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CHAPTER
Background Physics
YOU MUST UNDERSTAND SOME PHYSICS PRINCIPLES TO GRASP THE FUNDAMENTALS OF ELECTRICITY and electronics. In science, we can talk about qualitative things or quantitative things, that is, “what” versus “how much.” For now, let’s focus on “what” and worry about “how much” later!
Atoms
All matter consists of countless tiny particles in constant motion. These particles have density far greater than anything we ever see. The matter we encounter in our everyday lives contains mostly space, and almost no “real stuff.” Matter seems continuous to us only because of the particles’ submicroscopic size and incredible speed. Each chemical element has its own unique type of particle called its atom.
Atoms of different elements always differ! The slightest change in an atom can make a tremendous difference in its behavior. You can live by breathing pure oxygen, but you couldn’t survive in an atmosphere comprising pure nitrogen. Oxygen will cause metal to corrode, but nitrogen will not. Wood will burn in an atmosphere of pure oxygen, but won’t even ignite in pure nitrogen. Nevertheless, both oxygen and nitrogen are gases at room temperature and pressure. Neither gas has any color or odor. These two substances differ because oxygen has eight protons, while nitrogen has only seven.
Nature provides countless situations in which a slight change in atomic structure makes a major difference in the way a sample of matter behaves. In some cases, we can force such changes on atoms (hydrogen into helium, for example, in a nuclear fusion reaction); in other cases, a minor change presents difficulties so great that people have never made them happen (lead into gold, for example).
Protons, Neutrons, and Atomic Numbers
The nucleus, or central part, of an atom gives an element its identity. An atomic nucleus contains two kinds of particles, the proton and the neutron, both of which have incredible density. A teaspoonful of protons or neutrons, packed tightly together, would weigh tons at the earth’s surface. Protons and neutrons have nearly identical mass, but the proton has an electric charge while the neutron does not.
The simplest and most abundant element in the universe, hydrogen, has a nucleus containing one proton. Sometimes a nucleus of hydrogen has a neutron or two along with the proton, but not very often. The second most common element is helium. Usually, a helium atom has a nucleus with two protons and two neutrons. Inside the sun, nuclear fusion converts hydrogen into helium, generating the energy that makes the sun shine. The process is also responsible for the energy produced by a hydrogen bomb.
Every proton in the universe is identical to every other proton. Neutrons are all alike, too. The number of protons in an element’s nucleus, the atomic number, gives that element its unique identity. With three protons in a nucleus we get lithium, a light metal solid at room temperature that reacts easily with gases, such as oxygen or chlorine. With four protons in the nucleus we get beryllium, also a light metal solid at room temperature. Add three more protons, however, and we have nitrogen, which is a gas at room temperature.
In general, as the number of protons in an element’s nucleus increases, the number of neutrons also increases. Elements with high atomic numbers, such as lead, are therefore much more dense than elements with low atomic numbers, such as carbon. If you hold a lead shot in one hand and a similar-sized piece of charcoal in the other hand, you’ll notice this difference.
Isotopes and Atomic Weights
For a given element, such as oxygen, the number of neutrons can vary. But no matter what the number of neutrons, the element keeps its identity, based on the atomic number. Differing numbers of neutrons result in various isotopes for a given element.
Each element has one particular isotope that occurs most often in nature, but all elements have multiple isotopes. Changing the number of neutrons in an element’s nucleus results in a difference in the weight, and also a difference in the density, of the element. Chemists and physicists call hydrogen whose atoms contain a neutron or two in the nucleus (along with the lone proton) heavy hydrogen for good reason!
The atomic weight of an element approximately equals the sum of the number of protons and the number of neutrons in the nucleus. Common carbon has an atomic weight of 12. We call it carbon 12 (symbolized C12). But a less-often-found isotope has an atomic weight very close to 14, so we call it carbon 14 (symbolized C14).
Electrons
Surrounding the nucleus of an atom, we usually find a “swarm” of particles called electrons. An electron carries an electric charge that’s quantitatively equal to, but qualitatively opposite from, the charge on a proton. Physicists arbitrarily call the electron charge negative, and the proton charge positive. The charge on a single electron or proton constitutes the smallest possible quantity of electric charge. All charge quantities, no matter how great, are theoretically whole-number multiples of this so-called unit electric charge.
One of the earliest ideas about the atom pictured the electrons embedded in the nucleus, like raisins in a cake. Later, scientists imagined the electrons as orbiting the nucleus, making the atom resemble a miniature solar system with the electrons as “planets,” as shown in Fig. 1-1.
1-1 An early model of the atom, developed around the year 1900. Electrostatic attraction holds the electrons in “orbits” around the nucleus.
Today, we know that the electrons move so fast, with patterns of motion so complex, that we can’t pinpoint any single electron at any given instant of time. We can, however, say that at any moment, a particular electron will just as likely “reside” inside a defined sphere as outside it. We call an imaginary sphere of this sort, centered at the nucleus of an atom, an electron shell. These shells have specific, predictable radii. As a shell’s radius increases, the amount of energy in an electron “residing in” the shell also increases. Electrons commonly “jump” from one shell to another within an atom, thereby gaining energy, as shown in Fig. 1-2. Electrons can also “fall” from one shell to another within an atom, thereby losing energy.
1-2 Electrons move around the nucleus of an atom at defined levels, called shells, which correspond to discrete energy states. Here, an electron gains energy within an atom.
Electrons can move easily from one atom to another in some materials. In other substances, it is difficult to get electrons to move. But in any case, we can move electrons a lot more easily than we can move protons. Electricity almost always results, in some way, from the motion of electrons in a material. Electrons are much lighter than protons or neutrons. In fact, compared to the nucleus of an atom, the electrons weigh practically nothing.
Quite often, the number of electrons in an atom equals the number of protons. The negative charges, therefore, exactly cancel out the positive ones, and we get an electrically neutral atom, where “neutral” means “having a net charge of zero.” Under some conditions, an excess or shortage of electrons can occur. High levels of radiant energy, extreme heat, or the presence of an electric field (discussed later) can “knock” or “throw” electrons loose from atoms, upsetting the balance.
Ions
If an atom has more or fewer electrons than protons, then the atom carries an electrical charge. A shortage of electrons produces a positive charge; an excess of electrons produces a negative charge. The element’s identity remains the same no matter how great the excess or shortage of electrons. In the extreme, all the electrons might leave the influence of an atom, leaving only the nucleus; but even then, we still have the same element. We call an electrically charged atom an ion. When a substance contains many ions, we say that the substance is ionized.
The gases in the earth’s atmosphere become ionized at high altitudes, especially during the daylight hours. Radiation from the sun, as well as a constant barrage of high-speed subatomic particles from space, strips electrons from the nuclei. The ionized gases concentrate at various altitudes, sometimes returning signals from surface-based radio transmitters to the earth, allowing for long-distance broadcasting and communication.
An ionized material can conduct electricity fairly well even if, under normal conditions, it conducts poorly or not at all. Ionized air allows a lightning stroke (a rapid electrical discharge that causes a visible flash) hundreds or even thousands of meters long to occur, for example. The ionization, caused by a powerful electric field, takes place along a jagged, narrow path called the channel. During the stroke, the atomic nuclei quickly attract stray electrons back, and the air returns to its electrically neutral, normal state.
An element can exist as an ion and also as an isotope different from the most common isotope. For example, an atom of carbon might have eight neutrons rather than the usual six (so it’s C14 rather than C12), and it might have been stripped of an electron, giving it a positive unit electric charge (so it’s a positive ion). Physicists and chemists call a positive ion a cation (pronounced “cat-eye-on”) and a negative ion an anion (pronounced “an-eye-on”).
Compounds
Atoms of two or more different elements can join together by sharing electrons, forming a chemical compound. One of the most common compounds is water, the result of two hydrogen atoms joining with an atom of oxygen. As you can imagine, many chemical compounds occur in nature, and we can create many more in chemical laboratories.
A compound differs from a simple mixture of elements. If we mix hydrogen gas with oxygen gas, we get a colorless, odorless gas. But a spark or flame will cause the atoms to combine in a chemical reaction to give us the compound we call water, liberating light and heat energy. Under ideal conditions, a violent explosion will occur as the atoms merge almost instantly, producing a “hybrid” particle, as shown in Fig. 1-3.
1-3 Two hydrogen atoms readily share electrons with a single atom of oxygen.
Compounds often, but not always, have properties that drastically differ from either (or any) of the elements that make them up. At room temperature and pressure, both hydrogen and oxygen are gases. But under the same conditions, water exists mainly in liquid form. If the temperature falls enough, water turns solid at standard pressure. If it gets hot enough, water becomes a gas, odorless and colorless, just like hydrogen or oxygen.
Another common example of a compound is rust, which forms when iron joins with oxygen. While iron appears to us as a dull gray solid and oxygen appears as a gas, rust shows up as a red-brown powder, completely unlike either iron or oxygen. The chemical reaction that produces rust requires a lot more time than the reaction that produces water.
Molecules
When atoms of elements join in groups of two or more, we call the resulting particles molecules. Figure 1-3 portrays a molecule of water. Oxygen atoms in the earth’s atmosphere usually pair up to form molecules, so you’ll sometimes see oxygen symbolized as O2. The “O” represents oxygen, and the subscript 2 indicates two atoms per molecule. We symbolize water by writing H2O to show that each molecule contains two atoms of hydrogen and one atom of oxygen.
Sometimes oxygen atoms exist all by themselves; then, we denote the basic particle as O, indicating a lone atom. Sometimes, three atoms of oxygen “stick” together to produce a molecule of ozone, a gas that has received attention in environmental news. We symbolize ozone by writing O3. When an element occurs as single atoms, we call the substance monatomic. When an element occurs as two-atom molecules, we call the substance diatomic. When an element occurs as three-atom molecules, we call the substance triatomic.
Whether we find it in solid, liquid, or gaseous form, all matter consists of molecules or atoms that constantly move. As we increase the temperature, the particles in any given medium move faster. In a solid, we find molecules interlocked in a rigid matrix so they can’t move much (Fig. 1-4A), although they vibrate continuously. In a liquid, more space exists between the molecules (Fig. 1-4B), allowing them to slide around. In a gas, still more space separates the molecules, so they can fly freely (Fig. 1-4C), sometimes crashing into each other.
1-4 Simplified renditions of molecular arrangements in a solid (A), a liquid (B), and a gas (C).
Conductors
We define an electrical conductor as a substance in which the electrons can move with ease. The best known conductor at room temperature is pure elemental silver. Copper and aluminum also conduct electricity well at room temperature. Various other metals constitute fair to good conductors. In most electrical circuits and systems, we find copper or aluminum wire.
Some liquids conduct electricity quite well. Mercury provides a good example. Salt water conducts fairly well, but it depends on the concentration of dissolved salt. Gases or mixtures of gases, such as air, usually fail to conduct electricity because the large distances between the atoms or molecules prevent the free exchange of electrons. If a gas becomes ionized, however, it can conduct fairly well.
In an electrical conductor, the electrons “jump” from atom to atom (Fig. 1-5), predominantly from negatively charged locations toward positively charged locations. In a typical electrical circuit, many trillions, quadrillions, or quintillions of electrons pass a given point every second.
1-5 In an electrical conductor, some electrons pass easily from atom to atom.
Insulators
An electrical insulator prevents electron movement among atoms, except occasionally in tiny amounts. Most gases make good electrical insulators. Glass, dry wood, dry paper, and plastics also insulate well. Pure water normally insulates, although some dissolved solids can cause it to conduct. Certain metal oxides can function as good insulators, even if the metal in its pure form makes a good conductor.
Sometimes, you’ll hear an insulating material called a dielectric. This term arises from the fact that a sample of the substance can keep electrical charges apart to form an electric dipole, preventing the flow of electrons that would otherwise equalize the charge difference. We encounter dielectrics in specialized components, such as capacitors, through which electrons should not directly travel.
Engineers commonly use porcelain or glass in electrical systems. These devices, called insulators in the passive rather than the active sense, are manufactured in various shapes and sizes for different applications. You can see them on utility lines that carry high voltage. The insulators hold the wire up without risking a short circuit with a metal tower or a bleedoff (slow discharge) through a salt-water-soaked wooden pole.
If we try hard enough, we can force almost any electrical insulator to let electrons move by forcing ionization to occur. When electrons are stripped away from their atoms, they can roam more or less freely. Sometimes a normally insulating material gets charred, or melts down, or gets perforated by a spark. Then it loses its insulating properties, and electrons can move through it.
Resistors
Some substances, such as carbon, allow electrons to move among atoms fairly well. We can modify the conductivity of such materials by adding impurities such as clay to a carbon paste, or by winding a long, thin strand of the material into a coil. When we manufacture a component with the intent of giving it a specific amount of conductivity, we call it a resistor. These components allow us to limit or control the rate of electron flow in a device or system. As the conductivity improves, the resistance decreases. As the conductivity goes down, the resistance goes up. Conductivity and resistance vary in inverse proportion.
Engineers express resistance in units called ohms. The higher the resistance in ohms, the more opposition a substance offers to the movement of electrons. For wires, the resistance is sometimes specified in terms of ohms per unit length (foot, meter, kilometer, or mile). In an electrical system, engineers strive to minimize the resistance (or ohmic value) because resistance converts electricity into heat, reducing the efficiency that the engineers want and increasing the loss that they don’t want.
Semiconductors
In a semiconductor, electrons flow easily under some conditions, and with difficulty under other conditions. In their pure form, some semiconductors carry electrons almost as easily as good conductors, while other semiconductors conduct almost as poorly as insulators. But semiconductors differ fundamentally from plain conductors, insulators, or resistors. In the manufacture of a semiconductor device, chemists treat the materials so that they conduct well some of the time, and poorly some of the time—and we can control the conductivity by altering the conditions. We find semiconductors in diodes, transistors, and integrated circuits.
Semiconductors include substances, such as silicon, selenium, or gallium, that have been “doped” by the addition of impurities, such as indium or antimony. Have you heard of gallium-arsenide diodes, metal-oxide transistors, or silicon rectifiers? Electrical conduction in these materials occurs as a result of the motion of electrons, but the physical details of the process are rather complicated. Sometimes engineers speak of the movement of holes rather than electrons. A hole is a sort of electron deficiency. You might think of it as a place where an electron normally belongs, but for some reason it’s missing. Holes travel opposite to the flow of electrons, as shown in Fig. 1-6.
1-6 In a sample of semiconductor material, the holes travel in a direction opposite the electron motion.
When electrons make up most of the charge carriers in a substance, we have an N-type semiconductor. When most of the charge carriers are holes, we have a P-type semiconductor. A sample of P-type material passes some electrons, and a sample of N-type material carries some holes. We call the more abundant charge carrier the majority carrier, and the less abundant one the minority carrier.
Current
Whenever charge carriers move through a substance, an electric current exists. We express and measure current indirectly in terms of the number of electrons or holes passing a single point in one second. Electric current flows rapidly through any conductor, resistor, or semiconductor. Nevertheless, the charge carriers actually move at only a small fraction of the speed of light in a vacuum.
A great many charge carriers go past any given point in one second, even in a system carrying relatively little current. In a household electric circuit, a 100-watt (100-W) light bulb draws about six quintillion (6 followed by 18 zeroes or 6 × 1018) charge carriers per second. Even the smallest bulb carries quadrillions (numbers followed by 15 zeros) of charge carriers every second. Most engineers find it inconvenient to speak of current in terms of charge carriers per second, so they express current in coulombs per second instead. We might think of a coulomb as an “engineer’s superdozen”—approximately 6,240,000,000,000,000,000 (6.24 × 1018) electrons or holes. When 1 coulomb (1 C) of charge carriers passes a given point per second, we have an ampere, the standard unit of electric current. A 60-W bulb in your desk lamp draws about half an ampere (0.5 A). A typical electric utility heater draws 10 A to 12 A.
When a current flows through a resistance—always the case because even the best conductors have finite, nonzero resistance—we get heat. Sometimes we observe light as well. Old-fashioned incandescent lamps are deliberately designed so that the currents through their filaments produce visible light.
Static Electricity
When you walk on a carpeted floor while wearing hard-soled shoes, an excess or shortage of electrons can develop on your body, creating static electricity. It’s called “static” because the charge carriers don’t flow—until you touch a metallic object connected to earth ground or to some large fixture. Then an abrupt discharge occurs, accompanied by a spark, a snapping or popping noise, and a startling sensation.
If you acquire a much greater charge than you do under ordinary circumstances, your hair will stand on end because every strand will repel every other as they all acquire a static charge of the same polarity. When the discharge takes place, the spark might jump a centimeter or more. Then it will more than startle you; you could actually get hurt. Fortunately, charge buildups of that extent rarely, if ever, occur with ordinary carpet and shoes. However, a device called a Van de Graaff generator (Fig. 1-7), found in physics labs, can cause a spark several centimeters long. Use caution if you work around these things. They can be dangerous.
1-7 Simplified illustration of a Van de Graaff generator. This machine can create a charge sufficient to produce a spark several centimeters long.
Lightning provides the most spectacular example of the effects of static electricity on this planet. Lightning strokes commonly occur between clouds, and between clouds and the ground. The stroke is preceded by a massive static charge buildup. Figure 1-8 illustrates cloud-to-cloud (A) and cloud-to-ground (B) electric dipoles caused by weather conditions. In the scenario shown at B, the positive charge in the earth follows along beneath a storm cloud.
1-8 Electrostatic charges can build up between clouds (A) or between a cloud and the earth’s surface (B).
Electromotive Force
Charge carriers can move in an orderly fashion only if they experience a well-defined directional force in the form of a “push” or a “pull.” This force can result from a buildup of static electric charges, as in the case of a lightning stroke. When the charge builds up, attended by positive polarity (shortage of electrons) in one place and negative polarity (excess of electrons) in another place, a powerful electromotive force (EMF) exists. We express and measure EMF in units called volts.
Ordinary household electricity has an effective EMF, or voltage, of between 110 volts (110 V) and 130 V; usually it’s about 117 V. In the United States and most other countries, a new, fully charged car battery has an EMF of very close to 12.6 V. The static charge that you acquire when walking on a carpet with hard-soled shoes on a dry afternoon can reach several thousand volts. Before a discharge of lightning, millions of volts exist.
An EMF of 1 V, across a component having a resistance of 1 ohm, will cause a current of 1 A to flow through that component. In a DC circuit, the current (in amperes) equals the voltage (in volts) divided by the resistance (in ohms). This fact forms the cornerstone for a classic relationship in electricity called Ohm’s Law. If we double the voltage across a component whose resistance remains constant, then the current through that component doubles. If we keep the voltage constant but double the resistance, then the current goes down by half. We’ll examine Ohm’s law more closely later in this course.
Electromotive force can exist without any flow of current, producing static electricity, as we’ve seen. However, an EMF without current also exists between the two wires of an electric lamp when the switch is off. An EMF without current exists between the terminals of a common flashlight cell when we don’t connect it to anything. Whenever we have an EMF between two points, an electric current will flow if we provide a conductive path between those points. Voltage, or EMF, is sometimes called electric potential or potential difference for this reason. An EMF has the potential (that is, the ability) to move charge carriers, given the right conditions.
A huge EMF doesn’t necessarily drive a lot of current through a conductor or resistance. Think of your body after you’ve spent some time walking around on the carpet. Although the EMF might seem deadly in terms of sheer magnitude (thousands of volts), relatively few coulombs of charge carriers accumulate on your body. In relative terms, not that many electrons flow through your finger when you touch an external object. That’s why you don’t get a severe shock. However, if plenty of coulombs are available, then even a modest EMF, such as 117 V (typical of a household utility outlet), can drive a lethal current through your body. That’s why it’s dangerous to repair an electrical device when it’s connected to a source of power. The utility plant can deliver an unlimited number of coulombs.
Non-Electrical Energy
In scientific experiments, we often observe phenomena that involve energy in non-electrical form. Visible light provides an excellent example. A light bulb converts electricity into radiant energy that we can see. This fact motivated people like Thomas Edison to work with electricity, making discoveries and refining devices that make our lives convenient today. We can also convert visible light into electricity. A photovoltaic cell (also called a solar cell) works this sort of magic.
Light bulbs always give off heat as well as light. In fact, incandescent lamps actually give off more energy as heat than as light. You’ve probably had experience with electric heaters, designed for the purpose of changing electrical energy into heat energy. This “heat” is actually a form of radiant energy called infrared (IR), which resembles visible light, except that IR has a longer wavelength and you can’t see the rays.
We can convert electricity into radio waves, ultraviolet (UV) rays, and X rays. These tasks require specialized devices such as radio transmitters, mercury-vapor lamps, and electron tubes. Fast-moving protons, neutrons, electrons, and atomic nuclei also constitute non-electrical forms of energy.
When a conductor moves in a magnetic field, electric current flows in that conductor. This effect allows us to convert mechanical energy into electricity, obtaining an electric generator. Generators can also work backwards, in which case we have an electric motor that changes electricity into mechanical energy.
A magnetic field contains energy of a unique kind. The science of magnetism is closely related to electricity. The oldest and most universal source of magnetism is the geomagnetic field surrounding the earth, which arises as a result of the alignment of iron atoms in the core of the planet.
A changing magnetic field creates a fluctuating electric field, and a fluctuating electric field produces a changing magnetic field. This phenomenon, called electromagnetism, makes it possible to send wireless signals over long distances. The electric and magnetic fields keep producing one another over and over again through space.
Dry cells, wet cells, and batteries convert chemical energy into electrical energy. In an automotive battery, for example, acid reacts with metal electrodes to generate a potential difference. When we connect the poles of the battery to a component having finite resistance, current flows. Chemical reactions inside the battery keep the current going for a while, but the battery eventually runs out of energy. We can restore the chemical energy to a lead-acid automotive battery (and certain other types) by driving current through it for a period of time, but some batteries (such as most ordinary flashlight cells and lantern batteries) become useless when they run out of chemical energy.
Quiz
Refer to the text in this chapter if necessary. A good score is at least 18 correct answers out of these 20 questions. The answers are listed in the back of this book.
1. The number of protons in the nucleus of an atom always
(a) equals its atomic number.
(b) equals its atomic weight.
(c) equals the number of electrons.
(d) equals the number of neutrons plus the number of electrons.
2. The number of neutrons in the nucleus of an atom sometimes
(a) equals its atomic number.
(b) equals its atomic weight.
(c) equals the number of protons.
(d) More than one of the above
3. The atomic weight of an atom always
(a) equals the number of electrons.
(b) equals the number of protons.
(c) equals the number of neutrons.
(d) approximately equals the number of neutrons plus the number of protons.
4. When an atom has a net negative electric charge, we can call it
(a) an anion.
(b) a cation.
(c) diatomic.
(d) positronic.
5. An atom can have
(a) more than one isotope.
(b) only one isotope.
(c) no more protons than neutrons.
(d) no more neutrons than protons.
6. An element whose atoms can have more than one atomic weight
(a) cannot exist.
(b) always has an electric charge.
(c) shares protons with surrounding atoms.
(d) is a common occurrence in nature.
7. A compound comprising three atoms
(a) cannot exist.
(b) always has an electric charge.
(c) shares protons with surrounding atoms.
(d) is a common occurrence in nature.
8. Ionization by itself never causes
(a) the conductivity of a substance to improve.
(b) an atom to gain or lose protons.
(c) an electrically neutral atom to become charged.
(d) an atom to gain or lose electrons.
9. Which of the following substances is the worst electrical conductor?
(a) Mercury
(b) Aluminum
(c) Glass
(d) Silver
10. Which of the following substances allows electrons to move among its atoms with the greatest ease?
(a) Copper
(b) Pure water
(c) Dry air
(d) Porcelain
11. If we place 12 V across a component whose resistance equals 6 ohms, how much current will flow through the component?
(a) 0.5 A
(b) 2 A
(c) 72 A
(d) We need more information to say.
12. If we double the resistance in the situation of Question 11 but don’t change the voltage, the current will
(a) not change.
(b) get cut in half.
(c) double.
(d) quadruple.
13. The term static electricity refers to
(a) voltage with no current.
(b) current with no voltage.
(c) current through an infinite resistance.
(d) voltage that never changes.
14. Which of the following general statements applies to dielectric materials?
(a) They have extremely low resistance (practically zero).
(b) They have extremely high resistance (practically infinite).
(c) They have resistance that depends on the current through them.
(d) They produce two different voltages at the same time.
15. We can express the quantity of electrons flowing past a fixed point per unit of time in
(a) coulombs.
(b) volts.
(c) ohms.
(d) amperes.
16. In a lightning stroke, the term channel means
(a) a current-carrying path of ionized air.
(b) alternating-current frequency.
(c) a stream of moving protons and neutrons.
(d) a flowing stream of cool gas.
17. The term electromotive force (EMF) is an alternative expression for
(a) current.
(b) charge.
(c) voltage.
(d) resistance.
18. When you shuffle across a carpeted floor on a dry winter afternoon, you can acquire a potential difference, with respect to ground, of
(a) an ohm or two.
(b) up to about 200 ohms.
(c) millions of ohms.
(d) None of the above
19. Which of the following devices directly converts chemical energy to electricity?
(a) A generator
(b) A dry cell
(c) A motor
(d) A photovoltaic cell
20. Which of the following devices directly converts visible light to electricity?
(a) A generator
(b) A dry cell
(c) A motor
(d) A photovoltaic cell