Static Electricity
Basic
Terminology and Concepts
The
Structure of Matter
There is a large overlap of the world of static electricity
and the everyday world that you experience. Clothes tumble in the dryer and
cling together. You walk across the carpeting to exit a room and receive a door
knob shock. You pull a wool sweater off at the end of the day and see sparks of
electricity. During the dryness of winter, you step out of your car and receive
a car door shock as you try to close the door. Sparks of electricity are seen
as you pull a wool blanket off the sheets of your bed. You stroke your cat's
fur and observe the fur standing up on its end. Bolts of lightning dash across
the evening sky during a spring thunderstorm. And most tragic of all, you have
a bad hair day. These are all static electricity events - events that can only be
explained by an understanding of the physics of electrostatics.
Not only do electrostatic occurrences permeate the events of
everyday life, without the forces associated with static electricity, life as
we know it would be impossible. Electrostatic forces - both attractive and
repulsive in nature - hold the world of atoms and molecules together in perfect
balance. Without this electric force, material things would not exist. Atoms as
the building blocks of matter depend upon these forces. And material objects,
including us Earthlings, are made of atoms and the acts of standing and
walking, touching and feeling, smelling and tasting, and even thinking is the
result of electrical phenomenon. Electrostatic forces are foundational to our
existence.
One of the primary questions to be asked in this unit of The
Physics Classroom is: How can an object be charged and what affect does that
charge have upon other objects in its vicinity? The answer to this question
begins with an understanding of the structure of matter. Understanding charge
as a fundamental quantity demands that we have an understanding of the
structure of an atom. So we begin this unit with what might seem to many
students to be a short review of a unit from a Chemistry course.
History
of Atomic Structure
The search for the atom began as a philosophical question. It
was the natural philosophers of ancient Greece that began the search for the
atom by asking such questions as: What is stuff composed
of? What is the structure of material objects? Is there a basic unit from which
all objects are made? As 
early as 400 B.C., some Greek philosophers proposed that matter is made
of indivisible building blocks known as atomos. (Atomos in Greek
means indivisible.) To these early Greeks, matter could not be continuously
broken down and divided indefinitely. Rather, there was a basic unit or
building block that was indivisible and foundational to its structure. This
indivisible building block of which all matter was composed became known as the
atom.
The early Greeks were simply philosophers. They did not
perform experiments to test their theories. In fact, science as an experimental
discipline did not emerge as a credible and popular practice until sometime
during the 1600s. So the search for the atom remained a philosophical inquiry
for a couple of millennia. From the 1600s to the present century, the search
for the atom became an experimental pursuit. Several scientists are notable;
among them are Robert Boyle, John Dalton, J.J. Thomson, Ernest Rutherford,
and NeilsBohr.
Boyle's studies (middle to late 1600s) of gaseous substances
promoted the idea that there were different types of atoms known as elements.
Dalton (early 1800s) conducted a variety of experiments to show that different
elements can combine in fixed ratios of masses to form compounds. Dalton
subsequently proposed one of the first theories of atomic behavior that was supported by actual experimental
evidence.
English scientist J.J. Thomson's cathode ray experiments (end
of the 19th century) led to the discovery of the negatively charged electron
and the first ideas of the structure of these indivisible atoms. Thomson
proposed the Plum Pudding Model, suggesting that an atom's
structure resembles the favorite English
dessert - plum pudding. The raisins dispersed amidst the plum pudding are
analogous to negatively charged electrons immersed in a sea of positive charge.
Nearly a decade after Thomson, Ernest Rutherford's famous
gold foil experiments led to the nuclear model of atomic structure.
Rutherford's model suggested that the atom consisted of a densely packed core
of positive charge known as the nucleus surrounded
by negatively charged electrons. While the nucleus was unique to the Rutherford
atom, even more surprising was the proposal that an atom consisted mostly of
empty space. Most the mass was packed into the nucleus that was abnormally
small compared to the actual size of the atom.
Neils Bohr
improved upon Rutherford's nuclear model (1913) by explaining that the
electrons were present in orbits outside the nucleus. The electrons were
confined to specific orbits of fixed radius, each characterized by their own
discrete levels of energy. While electrons could be forced from one orbit to
another orbit, it could never occupy the space between orbits.
Bohr's view of quantized energy levels was the precursor to
modern quantum mechanical views of the atoms. The mathematical nature of
quantum mechanics prohibits a discussion of its details and restricts us to a
brief conceptual description of its features. Quantum mechanics suggests that
an atom is composed of a variety of subatomic particles. The three main
subatomic particles are the proton, electron and neutron. The proton and
neutron are the most massive of the three subatomic particles; they are located
in the nucleus of the atom, forming the dense core of the atom. The proton is
charged positively. The neutron does not possess a charge and is said to be
neutral. The protons and neutrons are bound tightly together within the nucleus
of the atom. Outside the nucleus are concentric spherical regions of space
known aselectron shells. The shells are the home of the negatively charged electrons. Each
shell is characterized by a distinct energy level. Outer shells have higher
energy levels and are characterized as being lower in stability. Electrons in
higher energy shells can move down to lower
energy shells; this movement is accompanied by the release of energy.
Similarly, electrons in lower energy shells can be induced to move to the
higher energy outer shells by the addition of energy to the atom. If provided
sufficient energy, an electron can be removed from an atom and be freed from
its attraction to the nucleus.
Application
of Atomic Structure to Static Electricity
This brief excursion into the history of atomic theory leads
to some important conclusions about the structure of matter that will be of
utmost importance to our study of static electricity. Those conclusions are
summarized here:
· All
material objects are composed of atoms. There are different kinds of atoms
known as elements; these elements can combine to form compounds. Different
compounds have distinctly different properties. Material objects are composed
of atoms and molecules of these elements and compounds, thus providing
different materials with different electrical properties.
· An atom
consists of a nucleus and a vast region of space outside the nucleus. Electrons
are present in the region of space outside the nucleus. They are negatively
charged and weakly bound to the atom. Electrons are often removed from and
added to an atom by normal everyday occurrences. These occurrences are the
focus of this Static Electricity unit of The Physics Classroom.
· The
nucleus of the atom contains positively charged protons and neutral neutrons.
These protons and neutrons are not removable or perturbable by
usual everyday methods. It would require some form of high-energy nuclear
occurrence to disturb the nucleus and subsequently dislodge its positively
charged protons. These high-energy occurrences are fortunately not an everyday
event and they are certainly not the subject of this unit of The Physics
Classroom. One sure truth of this unit is that the protons and neutrons will
remain within the nucleus of the atom. Electrostatic phenomenon can never be
explained by the movement of protons.
|
|
||
|
Proton |
Neutron |
Electron |
|
In nucleus Tightly
Bound Positive
Charge Massive |
In nucleus Tightly
Bound No Charge Massive |
Outside
nucleus Weakly
Bound Negative
Charge Not very
massive |
A variety of phenomena will be pondered, investigated and
explained through the course of this Static Electricity unit. Each phenomenon
will be explained using a model of matter described by the above three
statements. The phenomena will range from a rubber balloon sticking to a wooden
door to the clinging together of clothes that have tumbled in the dryer to the
bolt of lightning seen in the evening sky. Each of these phenomena will be
explained in terms of electron movement - both within the atoms and molecules
of a material and from the atoms and molecules of one material to those of
another. In the next section of Lesson 1 we will explore how electron movement can be
used to explain how and why objects acquire an electrostatic charge.
Check Your Understanding
Use your understanding of charge to answer the following
questions. When finished, click the button to view the answers.
1. ____ are the charged parts of an atom.
a. Only electrons
b. Only protons
c. Neutrons only
d. Electrons and neutrons
e. Electrons and protons
f. Protons and neutrons
Answer: E
Electrons are
negatively charged and protons are positively charged. The neutrons do not have
a charge.