In short, once the switch is pressed, there is a small jolt of current across the capacitor and then it all stops. If the switch is released the capacitor keeps its charge, so additional button pushes don’t do anything until the capacitor is neutralized somehow, draining excess electrons from the crowded side of the dielectric.
The result of this behavior is that the capacitor appears to conduct electricity while the voltage is changing, but does not conduct when the voltage is steady. If you apply an AC signal on one side of a capacitor, particularly one that goes negative as well as positive, a version of that signal will appear on the other side of the capacitor.
Another way to look at this is with a water analogy. A battery is like a large reservoir of water (Fig. 4-3), and a resistor is like a narrow spot in a connecting pipe (Fig. 10-14). A capacitor, then, is like a small reservoir with a rubber diaphragm across the middle (Fig. 10-15). Water can’t flow across it, but pressure on one side can push water out of the other side.
We explore the details of how the capacitor reacts to an AC signal in the section on capacitor networks. First, there is an experiment you can do to hear for yourself how a capacitor modifies an AC signal—if you dare.
Fig. 10-14. Water analogy for resistors.