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Unformatted text preview: PHY 2049: Introductory Electromagnetism Notes From the Text Chris Mueller Dept. of Physics, University of Florida 3 January, 2009 1 Introduction The subject of this course is the physics of electricity and magnetism. We have many different concepts and ideas to study before we will have a firm understanding of electromagnetism, but it is interesting to note from the outset that the entire theory can be written down in five fairly simple equations. ∇ · E = ρ ∇ × E =- ∂ ∂t B ∇ · B = 0 ∇ × B = 1 c 2 ∂ ∂t E + μ J F = q ( E + v × B ) 2 Chapter 21: Electric Charge 2.1 Electric Charge Electric charge is an intrinsic quantity of a particle. We can not ’see’ this charge, but we can experience its effects through the force that it exerts when placed near another charged object. Unlike gravity (which is always attractive), charged objects can exert both attractive and repulsive forces on each other. We distinguish the two types of charge by calling one negative and the other positive. Charges with the same sign repel each other while charges with opposite signs attract each other. Note that the direction of the force always lies along the line connecting the two charges. Recall that an atom is made up of a positively charged nucleus and negatively charged electrons, so that most atoms are electrically neutral. We can classify materials by their ability to transmit charge, i.e. how easily charge flows through them. Materials which allow charge to flow essentially unimpeded are called conductors, while those which do not allow charge to flow are called insulators. Most metals are good conductors which is the reason that electrical wires and circuits are made up of metal components. Plastics are generally insulators however which is why most wires are wrapped in a plastic coating. 2.2 Coulomb’s Law We can describe the force between two charged particles through Coulomb’s Law: ~ F = 1 4 π q 1 q 2 r 2 ˆ r (1) where is a universal constant, q 1 and q 2 are the charge on each particle, and r is the distance between the two. Notice the similarities to Newton’s Law of Gravitation, ~ F g = ( G ) m 1 m 2 r 2 ˆ r (2) The constant which appears in Coulomb’s Law is called the permittivity of free space and is written in such a strange way because shows up in other places alone. Given the similarity between these two force laws, we should expect other similarities as well. One of these similarities occurs with spherically symmetric objects, which have the following prop- erties: 1. A shell of uniform charge attracts or repels a charged particle that is outside the shell 1 as if all of the charge were located at its center. 2. A charged particle inside of a shell of uniform charge does not experience any net force from the shell. 3. Any excess charge placed on a spherical conductor will rearrange itself until it is uniformly distributed on the outside of the conductor....
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