Tab Electronics Guide to Understanding Electricity ... - Sciences Club

138 Chapter Five
As soon as the two levels are equal, all water flow from tank to tank will
cease. As the old proverb states, “water seeks its own level.”
If you were to monitor the level increase in the small tank with
respect to time, you would notice that the level does not rise in a linear
fashion; that is, it rises at a slower rate as it approaches the 10-foot top. If
you charted the rise in “level versus time” in the form of a graph, the
curve would be identical to the curve shown in Fig. 5-2.
The small water tank will have a holding capacity associated with it.
For instance, it can be specified as capable of holding 40 gallons. Capacitors
are also rated with respect to the quantity of charge they can hold.
The capacity (quantity of charge) of a capacitor is measured in units
called farads. A farad is the amount of capacitance required to store one
coulomb of electrical energy at a 1 volt potential. A coulomb is a volume
measurement unit of electrical energy (charge). It is analogous to other
volume measurement units such as quart, pint, or gallon. A gallon represents
4 quarts. A coulomb represents 6.28 10 18 electrons (or
6,280,000,000,000,000,000 electrons). The basic unit of current flow, the
ampere, can be defined in terms of coulombs. If one coulomb passes
through a conductor in one second, this is defined as one ampere of
current flow. Thus, 1 amp 1 coulomb/second.
A farad is generally too large of a quantity of energy to be stored by
only one capacitor. In the 1930s, a Gernsback magazine calculated the
size requirements for building a 1-farad paper-foil capacitor. Completely
fill the Empire State Building, from bottom to the top, with a stack of
paper and foil layers!
Therefore, the capacity of most capacitors is defined in terms of
microfarads (1 F 0.000,001 farad) or picofarads (1 pF 0.000,000,000,001
farad). As stated previously, capacitors also have an associated voltage rating.
If this voltage rating is exceeded, the capacitor could develop an
internal short (the term short means an undesired path of current flow.
Figure 5-2
Capacitor voltage
response of Fig. 5-1.
E Max 10 Volts
E
Across
C1
Time