SmartPhysics_em_1-4 - PA R T I E L E C T R I C I T Y UNIT 1 COULOMBS LAW 1.1 Overview Electricity and magnetism are the central topics to be developed

SmartPhysics_em_1-4 - PA R T I E L E C T R I C I T Y UNIT 1...

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P A R T I : E L E C T R I C I T Y U N I T 1 COULOMB’S LAW 1.1 Overview Electricity and magnetism are the central topics to be developed in this course. The one new big idea we will develop is that we will need to modify our description of the world to include the presence of a new kind of thing: fields. In particular, to account for new kinds of phenomena, we will need to introduce the concepts of electric and magnetic fields. In this first unit, we will introduce the concept of electric charge, the ultimate source of these electric and magnetic fields. We’ll discuss some basic features of electric charge and then introduce Coulomb’s law, the quantitative description of the forces between charges.
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Unit 1 2 We’ll then discuss the principle of superposition which allows us to determine the forces when more than two charges are present. We’ll then apply this principle of superposition to a couple of examples to see how it works. 1.2 Electric Charge We’ve just said that we will need to change our basic framework to include electric and magnetic fields as fundamental entities, and these fields are determined by the distribution and motion of electric charge. The first question then is what is this electric charge anyway? This question is not an easy one to answer. During the mechanics course, we talked about a box sliding down an inclined plane, but we never asked “what is a box?” What a box is seems obvious, and the reason a box slides down the plane is that the box has mass. What is this mass anyway? This question didn’t seem too important during the mechanics course because you have an intuitive sense of mass. However, your intuitive sense of mass probably comes from your understanding of weight, which we understand in terms of the gravitational force. In other words, we know about mass because it is the thing that is responsible for gravitational forces. Well, electric charge is the thing that is responsible for electric forces. That’s really about as good a description as we can make. Accepting that electric charge is the thing that is responsible for electric forces, we can learn more about charge by investigating its properties. The first important property of electric charge is that, unlike mass, there are two kinds of electric charge: positive and negative. Because there is only one kind of mass, the gravitational force exerted by one object on another object is always attractive. However, the existence of both positive and negative charges results in both attractive and repulsive electrical forces being exerted between charges. In particular, we say unlike charges attract and like charges repel. Figure 1.1 shows the magnitudes and directions of these forces. It is important to note that the forces on each pair of charges are in opposite directions and have equal magnitude, in accordance with Newton’s third law.
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