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Unformatted text preview: Chapter 1 Electric Charge; Coulomb’s Law 1.1 The Important Stuff 1.1.1 Introduction During the second semester of your introductory year of physics you will study two special types of forces which occur in nature as a result of the fact that the constituents of matter have electric charge; these forces are the electric force and the magnetic force . In fact, the study of electromagnetism adds something completely new to the ideas of the mechanics from first semester physics, namely the concept of the electric and magnetic fields . These entities are just as real as the masses and forces from first semester and they take center stage when we discuss the phenomenon of electromagnetic radiation, a topic which includes the behavior of visible light. The entire picture of matter and fields which we will have at the end of this study is known as classical physics , but this picture, while complete enough for many fields of engineering, is not a complete statement of the laws of nature (as we now know them). New phenomena which were discovered in the early 20th century demanded revisions in our thinking about the relation of space and time (relativity) and about phenomena on the atomic scale (quantum physics). Relativity and quantum theory are often known collectively as modern physics . 1.1.2 Electric Charge The phenomenon we recognize as “static electricity” has been known since ancient times. It was later found that there is a physical quantity known as electric charge that can be transferred from one object to another. Charged objects can exert forces on other charged objects and also on uncharged objects. Finally, electric charge comes in two types, which we choose to call positive charge and negative charge . Substances can be classified in terms of the ease with which charge can move about on their surfaces. Conductors are materials in which charges can move about freely; insula- tors are materials in which electric charge is not easily transported. 1 2 CHAPTER 1. ELECTRIC CHARGE; COULOMB’S LAW Electric charge can be measured using the law for the forces between charges (Coulomb’s Law). Charge is a scalar and is measured in coulombs 1 . The coulomb is actually defined in terms of electric current (the flow of electrons), which is measured in amperes 2 ; when the current in a wire is 1ampere, the amount of charge that flows past a given point in the wire in 1 second is 1 coulomb. Thus, 1ampere = 1A = 1 C s . As we now know, when charges are transferred by simple interactions (i.e. rubbing), it is a negative charge which is transferred, and this charge is in the form of the fundamental particles called electrons . The charge of an electron is 1 . 6022 × 10- 19 C, or, using the definition e = 1 . 602177 × 10- 19 C (1.1) the electron’s charge is- e . The proton has charge + e . The particles found in nature all have charges which are integral multiples of the elementary charge e : q = ne where n = 0 , ± 1 , ± 2 ... . Because of this, we say that charge is....
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