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Static Electricity: Electric Charge & Electric Field Ch 16 in your book Electric Charge Ideas 2 types of charge: + and – All of “electricity” is a result of charges and their interactions Most materials have equal numbers of positive and negative charges, so we don’t notice “electric affects” Objects become charged by friction, conduction (touching a charged object) or induction (we did this in a lab……) Electric charge Charge is quantized, it exists only in discrete amounts The smallest amount of charge is one electron = one proton = 1.6 x 10-19 C Conductors (metals typically) allow charges (electrons) to move about easily, insulators (plastic, glass, waffles) don't Electric charge on conductors If a conductor is charged (say has a bunch of excess electrons) where are the charges located A) evenly distributed? B) on the outside? C) anywhere they want? Electric Charge & Force Coulomb's Law: 2 charged objects experience an electric force. The electric force can attract (opposite charges) or repel (like charges) F = kQ1Q2/r2 k = koulomb constant = 9 x 109 Q = charge in coulombs r = distance between charged objects Notice electric Force looks a lot like the Universal Gravitation equation (coincidence?) Electric Force Electric force is a vector with direction. If there are multiple charges interacting, you can determine the total force by summing the force from each charged object. Principle of Superposition Electric Field Every charged object has an "electric field" around it. This means that a charge in that field will feel an electric force. E = F/q the electric field E equals the electric force F divided the charge of the object in the field (called the test charge). By dividing by q, the electric field is essentially the amount of Force per charge. E does not depend on the size of the test charge. Electric Field E = F/q but F = kQq/r2 So E = kQ/r2 , this is the amount of the electric field around a charge Q at a distance r away from Q. This is the electric field from a point source (as opposed to an electric field between 2 parallel plates which we'll talk about soon). Electric Field Lines Electric field lines point in the direction that a Positive (+) test charge would move. Electric field lines point away from positive (+) Q and towards a negative (-) Q.