In electricity and electronics, there are phenomena that involve other forms of energy besides elec- trical energy. Visible light is an example. A light bulb converts electricity into radiant energy that you can see. This was one of the major motivations for people like Thomas Edison to work with electricity. Visible light can also be converted into electric current or voltage. A photovoltaic cell
does this.
Light bulbs always give off some heat, as well as visible light. Incandescent lamps actually give off more energy as heat than as light. You are certainly acquainted with electric heaters, designed for the purpose of changing electricity into heat energy. This heat is a form of radiant energy called
infrared(IR). It is similar to visible light, except that the waves are longer and you can’t see them. Electricity can be converted into other radiant-energy forms, such as radio waves,ultraviolet
(UV), and X rays.This is done by specialized devices such as radio transmitters, sunlamps, and elec- tron tubes. Fast-moving protons, neutrons, electrons, and atomic nuclei are an important form of energy. The energy from these particles is sometimes sufficient to split atoms apart. This effect makes it possible to build an atomic reactor whose energy can be used to generate electricity.
When a conductor moves in a magnetic field, electric current flows in that conductor. In this way, mechanical energy is converted into electricity. This is how an electric generatorworks. Gener- ators can also work backward. Then you have a motorthat changes electricity into useful mechani- cal energy.
A magnetic field contains energy of a unique kind. The science of magnetismis closely related to electricity. Magnetic phenomena are of great significance in electronics. The oldest and most uni- versal source of magnetism is the geomagnetic fieldsurrounding the earth, caused by alignment of iron atoms in the core of the planet.
A changing magnetic field creates a fluctuating electric field, and a fluctuating electric field pro- duces 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 an- other over and over again through space.
Chemical energy is converted into electricity in dry cells,wet cells, and batteries.Your car battery is an excellent example. The acid reacts with the metal electrodes to generate an electromotive force. When the two poles of the batteries are connected, current results. The chemical reaction contin- ues, keeping the current going for a while. But the battery can only store a certain amount of chem- ical energy. Then it “runs out of juice,” and the supply of chemical energy must be restored by
charging.Some cells and batteries, such as lead-acid car batteries, can be recharged by driving cur- rent through them, and others, such as most flashlight and transistor-radio batteries, cannot.
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 atomic number of an element is determined by (a) the number of neutrons.
(b) the number of protons.
(c) the number of neutrons plus the number of protons. (d) the number of electrons.
2. The atomic weight of an element is approximately determined by (a) the number of neutrons.
(b) the number of protons.
(c) the number of neutrons plus the number of protons. (d) the number of electrons.
3. Suppose there is an atom of oxygen, containing eight protons and eight neutrons in the nucleus, and two neutrons are added to the nucleus. What is the resulting atomic weight?
(a) 8 (b) 10 (c) 16 (d) 18 4. An ion
(a) is electrically neutral. (b) has positive electric charge. (c) has negative electric charge.
(d) can have either a positive or negative charge. 5. An isotope
(a) is electrically neutral. (b) has positive electric charge. (c) has negative electric charge.
6. A molecule
(a) can consist of a single atom of an element. (b) always contains two or more elements. (c) always has two or more atoms. (d) is always electrically charged. 7. In a compound,
(a) there can be a single atom of an element. (b) there must always be two or more elements.
(c) the atoms are mixed in with each other but not joined. (d) there is always a shortage of electrons.
8. An electrical insulator can be made a conductor (a) by heating it.
(b) by cooling it. (c) by ionizing it. (d) by oxidizing it.
9. Of the following substances, the worst conductor is (a) air.
(b) copper. (c) iron. (d) salt water.
10. Of the following substances, the best conductor is (a) air.
(b) copper. (c) iron. (d) salt water.
11. Movement of holes in a semiconductor
(a) is like a flow of electrons in the same direction. (b) is possible only if the current is high enough. (c) results in a certain amount of electric current. (d) causes the material to stop conducting. 12. If a material has low resistance, then
(a) it is a good conductor. (b) it is a poor conductor.
(c) the current flows mainly in the form of holes. (d) current can flow only in one direction. 13. A coulomb
(a) represents a current of 1 ampere. (b) flows through a 100-watt light bulb. (c) is equivalent to 1 ampere per second.
(d) is an extremely large number of charge carriers.
14. A stroke of lightning
(a) is caused by a movement of holes in an insulator. (b) has a very low current.
(c) is a discharge of static electricity. (d) builds up between clouds. 15. The volt is the standard unit of
(a) current. (b) charge.
(c) electromotive force. (d) resistance.
16. If an EMF of 1 volt is placed across a resistance of 2 ohms, then the current is (a) half an ampere.
(b) 1 ampere. (c) 2 amperes.
(d) impossible to determine.
17. A backward-working electric motor, in which mechanical rotation is converted to electricity, is best described as
(a) an inefficient, energy-wasting device.
(b) a motor with the voltage connected the wrong way. (c) an electric generator.
(d) a magnetic field.
18. In a battery, chemical energy can sometimes be replenished by (a) connecting it to a light bulb.
(b) charging it. (c) discharging it.
(d) no means known; when a battery is dead, you must throw it away. 19. A fluctuating magnetic field
(a) produces an electric current in an insulator. (b) magnetizes the earth.
(c) produces a fluctuating electric field. (d) results from a steady electric current. 20. Visible light is converted into electricity
(a) in a dry cell. (b) in a wet cell.
(c) in an incandescent bulb. (d) in a photovoltaic cell.
THIS CHAPTER EXPLAINS,IN MORE DETAIL,STANDARD UNITS THAT DEFINE THE BEHAVIOR OF DIRECT-
current (dc) circuits. Many of these rules also apply to utility alternating-current (ac) circuits.
The Volt
In Chap. 1, you learned a little about the volt, the standard unit of electromotive force (EMF) or
potential difference.
An accumulation of electrostatic charge, such as an excess or shortage of electrons, is always as- sociated with a voltage. There are other situations in which voltages exist. Voltage can be generated at a power plant, produced in an electrochemical reaction, or caused by light rays striking a semi- conductor chip. It can be produced when an object is moved in a magnetic field, or is placed in a fluctuating magnetic field.
A potential difference between two points produces an electric field, represented by electric lines of flux(Fig. 2-1). There is a pole that is relatively positive, with fewer electrons, and one that is rel- atively negative, with more electrons. The positive pole does not necessarily have a deficiency of electrons compared with neutral objects, and the negative pole does not always have a surplus of electrons relative to neutral objects. But the negative pole always has more electrons than the posi- tive pole.
The abbreviation for volt (or volts) is V. Sometimes, smaller units are used. The millivolt(mV) is equal to a thousandth (0.001) of a volt. The microvolt(µV) is equal to a millionth (0.000001) of a volt. It is sometimes necessary to use units larger than the volt. One kilovolt(kV) is one thousand volts (1000 V). One megavolt (MV) is 1 million volts (1,000,000 V) or one thousand kilovolts (1000 kV).
In a dry cell, the voltage is usually between 1.2 and 1.7 V; in a car battery, it is 12 to 14 V. In household utility wiring, it is a low-frequency alternating current of about 117 V for electric lights and most appliances, and 234 V for a washing machine, dryer, oven, or stove. In television sets, transformers convert 117 V to around 450 V for the operation of the picture tube. In some broad- cast transmitters, the voltage can be several kilovolts.
17
2
CHAPTER