Working through this chapter of the study guide will enable you to:

1. Define the fundamental property known as electric charge.
2. Explain how and when electric charge can move from one place to another and thereby produce electric current.
3. Describe magnetic fields using the concepts of magnetic field lines and magnetic poles.
4. Show how the interaction of electric and magnetic effects can be incorporated into one unified theory known as electromagnetism.
5. Understand how the control of electron flow in circuits forms the basis for today's high technology lifestyle.

Discussion

Electric and magnetic phenomenon have been known for centuries, and many scientists have contributed to our understanding of these effects. The roots of both concepts lie deep in the atomic structure of matter, where the basic building blocks of atoms - electrons and protons - are the fundamental charge carriers. Electrons have been defined to carry negative charge, whereas protons are positively charged. It is the motion of these basic particles, primarily electrons, that constitutes electric current in conductors. Neutrons are also present in most atoms, but because they carry no electrical charge, they play no direct role in our understanding of electrical phenomenon.
The forces between charged objects can be calculated using Coulomb's law, and these forces give rise to electric potential energy that drives electrons through conductors to produce electric currents. The understanding of electric work and power is very important for us in today's areas of electronic communication, heating and cooling, computer control, and electronics. Not only is a large percentage of our fixed income spent on the electric power (really electric work) that is supplied to us for heating, lighting, cooking, and cooling by the commercial electric power industry, but much of our discretionary buying is done in the areas of television, electronic games, CDs and tapes, computers, and other types of electronic entertainment.
At first glance, magnetism may not appear to be as intimately involved in our everyday lives as electricity, but magnetic interactions in large generators produce nearly all of the commercial electricity available for our use. Magnetism also stores the sounds of our voices and music on tapes, our thoughts on computer disks, and even provides the force that overcomes gravity and reduces friction on high-speed trains that are used for mass transportation in some parts of the world.
It is not really surprising that electricity and electronics make up two of the fastest growing economic influences in today's society. No responsible citizen can afford to be ignorant of the basic concepts of electricity and magnetism if he or she is going to take their rightful place in our fast-paced world of modern technology. It is hard to even imagine what the next few years of space exploration, high speed communication, data storage, and electronic control will bring into our already fast-paced lives.

Section  8.1 Electric Charge and Current


It seems best to begin our study of electricity with the basic structure of the atom. Electrons, protons, and neutrons make up the atoms that comprise all matter, but only electrons and protons carry electric charge. Electrons are designated as negative (—) charge carriers, whereas protons carry positive (+) charge. Since normal matter is made up of equal numbers of electrons and protons, atoms are normally neutral in charge and so is the matter composed of these atoms, unless some of the charges (usually electrons) have been transferred from one place to another.
The continuous movement of electrical charge from one location to another, usually through a metal conducting wire, is called an electric current. Materials such as wood or plastic, which do not allow the flow of electric current, are known as insulators. Electric charge (q) is measured in units called coulombs, and the flow of electric current (I) is measure in amperes. By definition, electric current is the quantity of electric charge flowing through a conductor divided by the time during which this flow occurs.
I = q / t
If a net electric charge exists on any two objects, an electric force will always exist between them. The magnitude of these forces between charged objects can be calculated by using Coulomb's law, as given in Eq. 8.2 in the textbook. This law is ultimately responsible for such varied processes as holding atoms together and causing the flow of electrons in metallic conductors. Electric forces can be either attractive or repulsive. If the two charges involved are of opposite signs, they are attracted to each other. When the two charges are alike, either + to + or — to —, their interaction is repulsive.

Section  8.2 Voltage and Electric Power


For electricity to be useful in everyday situations, we must first separate electric charges. This is because the equal numbers of electrons and protons in normal atoms make most objects electrically neutral. Several common methods of electric charge separation are by mechanical means such as rubbing a glass rod with a silk cloth or in a motor-driven electric generator, by chemical means such as is observed in batteries of various kinds, and by direct solar interaction with photocells. Charge separation produces an electric potential, or voltage (V), which is defined as work per unit charge.
V = W / q
Once a voltage has been established, electric current (I) can flow through any appropriate conducting path that is provided. Even the best conductors of electricity tend to show some reluctance to having current flow through them. This is referred to as electrical resistance (R). Resistance varies greatly, depending on the type of material through which the current is flowing. A simple relationship among voltage, resistance, and electric current flow is expressed by Ohm's law.
V = I R
We can also define the concept of electrical power (P) using these same basic electrical quantities.
P = I V = I (I R) = I ² R

Section  8.3 Simple Electric Circuits and Electrical Safety


The flow of electrical current can occur through conductors in two basic ways. If the current always flows in the same direction, it is called direct current, or dc. If the current flows back and forth through the conductor, reversing its direction of travel several times each second, we have alternating current, or ac. Electrical components such as resistors can be arranged in ac and dc circuits either in series or in parallel.
Pay particular attention to the last part of Section  8.3, which deals with electrical safety. Many people are injured or even killed each year because of carelessness or faulty electrical equipment. You should not be afraid of electricity, but you must be aware of its potential hazards so that you can avoid them. Three-prong electric plugs and outlets, polarized plugs, and direct ground wires have become quite common and in many cases are required by law. These safety features should not be ignored or disconnected if you wish to maintain a high level of electrical safety in your home, school, and work site.

Section  8.4 Magnetism


Magnets have two distinct regions, called magnetic poles, that are designated as either north (N) or south (S). The law of magnetic poles is very similar to Coulomb's law in that like poles repel, whereas unlike poles attract. When we study magnetic effects, it is often helpful to think about the concept of magnetic fields, which can be drawn around and through magnetic materials. See Figs. 8.16 and 8.17 in the textbook. Electric fields are also quite often useful in describing the interaction of electric and magnetic phenomena.

Section  8.5 Electromagnetism


The interaction between electricity and magnetism is very important. Electromagnets lift heavy metal objects, electromagnetic generators produce the electrical power that we use in our homes and businesses, and electromagnetic tapes and disks store sound and information. All of these processes are possible because there is an interaction between moving electric charges and magnetic fields. Such interactions are also responsible for the operation of electric motors and generators, transformers for adjusting ac voltages in electrical devices such as calculators and television sets, and in many other applications that we often take for granted in our technology-assisted lives.
Electrical devices such as cathode ray tubes produce black-and-white or color pictures in television sets and computer monitors for our entertainment and education. Transistors and diodes have become indispensable in controlling the electronic devices that we have come to rely on. The microchips used in computers, automobile ignition systems, and many other control applications have revolutionized our ideas about data storage and equipment control in the last few years.

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