1-36
Q45. What do we call a conductor (or set of conductors) that radiates electromagnetic energy into
space?
Electric Field
Around every electrically charged object is a force field that can be detected and measured. This
force field can cause electric charges to move in the field. When an object is charged electrically, there is
either a greater or a smaller concentration of electrons than normal. Thus, a difference of potential exists
between a charged object and an uncharged object. An electric field is, therefore, associated with a
difference of potential, or a voltage.
This invisible field of force is commonly represented by lines that are drawn to show the paths along
which the force acts. The lines representing the electric field are drawn in the direction that a single
positive charge would normally move under the influence of that field. A large electric force is shown by
a large concentration of lines; a weak force is indicated by a few lines.
When a capacitor is connected across a source of voltage, such as a battery, it is charged by a
particular amount, depending on the voltage and the value of capacitance. (See figure 1-25.) Because of
the emf (electromotive force) of the battery, negative charges flow to the lower plate, leaving the upper
plate positively charged. Along with the growth of charge, the electric field is also building up. The flux
lines are directed from the positive to the negative charges and at right angles to the plates. When the
capacitor is fully charged, the voltage of the capacitor is equal to the voltage of the source and opposite in
polarity. The charged capacitor stores the energy in the form of an electric field. It can be said, therefore,
that an electric field indicates voltage.
Figure 1-25.—Electric fields between plates.
If the two plates of the capacitor are spread farther apart, the electric field must curve to meet the
plates at right angles (fig. 1-26). The straight lines in view A of figure 1-26 become arcs in view B, and
approximately semicircles in view C, where the plates are in a straight line. Instead of flat metal plates, as
in the capacitor, the two elements can take the form of metal rods or wires and form the basic antenna.
