Introduction to Understandable Physics
Volume III - Electricity, Magnetism and Light
by
Book Cover & Preview Text
20-1. Introduction
Electrostatics extends our earlier study of forces in equilibrium by including the nature of electrical forces. Accordingly we will examine stationary systems as done earlier, but with special attention to forces caused by electric charges. We are probably familiar with the fact that charges may be either positive or negative, and that opposite charges attract while same charges repel. In this chapter the quantitative nature of these forces will be explored. We will find that we can utilize the electrical force per unit charge as an electric field, and that the corresponding electrical potential energy per unit charge is a useful quantity called the electric potential. Furthermore, we will see how the electric energy per unit volume can be expressed as the electric field energy density
. The physical properties of electric charges, forces, and energy have resulted in many practical applications. Electric circuits store charge in capacitors, which we will examine in some detail. A Van de Graaff generator produces and stores charge on a large capacitor in the form of a metal sphere. The Millikan oil drop experiment, discussed earlier in Chapter 12, utilized electrostatic properties to determine the charge of the electron. An oscilloscope and TV use electrostatic forces to steer their electron beams to impact the desired locations on their image screens. Finally, Xerox copy machines utilize electrostatic attraction of ink particles called toner to reproduce images. We will explore the electrical properties of each of the above in this chapter.
20-2. Electric Charge
History
Evidence for electricity such as lightning, sparking, and simple electrostatic phenomena, have been observed since ancient times. History records Thales of Miletus (640? - 546 BC), a Greek natural philosopher, as the first to conduct systematic studies of electricity, whereby he examined how light objects such as feathers, fluff, and dried leaf bits were attracted to the fossil resin amber when rubbed with a cloth. It is interesting that the word electron is derived from the Greek word for amber. Much later in 1570 English physician William Gilbert (1540 -1603) began to discover this phenomena in many additional materials, including a number of gems, and in 1600 he correctly deduced that these forces were different from the forces produced by magnetic loadstones. Later in the 1660's, German physicist Otto von Guericke (1602 -1682) used sulfur as an inexpensive material for the developing larger electrostatic generators, which he subsequently used to show that electric forces can be either attractive or repulsive. In 1729 English electrician Stephen Gray (1696 -1736) conducted experiments that showed that the force could be transferred from one location to another, suggesting that there is some type of force carriers. Shortly later in 1733 French chemist Charles Francois Du Fay (1698 - 1739) interpreted from his experiments that the carriers constitute two types of fluids to account for both the attractive and repulsive forces.
American diplomat and scientist Benjamin Franklin (1706 -1790) began conducting electrical studies in 1746, and he modified the concept of two fluids to one fluid as follows. He assumed that when a material had the appropriate amount of this fluid, the material was neutral with no charge. When some of the fluid was removed from a neutral material, the material had a deficit of the fluid charge, and when fluid was added to a neutral material, the material had a surplus of the fluid charge. Today we understand that Franklin's fluid is made of electrons, which have negative charge. Thus, removal of electrons leaves the material with a positive charge and addition of electrons makes the material have a negative charge. Actually Franklin assigned a positive charge for the fluid, and as a result electricians have treated electrical currents as a flow of positive charge; however, because this is the same as flow of negative charge in the opposite direction, the same results occur regardless of how the signs of the charge are assigned. In fact, Franklin's understanding of lightning from his famous kite experiment in 1752 resulted in his invention of a lightning rod that is still useful, regardless of the sign he used for the electrical fluid charge!
In fairness to Du Fay's two fluid interpretation of electricity, it is valid under certain circumstances. For example, during electroplating of silver onto a surface, positively charged silver ions move through a solution and are deposited on the surface of interest. The motion of these ions constitutes a fluid with positive charge. Also many particle accelerators use beams of protons, which act as a fluid of positive charge. On the other hand, Franklin's single fluid and his assumption that matter is normally neutral leads to the concept of conservation of electric charge, which does not necessarily follow from du Fay's approach. Specifically, we now know that each electron has a unit of negative charge and each proton in the atomic nucleus has an identical unit of positive charge; because each atom has the same number of electrons as protons, matter as a whole has an equal amount of positive and negative charge. In certain situations, electrons from one type atom may be taken away by another type atom to create positively and negatively charged ions, in which the number of electrons and protons differ per atom; however, the total numbers of electrons and protons in all the atoms are still equal.
Formats
Book Details
About the Book
Will Winn has writtenIntroduction to Understandable Physics with the goal of presenting physics in a building-block fashion. Accordingly, Volume III. Electricity, Magnetism and Lightrequires a knowledge of Volume I. Mechanics and Volume II. Matter, Heat and Waves. Volume III begins with a study of electric charges, their electric fields/forces, and subsequently their motion as electric currents. These currents are shown to produce magnetic fields/forces, where electromagnets are studied as models for understanding permanent magnets. Next, the reverse process where magnetic fields produce current is examined and applied for generating electricity. AC and DC circuits exemplify further applications. Finally, electric and magnetic fields are found to produce electromagnetic waves that move at the speed of light.
The study of light begins with historical measurements of its speed and then examines its electromagnetic power intensity, light spectra, human response and color perception. Next, light reflection and refraction are applied to mirrors, lenses, rainbows, eyeglasses, telescopes and microscopes. Subsequently, the text examines the wave nature of light, as exhibited by its diffraction and interference phenomena. Furthermore, when the electric field amplitudes of waves are oriented along one dimension, light is polarized. Polaroids filter out such “glaring” light when used in sunglasses. Finally, various light experiments provided early clues for discovering relativity and quantum mechanics, which are examined in Volume IV.
Near the end of each chapter a Simple Projects section suggests experiments and/or field trips that can reinforce the physics covered. Some experiments are simple enough for students to explore alone, while others benefit from equipment available to physics instructors. Also {optional} text sections provide students with a deeper appreciation of the subject matter; however these are not required for continuity. Some of these optional topics can be candidates for term projects.
About the Author
Will Winn has enjoyed a 50-yr physics career, which includes various experiences in teaching , pure research, and industrial applications. Early in his career he taught physics at both secondary institutions and universities, namely Cornell and Virginia Commonwealth, and in later years he taught again at University of South Carolina Aiken. Upon receiving his Ph.D. in nuclear physics at Cornell in 1968, he had commenced a post-doctoral appointment at the Washington University Cyclotron facility to conduct research on nuclear energy levels, and he continued this work with another post-doctoral appointment at the MP Van de Graaff accelerator at the University of Rochester. Seeking to apply his background to practical applications, he completed a masters degree program in Nuclear Engineering at University of Virginia in 1974. Then he began his research at the Savannah River Site, which included reactor experiments, nuclear chemistry, non-destructive testing, environmental monitoring, and non-proliferation studies. In 2000 he retired from SRS to teach math and physics at USCA, after which he commenced writing this text. Here, he refined physics notes he prepared for his USCA students, as well as drawing from his career writing experience, which includes over 60 published papers.
Dr. Winn has encountered a wide range of activities related to physics over the years. He has conducted research with university professors, chemists, biologists, geologists, engineers, and other physicists. As a former premed student who switched to physics, he also has had a continuing interest in the physics involved in medical applications. Accordingly, he has a good appreciation for the role of physics in a considerable number of fields, and this has influenced his teaching of students who require an introductory physics course. Furthermore, he has applied this experience to writing Introduction to Understandable Physics.