Electronic circuits are integral parts of
nearly all the technological advances being
made in our lives today. Television, radio,
phones and computers immediately come to
mind, but electronics are also used in
automobiles, kitchen appliances, medical
equipment and industrial controls. At the
heart of these devices are active
components, or components of the circuit
that electronically control electron flow, like
semiconductors. However, these devices
could not function without much simpler,
passive components that predate
semiconductors by many decades. Unlike
active components, passive components,
such as resistors, capacitors and inductors,
can't control the electron flow with
electronic signals.
Resistance
As its name implies, a resistor is an
electronic component that resists the flow
of electric current in a circuit.
In metals such as silver or copper, which
have high electrical conductivity and
therefore low resistivity, electrons are able
to skip freely from one atom to the next,
with little resistance.
The electrical resistance of a circuit
component is defined as the ratio of the
applied voltage to the electric current that
flows through it, according to HyperPhysics ,
a physics resource website hosted by the
department of physics and astronomy at
Georgia State University. The standard unit
for resistance is the ohm, which is named
after German physicist Georg Simon Ohm . It
is defined as the resistance in a circuit with
a current of 1 ampere at 1 volt. Resistance
can be calculated using Ohm's law, which
states that resistance equals voltage divided
by current, or R = V/I (more commonly
written as V = IR), where R is resistance, V
is voltage and I is current.
Resistors are generally classified as either
fixed or variable. Fixed-value resistors are
simple passive components that always have
the same resistance within their prescribed
current and voltage limits. They are available
in a wide range of resistance values, from
less than 1 ohm to several million ohms.
Variable resistors are simple
electromechanical devices, such as volume
controls and dimmer switches, which change
the effective length or effective temperature
of a resistor when you turn a knob or move a
slide control.
An example of an inductor made from a
copper wire installed on a circuit board.
Credit: Shutterstock
Inductance
An inductor is an electronic component
consisting of a coil of wire with an electric
current running through it, creating a
magnetic field. The unit for inductance is
the henry (H), named after Joseph Henry , an
American physicist who discovered
inductance independently at about the same
time as English physicist Michael Faraday.
One henry is the amount of inductance that
is required to induce 1 volt of electromotive
force (the electrical pressure from an energy
source) when the current is changing at 1
ampere per second.
One important application of inductors in
active circuits is that they tend to block
high-frequency signals while letting lower-
frequency oscillations pass. Note that this is
the opposite function of capacitors.
Combining the two components in a circuit
can selectively filter or generate oscillations
of almost any desired frequency.
With the advent of integrated circuits, such
as microchips, inductors are becoming less
common, because 3D coils are extremely
difficult to fabricate in 2D printed circuits.
For this reason, microcircuits are designed
without inductors and use capacitors instead
to achieve essentially the same results,
according to Michael Dubson, a professor of
physics at the University of Colorado
Boulder.
Several examples of capacitors.
Capacitors store electric charge.
Credit: Peter Mathys, University of
Colorado
Capacitance
Capacitance is the ability of a device to
store electric charge, and as such, the
electronic component that stores electric
charge is called a capacitor. The earliest
example of a capacitor is the Leyden jar .
This device was invented to store a static
electric charge on conducting foil that lined
the inside and outside of a glass jar.
The simplest capacitor consists of two flat
conducting plates separated by a small gap.
The potential difference, or voltage, between
the plates is proportional to the difference in
the amount of the charge on the plates. This
is expressed as Q = CV, where Q is charge,
V is voltage and C is capacitance.
The capacitance of a capacitor is the
amount of charge it can store per unit of
voltage. The unit for measuring capacitance
is the farad (F), named for Faraday, and is
defined as the capacity to store 1 coulomb
of charge with an applied potential of 1 volt.
One coulomb (C) is the amount of charge
transferred by a current of 1 ampere in 1
second.
To maximize efficiency, capacitor plates are
stacked in layers or wound in coils with a
very small air gap between them. Dielectric
materials — insulating materials that partially
block the electric field between the plates —
are often used within the air gap. This
allows the plates to store more charge
without arcing and shorting out.
Capacitors are often found in active
electronic circuits that use oscillating
electric signals such as those in radios and
audio equipment. They can charge and
discharge nearly instantaneously, which
allows them to be used to produce or filter
certain frequencies in circuits. An oscillating
signal can charge one plate of the capacitor
while the other plate discharges, and then
when the current is reversed, it will charge
the other plate while the first plate
discharges.
In general, higher frequencies can pass
through the capacitor, while lower
frequencies are blocked. The size of the
capacitor determines the cutoff frequency
for which signals are blocked or allowed to
pass. Capacitors in combination can be used
to filter selected frequencies within a
specified range.
Supercapacitors are manufactured using
nanotechnology to create superthin layers of
materials, such as graphene , to achieve
capacities that are 10 to 100 times that of
conventional capacitors of the same size;
but they have much slower response times
than conventional dielectric capacitors, so
they can't be used in active circuits. On the
other hand, they can sometimes be used as
a power source in certain applications, such
as in computer memory chips, to prevent
data loss when the main power is cut.
Capacitors are also critical components of
timing devices, such as those developed by
SiTime, a company based in California.
These devices are used in a wide variety of
applications, from mobile phones to high-
speed trains and trading on the stock
market. Known as MEMS
(microelectromechanical systems), the tiny
timing device relies on capacitors to function
properly. "If the resonator [the oscillating
component within the timing device] doesn't
have the right capacitor and load
capacitance, the timing circuit will not start
up reliably and, in some cases, it stops
oscillating altogether," said Piyush Sevalia,
the executive vice president of marketing at
SiTime.
