Unlike gravity, which always pulls objects toward one
another, electric forces can be either attractive or repulsive.
You can experiment with electric forces using a
thin stream of water and an electrically charged comb.
First, open a water faucet slightly so that the fl ow of
water forms a thin but continuous strand below the
mouth of the faucet. Next, give your rubber or plastic
comb an electric charge by passing it rapidly through
your hair or rubbing it vigorously against a wool sweater.
Finally, hold the comb near the stream of water, just
below the faucet, and watch what happens to the stream.
Is the electric force that you’re observing attractive or
repulsive? Why does this force change the path of the
Rubbing the comb through your hair makes it electrically
charged. What other objects can acquire and hold a
charge when you rub them across hair or fabric? Which
works better: a metal object or one that’s an insulator? Why?
Although we often experience electric forces and currents as
novelties or nuisances, there are also many devices that depend
on them. In this chapter, we examine the mysteries of (1) static
electricity and study two modern devices based on electricity:
(2) xerographic copiers and (3) fl ashlights. In Static Electricity,
we look at how clothes and other objects acquire charges and
how they exert forces on one another as a result. In Xerographic
Copiers, we see how these same electric forces work together
with light to control the placement of black powder to reproduce
images on sheets of paper. In Flashlights, we look at how a
current of electric charges conveys power from batteries to a
lightbulb. For a more complete preview of the chapter, turn
ahead to the Chapter Summary and Important Laws and Equations
at the end of the chapter.
This chapter concentrates on electricity and its charges,
but as we will see in Chapter 11, electricity is closely related to
magnetism and its poles. While we’ll leave the relationships
between electricity and magnetism for that next chapter, you
may already begin seeing similarities between those two seemingly
Electricity may be diffi cult to see, but you can easily observe
its effects. How often have you found socks clinging to a shirt
as you remove them from a hot dryer or struggled to throw away
a piece of plastic packaging that just won’t leave your hand or
stay in the trash can? The forces behind these familiar effects
are electric in nature and stem from what we commonly call
static electricity. Static electricity does more than just push
things around, however, as you’ve probably noticed while
reaching for a doorknob or a friend’s hand on a cold, dry day. In
this section, we’ll examine static electricity and the physics
behind its intriguing forces and often painful shocks.
Questions to Think About: How does a dryer produce static
electricity, and why do some clothes cling while others repel
each other? Why does walking across a carpet on a cold, dry
day put you at great risk of a shock as you reach for a doorknob?
Why do you get only a single brief shock from that knob
and not a long sustained one? When you touch a friend and get
a shock, did one of you cause that shock or are you both
responsible? If rubbing is required to develop static electricity,
why does the plastic wrap produce so much of it when you
open a package? Why do moist air and antistatic chemicals
reduce static electricity?
Experiments to Do: You can study static electricity by rubbing
a toy balloon vigorously through your hair or against a wool
sweater. Though its appearance won’t change, the balloon will
begin to attract other things, particularly your hair. What has
happened to the balloon? to your hair? Why does the balloon
also attract things that weren’t rubbed?
Try to get rid of the balloon’s attractiveness by letting a thick
stream of water fl ow over its surface. Why does this process return
the balloon to normal? What did you “wash” off the balloon? Now
rub two identical balloons through your hair and see whether they
attract or repel one another. Does the result make sense?
Finally, draw two long strips of transparent tape from a
dispenser without rubbing them on anything, and see if they
attract or repel. Is rubbing essential to the development of
Electric Charge and Freshly Laundered Clothes
Unless you have always lived in a damp climate and avoided synthetic materials, you have
experienced the effects of static electricity. Seemingly ordinary objects have pushed or
pulled on one another mysteriously, and you’ve received shocks while reaching for light
switches, car doors, or friends’ hands. Static electricity is more than an interesting nuisance,
though; it’s a simple window into the inner workings of our universe and worthy of
a serious look. It will take some time to lay the groundwork, but soon you’ll be able to
explain most of the effects of static electricity and even to control it to some extent.
The existence of static electricity has been known for several thousand years. About
600 bc, the Greek philosopher Thales of Miletus (ca. 624–546 bc) observed that when
amber is rubbed vigorously with fur, it attracts light objects such as straw and feathers.
Known in Greek as elektron (,η’λεκτρου), amber is a fossil tree resin with properties similar
to those of modern plastics. The term static electricity, like many others in this chapter,
derives from that Greek root.
Static electricity begins with electric charge, an intrinsic property of matter. Electric
charge is present in many of the subatomic particles from which matter is constructed,
and these particles incorporate their charges into nearly everything. No one knows why
charge exists; it’s simply one of the basic features of our universe and something that people
discovered through observation and experiment. Because electric charge has so much
influence on the objects that contain it, we sometimes refer to those objects as electric
charges, or simply as charges.
Charges exert forces on one another, and these forces are what you observe with static
electricity. Next time you’re doing laundry, experiment with your clothes as they come out
of the dryer. You’ll fi nd that some electrically charged garments attract one another, while
others repel each other. Evidently, there are two different types of charge. Although this
dichotomy has been known since 1733, when it was discovered by French chemist Charles-
François de Cisternay du Fay (1698–1739), it was Benjamin Franklin 1 who fi nally gave
the two charges their present names. Franklin called what appears on glass when it’s rubbed
with silk “positive charge” and what appears on hard rubber when it’s rubbed with animal
fur “negative charge.”
Two like charges (both positive or both negative) push apart, each experiencing a
repulsive force that pushes it directly away from the other (Figs. 10.1.1a, b). Two opposite
charges (one positive and one negative) pull together, each experiencing an attractive force
that pulls it directly toward the other
When you fi nd that two freshly laundered socks push apart, it’s because they both have
the same type of charge. Whether that charge is positive or negative depends on the fabrics
involved (more on that later), so let’s just suppose that the dryer has given each sock a
negative charge. Since like charges repel, the socks push apart. What does it mean for the
dryer to give each sock a negative charge?
The answer to that question has several parts. First, the dryer didn’t create the negative
charge that it gave to a sock. Like momentum, angular momentum, and energy, electric
charge is a conserved physical quantity—it cannot be created or destroyed, only transferred.
The negative charge that the dryer gave to the sock must have come from something
else, perhaps a shirt.
Second, positive charge and negative charge aren’t actually separate entities—they’re
just positive and negative amounts of the same physical quantity: electric charge. Positive
charges have positive amounts of electric charge, while negative charges have negative
amounts. Like most physical quantities, we measure charge in standard units. The SI unit
of electric charge is the coulomb (abbreviated C). Small objects rarely have a whole coulomb
of charge, and your sock’s charge is only about −0.0000001 C.
Third, the sock’s negative charge refers to the sock as a whole, not to its internal
pieces. As with all ordinary matter, the sock contains an enormous number of positively
and negatively charged particles. Each of the sock’s atoms consists of a dense central core
or nucleus, containing positively charged protons and uncharged neutrons, surrounded by
a diffuse cloud of negatively charged electrons. The electrostatic forces between those tiny
charged particles hold together not only the atoms but also the entire sock. However, in
giving the sock a negative charge, the dryer saw to it that the sock’s net electric charge, the
sum of all its positive and negative amounts of charge, is negative. With its negative net
charge, the sock behaves much like a simple, negatively charged object.
Last, the sock became negatively charged when it contained more electrons than protons.
Underlying that seemingly simple statement is a great deal of painstaking scientifi c
study. To begin with, experiments have shown that electric charge is quantized, that is,
charge always appears in integer multiples of the elementary unit of electric charge. This
elementary unit of charge is extremely small, only about 1.6 × 10−19 C, and is the magnitude
of the charge found on most subatomic particles. An electron has a −1 elementary unit
of charge, while a proton has a +1 elementary unit of charge. Since the only charged subatomic
particles in normal matter are electrons and protons, the sock becomes negatively
charged simply by having more electrons than protons.
Returning to the original question, we now know what the dryer did that gave a sock a
negative charge. Assuming the sock was electrically neutral to start—it had zero net charge—
the dryer must have added electrons to the sock or removed protons from the sock or both.
These transfers of charge upset the sock’s charge balance and gave it a negative net charge.
In keeping with our convention regarding conserved quantities, all unsigned references
to charge in this book imply a positive amount. For example, if the dryer gave charge to a
jacket, we mean it gave a positive amount of charge to that jacket. We follow this same
convention with money: when you say that you gave money to a charity, we assume that
you gave a positive amount.
Finally, Franklin’s charge-naming scheme was brilliant in concept but unlucky in execution.
Although it reduced the calculation of net charge to a simple addition problem, it
required Franklin to choose which type of charge to call “positive” and which to call “negative.”
Unfortunately, his seemingly arbitrary choice made electrons, the primary constituents
of electric current in wires, negatively charged. By the time physicists had recognized
the mistake, it was too late to fi x. Scientists and engineers have had to deal with negative
amounts of charge fl owing through wires ever since. Imagine the awkwardness of having
to carry out business using currency printed only in negative denominations!