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Unformatted text preview: 1 8/28/02 ECE 2317 Course Notes Electrostatics Charge and Current Motivation: The basic sources of electromagnetic (electrical) forces are stationary and moving electric charges, which exert forces on other stationary and moving electric charges. To study these forces, the concept of force fields has been devised. When charges are stationary, the force field is an electric ( electrostatic ) field. When charges are moving with a constant velocity, the force field is a magnetic ( magnetostatic ) field. Accelerating charges produce electromagnetic fields. Brief historical perspective: Observations of the forces between electric charges have been recorded back to at least 600 BC. Rubbing together fur and glass, resin and silk, putting two dissimilar metals together, and even flying kites in lightning storms (dont try this at home!!) created electrical effects that mystified and amazed people for centuries. Two glass rods rubbed with silk and placed near one another were observed to repel each other. On the other hand, if a resin (plastic) rod rubbed with fur is placed near one of the glass rods, the force was attractive. However, it was not until the mid 1700s that many of these effects began to be quantified and widely referenced. During this time Benjamin Franklin is credited with naming electrical charge; that appearing on the glass rod was designated positive and that on the resin (plastic) rod, negative . Between 1755-1766 Franklin and Joseph Priestley made several fundamental experimental observations and postulated an inverse square law dependence for the electrical force between electric charges: The attraction of electricity is subject to the same laws as that of gravitation. In 1769 John Robison and in 1773 Henry Cavendish experimentally verified that the electrical force between two charges varies inversely as the square of the distance between them. Unfortunately, both of these men never published their results. Their work was not discovered until after 1785 when Charles Coulomb experimentally measured this force using a torsion balance. He published his results and was properly credited with the discovery. In 1800, Alessandro Volta invented a voltaic piller or pile , the worlds first manmade battery. Now sparks could be generated, wires heated, and new electric and magnetic effects observed, all due to the movement of electric charge ( current ). 2 Charge Distributions: The smallest unit of charge is that of an electron and proton, Electron: 19 1.602 10 e q = [C, Coulombs] Proton: 19 1.602 10 e q = + [C] The concept of a point charge is used when the dimensions of the electric charge distribution are very small....
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- Spring '08