handbook of modern sensors

38 3 Physical Principles of Sensing
This chapter examines various physical effects that can be used for a direct conversion
of stimuli into electric signals. Because all such effects are based on fundamental
principles of physics, we briefly review these principles from the standpoint of sensor
technologies.
3.1 Electric Charges, Fields, and Potentials
There is a well-known phenomenon to those who live in dry climates—the possibility
of the generation of sparks by friction involved in walking across the carpet. This
is a result of the so-called triboelectric effect, 1 which is a process of an electric
charge separation due to object movements, friction of clothing fibers, air turbulence,
atmosphere electricity, and so forth. There are two kinds of charge. Like charges
repel each other and the unlike charges attract each other. Benjamin Franklin (1706–
1790), among his other remarkable achievements, was the first American physicist.
He named one charge negative and the other positive. These names have remained to
this day. He conducted an elegant experiment with a kite flying in a thunderstorm to
prove that the atmospheric electricity is of the same kind as produced by friction. In
doing the experiment, Franklin was extremely lucky, as several Europeans who were
trying to repeat his test were severely injured by the lightning and one was killed.
Atriboelectric effect is a result of a mechanical charge redistribution. For instance,
rubbing a glass rod with silk strips electrons from the surface of the rod, thus leaving
an abundance of positive charges (i.e., giving the rod a positive charge). It should
be noted that the electric charge is conserved: It is neither created nor destroyed.
Electric charges can be only moved from one place to another. Giving negative charge
means taking electrons from one object and placing them onto another (charging it
negatively). The object which loses some amount of electrons is said gets a positive
charge.
A triboelectric effect influences an extremely small number of electrons as compared
with the total electronic charge in an object. The actual amount of charges
in any object is very large. To illustrate this, let us consider the total number
of electrons in a U.S. copper penny 2 [1]. The coin weighs 3.1 g; therefore, it
can be shown that the total number of atoms in it is about 2.9 × 10 22 . A copper
atom has a positive nuclear charge of 4.6 × 10 −18 C and the same electronic
charge of the opposite polarity. A combined charge of all electrons in a penny is
q = (4.6 × 10 −18 C/atom)(2.9 × 10 22 atoms) = 1.3 × 10 5 C, a very large charge indeed.
This electronic charge from a single copper penny may generate a sufficient
current of 0.91 A to operate a 100-W light bulb for 40 h.
With respect to electric charges, there are three kinds of material: conductors,
isolators, and semiconductors. In conductors, electric charges (electrons) are free to
move through the material, whereas in isolators, they are not. Although there is no
1 The prefix tribo means “pertinent to friction.”
2 Currently, the U.S. pennies are just copper-plated zinc alloy, but before 1982 they were
made of copper.