ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 1
As electrons accumulate on the top 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. The
source voltage (emf) forces current around the circuit' (figure
32 on the previous page in a clockwise direction. The emf
developed across the capacitor, however, has a tendency to force
the current in a counterclockwise direction, opposing the source
emf. As the capacitor continues to charge, the voltage across
the capacitor rises until it is equal to the source voltage.
Once the capacitor voltage equals the source voltage, the two
voltages balance one another and current ceases to flow in the
In studying the charging process of a capacitor, it must be
noted 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 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 28 on page 42.
Since the electrostatic field is maximum, the energy stored in
the dielectric is called maximum.
If the switch is now opened, as shown in figure 33, view A, on
the following page, 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. It should be noted, at this point, that the