
customary to say that current through a capacitor leads the applied
voltage.
b. Capacitive Reactance. Note that the capacitor develops a voltage that is
in opposition to the generator output voltage polarity.
This is the counter EMF
developed by the capacitor; this is also the opposition that a capacitor offers to
the flow of current in an ac circuit. We call this opposition
308
27
Previous Page
