Why Are Capacitors Important?
The capacitor is a
widely used electrical component. It has several features that make it
useful and important:
-
A capacitor can store energy, so
capacitors are often found in power supplies.
-
A capacitor has a voltage that is
proportional to the charge (the integral of the current) that is stored in the
capacitor, so a capacitor can be used to perform
interesting computations in op-amp circuits, for
example.
-
Circuits with capacitors exhibit
frequency-dependent behavior
so that circuits that amplify certain frequencies selectively can be built.
What Is A Capacitor?
Capacitors are two-terminal electrical elements. Capacitors are
essentially
two conductors, usually conduction plates - but any
two conductors -
separated by an insulator - a dielectric - with
conection wires connected to the two conducting plates.
Capacitors occur naturally. On printed circuit boards two wires running parallel
to each other on opposite sides of the board form a capacitor. That's a
capacitor that comes about inadvertently, and we would normally prefer that it
not be there. But, it's there. It has electrical effects, and it will
affect your circuit. You need to understand what it does.
At other
times, you specifically want to use capacitors because of their frequency
dependent behavior. There are lots of situations where we want to design for
some specific frequency dependent behavior. Maybe you want to filter out some
high frequency noise from a lower frequency signal. Maybe you want to filter out
power supply frequencies in a signal running near a 60 Hz line. You're almost
certainly going to use a circuit with a capacitor.
Sometimes
you can use a capacitor to store energy. In a subway car, an insulator at
a track switch may cut off power from the car for a few feet along the line. You
might use a large capacitor to store energy to drive the subway car through the
insulator in the power feed.
Capacitors are used for all these purposes, and more. In this chapter you're
going to start learning about this important electrical component. Remember
capacitors do the following and more.
Goals
You need to
know what you should get from this lesson on capacitors. Here's the story.
Capacitors and inductors are both elements
that can store energy in purely electrical forms. These two elements were both
invented early in electrical history. The capacitor appeared first as the
legendary Leyden jar, a device that consisted of a glass jar with metal foil on
the inside and outside of the jar, kind of like the picture below. This
schematic/picture shows a battery attached to leads on the Leyden jar capacitor.
Although this device first appeared in Leyden, a city in the Netherlands
sometime before 1750. It was discovered by E. G. von Kleist and Pieter van
Musschenbroek. Although it has been around for about 250 years, it has all of
the elements of a modern capacitor, including:
The way the Leyden jar operated was that charge could be put onto both foil
elements. If positive charge was put onto the inside foil, and negative charge
on the outside foil, then the two charges would tend to hold each other in
place. Modern capacitors are no different and usually consist of two metallic or
conducting plates that are arranged in a way that permits charge to be bound to
the two plates of the capacitor. A simple physical situation is the one shown at
the right.
If the top
plate contains positive charge, and the bottom plate contains negative charge,
then there is a tendency for the charge to be bound on the capacitor plates
since the positive charge attracts the negative charge (and thereby keeps the
negative charge from flowing out of the capacitor) and in turn, the negative
charge tends to hold the positive charge in place. Once charge gets on the
plates of a capacitor, it will tend to stay there, never moving unless there is
a conductive path that it can take to flow from one plate to the other.
There is also a standard circuit symbol for a
capacitor. The figure below shows a sketch of a physical capacitor, the
corresponding circuit symbol, and the relationship between Q and V. Notice how
the symbol for a capacitor captures the essence of the two plates and the
insulating dielectric between the plates.
Now, consider
a capacitor that starts out with no charge on either plate. If the capacitor is
connected to a circuit, then the same charge will flow into one plate as flows
out from the other. The net result will be that the same amount of charge, but
of opposite sign, will be on each plate of the capacitor. That is the usual
situation, and we usually assume that if an amount of charge,
Q, is on the positive plate then -Q is the
amount of charge on the negative plate.
The essence of a capacitor is that it stores charge. Because they
store charge they have the properties mentioned earlier - they store energy and
they have frequency dependent behavior. When we examine charge storage in a
capacitor we can understand other aspects of the behavior of capacitors.
In a
capacitor charge can accumulate on the two plates. Normally charge of opposite
polarity accumulates on the two plates, positive on one plate and negative on
the other. It is possible for that charge to stay there. The positive charge on
one plate attracts and holds the negative charge on the other plate. In that
situation the charge can stay there for a long time.
That's it
for this section. You now know pretty much what a capacitor is. What you need to
learn yet is how the capacitor is used in a circuit - what it does when you use
it. That's the topic of the next section. If you can learn that then you can
begin to learn some of the things that you can do with a capacitor. Capacitors
are a very interesting kind of component. Capacitors are one large reason why
electrical engineers have to learn calculus, especially about derivatives.
In the next section you'll learn how capacitors influence voltage and current.
Voltage-Current Relationships In Capacitors
There is a
relationship between the charge on a capacitor and the voltage across the
capacitor. The relationship is simple. For most dielectric/insulating
materials, charge and voltage are linearly related.
Q
= C V
where:
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