# The World That Came Together

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8/26/2013 Introduction to Electric Charge Dr. DiFabio Physics 202 Lecture UL-Lafayette, Fall 2013 Halliday, Resnick, Walker (Chapter 21) Introduction to Electric Charge • Electric charge is an intrinsic property of the elementary particles that make up atoms. • Atoms • Nucleus consisting of protons (positive charge) and neutrons (no charge) • Electrons (negative charge) orbit the nucleus. • Like charges repel, while opposite charges attract. • Everyday objects have equal amounts of positive and negative charge, so they have no net charge. 1 8/26/2013 Charging objects • Rubbing glass rod with silk causes electrons to be transferred from glass to silk giving the glass a positive charge • Rubbing plastic rod with fur…show more content…
3 8/26/2013 Coulomb’s Law Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. Calculate electric force between two point charges (q1 and q2) ̂ • k = 1/(4πε0) = 8.99x109 N·m2/C2 ; Electrostatic Constant • ε0 = 8.85x10-12 C2/N·m2 ; Permittivity Constant • ̂ is a “unit vector along an axis extending through the two particles” (page 566). – No units and has a magnitude of one. – When calculating the force that q2 exerts on q1, ̂ points directly away from charge q2. Coulomb’s Law ̂ • Equation is similar to the gravitational force • Apply to more than two point charges (n point charges) – Calculate electric force on q1 due to charges q2, q3, …, qn. – Use Coulomb’s law to determine the force q2 exerts on q1 , the force q3 exerts on q1 , etc. – Use superposition to get the net force: ⋯ – Remember, the electric force is a vector and you must perform the vector sum to get the net force. 4 8/26/2013 Question 21.1 Question 21.2 A positively charged insulating rod is brought close to an object that is suspended by a string. If the object is attracted toward the rod, we can conclude: a. The object is positively charged. b. The object is negatively charged. c. The object is an insulator. d. The object is a conductor. e. None of the above 5 8/26/2013 Question 21.3 Two electrons (e1 and e2) and a proton (p) lie on a straight line, as shown. The directions of the force of e2 on e1, the force of p on e1, and the total