Capacitors in parallel
Capacitances in parallel add like resistances in series. That is, the total capacitance is
the sum of the individual component values. Again, you need to be sure that you use the
same size units all the way through.
If two or more capacitors are connected in parallel, and one of the components is
much, much larger than any of the others, the total capacitance can be taken as simply
the value of the biggest one.
Problem 11-4
Three capacitors are in parallel, having values of C1 = 0.100 μF, C2,= 0.0100 μF, and
C3 0.00100 μF, as shown in Fig. 11-4. What is the total capacitance?
Just add them up: C = 0.100 + 0.0100 = 0.00100 = 0. 111000. Because the values
are given to three significant figures, the final answer should be stated as C = 0.111 μF.
Problem 11-5
Two capacitors are in parallel, one with a value of 100 μF and one with a value of 100 pF.
What is the effective total capacitance?
In this case, without even doing any calculations, you can say that the total is 100 μF for practical purposes. The 100-pF unit is only a millionth of the capacitance of the
100-μF component; therefore, the smaller capacitor contributes essentially nothing to
the composite total.
Dielectric materials
Just as certain solids can be placed within a coil to increase the inductance, materials
exist that can be sandwiched in between the plates of a capacitor to increase the
capacitance. The substance between the plates is called the dielectric of the capacitor.
Air works quite well as a dielectric. It has almost no loss. But it is difficult to get very
much capacitance using air as the dielectric. Some solid material is usually employed as
the dielectric for most fixed capacitors, that is, for types manufactured to have a constant,
unchangeable value of capacitance.
Dielectric materials conduct electric fields well, but they are not good conductors
of electric currents. In fact, the materials are known as good insulators.
Solid dielectrics increase the capacitance for a given surface area and spacing of
the plates. Solid dielectrics also allow the plates to be rolled up, squashed, and placed very close together (Fig. 11-5). Both of these act to increase the capacitance per unit
volume, allowing reasonable capacitances to exist in a small volume.
