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Electricity and Magnetism Physics 102 Professor Lee Carkner Lecture 8 Electricity and Magnetism In E&M, we will deal with forces that depend upon charge Charged particles generate E&M forces Stationary charges exert an electric force E&M forces only affect charged particles Forces The electric force is much stronger than the gravitational force Why don’t we feel this strong force? Most things have roughly equal numbers of positive and negative particles Like charges repel, opposite attract Charges want to arrange themselves so there is no force e.g. lightning, static electric shock Plus and Minus The basic particle of negative charge is the electron The basic particle of positive charge is the proton They are bound in the nucleus Adding electrons makes something negative All E&M forces depend on what the electrons are doing Using Electricity Why is the electrical force important? Convert electrical energy into work Convert electrical energy into heat and light Convert electrical energy into sound Electrical energy can be very finely controlled Computer, internet Units of Charge The unit of charge is the Coulomb (C) The electron and the proton have equal and opposite charges: 1 e = 1.60 X10-19 C Charge is represented by the variable q (or sometimes Q) Electric Force F = k q1 q2/r2 Where: k is the Coulomb constant (8.99 X 109 N m2/C2) q1 and q2 are the two charges (in Coulombs) You must assign a sign to F at the end r is the distance between them (in meters) Direction of Forces but, to find the direction of the force we need to use the rule: opposites attract, like repel n.b., the direction of the force does not come out of the equation, you have to find it yourself Making Electricity Three ways to do this Chemically e.g. Magnetically e.g. Physically Four basic methods: friction, conduction, induction, polarization Triboelectricity Rubbing will transfer electrons from one substance to the other by friction Example: Glass rubbed with silk becomes positive, rubber rubbed with fur becomes negative Triboelectric Charging eFur Rubber Rubbing a piece of rubber with fur transfers the electrons from the fur to the rubber due to friction. How Does Charge Move? Conductors When you charge a conductor, the electrons will flow through it Other types do not allow electrons to flow (e.g. glass, rubber) Insulators If you charge them, the charge stays put Conduction Both end up with the same sign charge Example: shocking your friend Conductive Charging Neutral Metal Charged Metal e- Both Metal Rods Now Charged Charge will move from one conductor to another Induction attract the opposite sign charges to the near end The whole conductor has no net charge, but each end does Will always attract the original charged object Inductive Charging Charged Metal Induced Charge on Metal A charged conductor will split the charge on a near-by conductor Induction via Grounding If you connect a conductor to the Earth, an endless amount of charge can flow from it to the ground If you place a charged object near a grounded conductor it will repel the same sign charges to the ground Charge is opposite that of the inductor Inductive Charging with Grounding Charged Metal Metal becomes positive Electrons pushed to ground A charged conductor will push out the same charge to the ground, leaving the other conductor with an opposite charge Polarization You have charged the balloon but not the wall The negative charge on the balloon attracts the positive charged parts of the molecules of the wall, polarizing it Polarization Origins of Electricity Moving charges by rubbing has been known since ancient times Benjamin Franklin proposed the terms positive and negative for the two types of charge Franklin’s Kite Franklin demonstrated that lightning is a form of electricity He flew a kite in a thunderstorm and saw that charge flowed down the string Next Time Read: 16.5-16.7 Homework: Ch 16: P 8, 12, 23, 24