Capacitance and Inductance

Author: J.B. Hoag

Suppose we rub a glass rod with a piece of silk. The glass rod will become positively charged and the silk negatively charged. We have not created electricity. Nor can it be destroyed. It can only be transferred from one body to another. A neutral body contains equal amounts of positive and negative electricity, and when electrons are added to it, it is negatively charged; when they are removed, it is positively charged.

All around the positively charged glass rod there are electrical forces, strong near the glass and weaker farther away. The strength of the electrical forces can be represented by lines; the more lines of force, the stronger the field. If we connect the terminals of a battery with wires to two metal plates, an electrical field or region of electrical force is created between the plates. This field is stronger (more lines of force) when the plates are close together or when a higher voltage battery is used.

The directions of the lines of force are very important, for they represent the paths along which free electrical charges tend to move. We can control the motion of the electrons in radio and cathode-ray tubes by sending them through properly designed electrical fields. Figure 3 A shows some electrical fields around metal balls and plates connected to batteries of various potentials, together with the apparatus used to measure the fields (an electrolytic trough). Also in this figure, there are some lines everywhere at right angles to the lines of force. These correspond to the contour lines of a map and are called equipotential lines, for the voltage or potential is everywhere equal along a given line. We shall return to the discussion and use of lines of force and equipotential lines in a later chapter.

Fig. 3 A. The electrical fields and equipotential lines around several charged bodies together with the apparatus used to measure them. The circuit is taken from the author's book " Electricity Laboratory," distributed by the University of Chicago Bookstore.