WHAT A CAPACITOR IS

A capacitor (called a condenser in older literature) consists of metal plates, separated by a thin layer of insulation, which could be vacuum or air, but usually is plastic or ceramic. When attached to a battery, electrons will flow onto one metal plate, and they are pulled off the other one, leaving positive charges there. 

A small capacitor allows a small ac current. Thus the human body conduction on page 6. 

When the charge accumulates on the metal plates of a capacitor (see top of Fig. 9.2), a voltage appears, and this is equal to the total charge divided by the distance between the plates, as previously described in the dimensions at the bottom of page 15. If there is just vacuum between the plates, this situation is shown at the top of the figure. The meter is a high quality multimeter, which can measure the capacitance directly. (The meters used in this course do not, unless the instructor is able to obtain a special one for demonstrations.) It determines the 

Capacitance, C, is the amount of charge, Q, per voltage, V, as in the following equation.  

Another useful relationship, somewhat difficult to find in most physics or electronics books, is 

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A charge distribution of this type is completely analogous to that of a capacitor. Figure 23 illustrates what a capacitor is two parallel metal plates separated by a dielectric material. 

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