Vern J. OstdiekDonald J. Bord

Chapter 7Electricity(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.