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Vern J. Ostdiek
Donald J. Bord
Chapter 7
Electricity
(Section 1)
iProducts: iPods, iPhones, and iPads
• Within a few years of its introduction in 2001, the
iPod revolutionized the way people purchase and
enjoy music.
iProducts: iPods, iPhones, and iPads
• Later models and derivative products such as the
iPhone and iPad have brought the same portability
to images, video, and the Internet, and they now
offer hundreds of “apps” that you can use to locate
the nearest coffee shop, monitor your blood
pressure, manage your bank accounts, find the
most recent results from the world of sports, follow
the latest “tweets” from your favorite celebrities,
and much, much more.
• The iPod and its relatives represent 21st-century
triumphs in the application of electricity,
magnetism, materials science, and optics.
iProducts: iPods, iPhones, and iPads
• Several key components of these iProducts
function by exploiting electric fields, one of the
principal topics in this chapter.
• An electric field, which shares many of the same
characteristics as those of the gravitational field
studied in Chapter 3, is familiar to anyone who has
noticed laundry items clinging together after their
removal from a clothes dryer or who has received
a sudden shock when reaching for a metal
doorknob in winter after crossing a carpeted floor.
iProducts: iPods, iPhones, and iPads
• For example, the click wheel on an iPod or the
screen-scroll feature of an iPad each detects the
presence and motion of the user’s finger by
sensing the changes the finger causes in the
electric field maintained by a fine conducting grid
embedded in the device.
• Similarly, the liquid crystal displays (LCDs) on
these devices, like those on iPhones and laptop
computers, create images using by using electric
fields to selectively activate individual picture
elements (pixels) embedded in the screens.
iProducts: iPods, iPhones, and iPads
• What makes these “iProducts” so powerful, of
course, is the incredible quantity of digital
information stored on them and precisely
converted into audio or video signals ultimately
detected by the users’ ears and eyes.
• The enormous computational task required to
carry out this conversion is itself accomplished by
millions of miniaturized transistors embedded in
integrated circuit chips, each of which is subject to
electric fields that control the flow of electricity
through them.
iProducts: iPods, iPhones, and iPads
• It is no exaggeration to say, then, that it is the
common electric field that serves as the workhorse
for some of the most desirable and ubiquitous
electronic devices now in use by you and millions
of your fellow human beings around the world.
• In this chapter, we consider some of the basic
aspects of electricity, starting with electrostatic
phenomena involving stationary electric charges.
iProducts: iPods, iPhones, and iPads
• Following that, we begin a discussion of the
physics of moving charges that will continue into
Chapter 8.
• We also introduce the important quantities of
voltage, resistance, and electric current and show
how they are involved in many devices around us,
as well as in living things.
• The final two sections of the chapter deal with
electric power and energy and the two different
types of electric current: AC and DC.
7.1 Electric Charge
• How many electrical devices have you used so far
today?
•
How many are operating around you right now?
• Electricity governs your life in ways you probably
rarely think about.
• Even the properties of all matter that surrounds
you—the air you breathe, the water you drink, the
clothes that protect and insulate your body—are
largely determined by electrical forces acting in
and between atoms.
•
Most of the material in the remainder of this text is
connected to electricity to one degree or another.
• So let’s take a closer look at what electricity is.
7.1 Electric Charge
• The word electricity comes from electron, which
itself is based on the Greek word for amber.
•
Amber is a fossil resin that attracts bits of thread,
paper, hair, and other things after it has been
rubbed with fur.
• You may have noticed that a comb can do the
same after you run it through your hair.
7.1 Electric Charge
• This phenomenon, known as the amber effect,
was documented by the ancient Greeks, but its
cause remained a mystery for more than two
millennia.
• The results of numerous experiments, some
conducted by American scientist and statesman
Benjamin Franklin, indicated that matter
possessed a “new” property not connected to
mass or gravity.
7.1 Electric Charge
• This property was eventually traced to the atom
and is called electric charge.
Electric Charge An inherent physical property of
certain subatomic particles that is responsible for
electrical and magnetic phenomena.
•
Charge is represented by q, and the SI unit of
measure is the coulomb (C).
7.1 Electric Charge
• Early on, we stated that there are three
fundamental things that physicists can quantify or
measure:
•
space, time, and properties of matter.
• Until now, mass has been the only fundamental
property of matter that we have used.
• Electric charge is another basic property of matter,
but it is intrinsically possessed only by electrons,
protons, and certain other subatomic or
“elementary” particles.
7.1 Electric Charge
• Unlike mass, there are two different (and opposite)
types of electric charge, positive and negative.
• One coulomb of positive charge will “cancel” 1
coulomb of negative charge.
•
In other words, the net electric charge would be
zero:
q = –1 C + 1 C = 0 C
7.1 Electric Charge
• Recall that every atom is composed of a nucleus
surrounded by one or more electrons (Section 4.1).
•
The nucleus itself is composed of two types of
particles: protons and neutrons.
7.1 Electric Charge
• Every electron has a charge of –1.6×10–19 C, and
every proton has a charge of +1.6×10–19 C.
•
To have a total charge of –1 C, more than 6 billion
billion electrons are needed.
• Neutrons are so named because they are neutral;
they have no net electric charge.
• Normally, an atom will have the same number of
electrons as it has protons, which means the atom
as a whole is neutral.
7.1 Electric Charge
• The number of protons in the nucleus is the
element’s atomic number, which determines the
atom’s identity.
•
•
For example, an atom of the element helium has
two protons in the nucleus and two electrons in
orbit around the nucleus.
The positive charge possessed by the two protons
is exactly balanced by the negative charge
possessed by the two electrons, so the net charge
is zero.
• Most of the substances that we normally
encounter are electrically neutral simply because
the total number of electrons in all of the atoms is
equal to the total number of protons.
7.1 Electric Charge
• A variety of physical and chemical interactions can
cause an atom to gain one or more electrons or to
lose one or more of its electrons.
•
In these cases, the atom is said to be ionized.
7.1 Electric Charge
• For example, if a helium atom gains one electron,
it has three negative particles (electrons) and two
positive particles (protons).
•
•
This atom is said to be a negative ion, because it
has a net negative charge
The value of its net charge is just the charge on the
“extra” electron, q = –1.6×10–19 C.
7.1 Electric Charge
• Similarly, if a neutral helium atom loses one
electron, it becomes a positive ion, because it
has two positive particles and only one negative
particle. Its net charge is +1.6×10–19 C.
7.1 Electric Charge
• In many situations, ions are formed on the surface
of a substance by the action of friction.
• When a piece of amber, plastic, or hard rubber is
rubbed with fur, negative ions are formed on its
surface.
•
The contact between the fur and the material
causes some of the electrons in the atoms of the fur
to be transferred to some of the atoms on the
surface of the solid.
• The fur acquires a net positive charge, because it
has fewer electrons than protons.
7.1 Electric Charge
• Similarly, the amber, plastic,
or hard rubber acquires a net
negative charge because it
has an excess of electrons.
•
Combing your hair can
charge the comb in the same
way.
7.1 Electric Charge
• Rubbing glass with silk causes the glass to
acquire a net positive charge.
•
Some of the electrons in the surface atoms of the
glass are transferred to the silk, which becomes
negatively charged.
• Ion formation by friction is a complicated
phenomenon that is still not completely
understood.
•
It is affected by many factors including what
materials are used and how high the relative
humidity is.