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Ch0201

# Ch0201 - Chapter 1 Charge Coulomb's Law 1 Charge Coulomb's...

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Chapter 1 Charge & Coulomb's Law 2 1 Charge & Coulomb's Law Charge is a property of matter. There are two kinds of charge, positive “+” and negative “ ”. 1 An object can have positive charge, negative charge, or no charge at all. A particle which has charge causes a force-per-charge-of-would-be-victim vector to exist at each point in the region of space around itself. The infinite set of force-per-charge-of-would-be-victim vectors is called a vector field. Any charged particle that finds itself in the region of space where the force-per- charge-of-would-be-victim vector field exists will have a force exerted upon it by the force-per- charge-of-would-be-victim field. The force-per-charge-of-would-be-victim field is called the electric field. The charged particle causing the electric field to exist is called the source charge. (Regarding jargon: A charged particle is a particle that has charge. A charged particle is often referred to simply as “a charge.”) The source charge causes an electric field which exerts a force on the victim charge. The net effect is that the source charge causes a force to be exerted on the victim. While we have much to discuss about the electric field, for now, we focus on the net effect, which we state simply (neglecting the “middle man”, the electric field) as, “A charged particle exerts a force on another charged particle.” This statement is Coulomb’s Law in its conceptual form. The force is called the Coulomb force , a.k.a. the electrostatic force . Note that either charge can be viewed as the source charge and either can be viewed as the victim charge. Identifying one charge as the victim charge is equivalent to establishing a point of view, similar to identifying an object whose motion or equilibrium is under study for purposes of applying Newton’s 2 nd Law of motion, m = F a harpoonrightnosp harpoonrightnosp . In Coulomb’s Law, the force exerted on one charged particle by another is directed along the line connecting the two particles, and, away from the other particle if both particles have the same kind of charge (both positive, or, both negative) but, toward the other particle if the kind of charge differs (one positive and the other negative). This fact is probably familiar to you as, “like charges repel and unlike attract.” The SI unit of charge is the coulomb, abbreviated C. One coulomb of charge is a lot of charge, so much that, two particles, each having a charge of +1 C and separated by a distance of 1 meter exert a force of 9 10 9 × N, that is, 9 billion newtons on each other. This brings us to the equation form of Coulomb’s Law which can be written to give the magnitude of the force exerted by one charged particle on another as: 1 It can be argued that, since the net charge on an object consisting of a bunch of particles, each of which has a positive amount of charge, and a bunch of particles, each of which has a negative amount of charge, is simply the

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