Lecture 2Electric ChargeChapter 22Electric Charge (7)• Review from yesterday– There are 2 types of charge pos. and neg. – Like charges repel; unlike charges attract– Most objects are electrically neutral; there are equal numbers of neg. and pos. charges so the net charge = 0 – An object becomes charged (net charge ≠ 0) by adding or removing electrons – Conductors are materials where some of the electrons can move freely– Insulators are materials where none of the charges can move freelyElectric Charge (8)• An object can be given a charge by conduction or induction•In conduction the charge is transferred between objects by direct contact. For example, – Rubbing a glass rod (an insulator) with silk – Connecting 2 conductors through a conducting pathway (such as a wire) or by grounding the object • See Sample Problem 22-3Electric Charge (9)• An electrically neutral object can have an induced charge when some of its positive and negative charges separate due to a nearby charge• Neutral object will display characteristics of a charged object even though there is no net charge • Can we get an induced charge with an insulator?Electric Charge (10)• Take induction of charge one step further• Induce a charge on a conducting sphere by bringing a charged object nearby• Ground the sphere keeping the charged object nearby• Remove the connection to ground, move the charged object away and conducting sphere becomes uniformly charged• See diagram on boardElectric Charge (11)• The magnitude of the electrostatic force, F, between 2 charged particles with charges q1and q2, respectively, and separated by a distance r is defined as •This is Coulomb’s law where k is a constant• The forces on 2 point charges are equal and opposite, pointing to (away from) the other particle for unlike (like) charges221rqqkF =Electric Charge (12)• Coulomb’s law should remind you of Newton’s equation for the gravitational force• k is called the electrostatic constant• ε0is called the permittivity constant221rmGmF =2290/1099.841CmNk ⋅×==πεElectric Charge (13)• Electrostatic force and gravitational force are both inverse square laws involving a property of the interacting particles• Both obey the superposition principle: – The net force acting on any charge is the vector sum of the forces due to each of the remaining charges in a given distributionnnetFFFFF1141312,1...rrrrr++++=Electric Charge (14)• Have 2 shell theorems for electrostatics– A shell of uniform charge attracts or repels a charged particle that is outside the shell as if all the shell’s charge were concentrated at its center– If a charged particle is located inside a shell of uniform charge, there is no net electrostatic force on the particle from the shell• Electrostatic force differs from gravitational:– Can be either attractive or repulsive– Holds for all experimental tests and over all rangesElectric Charge (15)• Charge is quantized – comes in discrete values– Proven by Millikan oil-drop experiment (section 23-8)• Electric charge q is an integer multiple of the fundamental (or elementary) charge constant e• q=ne where n = ±1, ±2, ±3 and e = 1.60 x 10-19CMassElectric ChargeParticleMn=1.674E10-27kg0NeutronMp=1.672E10-27kg+e = 1.6E10-19CProtonMe=9.11E10-31 kg-e = -1.6E10-19 CElectronElectric Charge (16)• Net charge of an object is the difference between the number of protons and electrons in it times e• Charge is conserved– Net charge of any isolated system cannot change– Same as energy, linear and angular
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