Invention of the first Invention of the first TransistorTransistorin Nov.17in Nov.17--Dec.23 1947, Dec.23 1947, the most important invention of 20the most important invention of 20ththcenturycenturyGoalsGoals Knowledge of physics Critical thinking, idealization, approximation, mathematical and graphical representations of phenomena. Physics 213 Electric Charge, Field, Energy, Currents Magnetism Electromagnetism Light and Optics Modern Physics including Special Relativity, Quantum Mechanics and Nuclear Physics Chapter 15Electric Forces Electric Fields16_08a.jpg16_22.jpg Electric Charge  Coulomb’s LawOutline for TodayWhat is electric charge?What is electric charge?Like rest mass, it is a fundamental property of some of the elementary particles of which all matter is composedElectric charge is the fourth quantity we have learned (energy, linear momentum, and angular momentum)Properties of Electric Charges Two types of charges exist They are called positive and negative Named by Benjamin Franklin Like charges repel and unlike charges attract one another Nature’s basic carrier of positive charge is the proton Protons do not move from one material to another because they are held firmly in the nucleusMore Properties of Charge Nature’s basic carrier of negative charge is the electron Gaining or losing electrons is how an object becomes charged Electric charge is always conserved Charge is not created, only exchanged Objects become charged because negative charge is transferred from one object to anotherProperties of Charge Charge is quantized All charge is a multiple of a fundamental unit of charge, symbolized by e Quarks are the exception Electrons have a charge of –e Protons have a charge of +e The SI unit of charge is the Coulomb (C) e = 1.6 x 10-19CQuestionQuestion: Electric charge(a) is a continuous quantity that can be subdivided indefinitely(b) is a continuous quantity but it cannot be subdivided into smaller parcels than 1.6x10-19C(c) occurs only in separate parcels, each of 1.6x10-19C(d) occurs only in separate parcels, each of 1 C Answer: cQuestion:A negative electric charge (a) interacts only with positive charges (b) interacts only with negative charges(c) interacts with both positive and negative charges(d) may interact with either positive and negative charges, depending on circumstancesAnswer: cQuestion: An object has a positive electric charge whenever (a) it has an excess of electrons(b) it has a deficiency of electrons (c) the nuclei of its atoms are positively charged (d) the electrons of its atoms are positively chargedAnswer: bConductors Conductors are materials in which the electric charges move freely Copper, aluminum and silver are good conductors When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material00.10.20.30.40.50 200 400 600 800 1000RuO2T(K)CaRuO3SrRuO3Mott-Ioffe-Regel limitρ ~T2ab-planeMetal: dρ/dT>0Insulators Insulators are materials in which electric charges do not move freely Glass and rubber are examples of insulators When insulators are charged by rubbing, only the rubbed area becomes charged There is no tendency for the charge to move into other regions of the material0.110100010510710950 100 150 200 250 300T (K)Ca2RuO4Insulator: dρ/dT<0Semiconductors The characteristics of semiconductors are between those of insulators and conductors Silicon and germanium are examples of semiconductorsPolarizationCharging by RubbingCharging by InductionCoulomb’s Lawr12Fq=ke2q121221+q+qF12F21r12Coulomb’s Law, cont. keis called the Coulomb Constant ke= 8.99 x 109N m2/C2Typical charges can be in the µC range Remember, Coulombs must be used in the equation Remember that force is a vector quantityCoulomb force Gravitational forceBut electric forces may be either attractive or repulsive, whereas gravitational forces are always attractive. That is why matter in the universe tends to come together to form large bodies, these bodies are always found in groups, such as galaxies of stars and families of planets. On an atomic scale, electricity is much more important than gravityr12Fm=G2m1212r12FQ=ke2Q1212Example:The hydrogen atom has the simplest structure of all atom, consisting of a proton and an electron whose average separation is 5.3x10-11m.The mass of electron and proton is 9.1x10-31kg, 1.67x10-27kg, respectivelyFe=kQeQp/r2=(9.0x109N•m/C2)(1.6x10-19C2)/(5.3x10-11m)2=8.2x10-8NFg=Gmemp/r2=(6.7x10-11N•m/kg2)(9.11x10-31kg)(1.67x10-27kg)/(5.3x10-11m)2=3.7x10-47Nme=9.11x10-31kg, mp=1.673x10-27kgThe electric force is over 10103939times greater than the gravitational force16_11.jpgFour point charges, each of the same magnitude, with varying signs are arranged at the corners of a square as shown. Which of the arrows labeled A, B, C, and D gives the correct direction of the net force that acts on the charge at the upper right corner?a Ab. Bc. Cd. De. The net force on that charge is zeroAnswer: bQuestion:QUICK QUIZ 15.2Object A has a charge of +2 µC, and object B has a charge of +6 µC. Which statement is true:(a) FAB= –3FBA, (b) FAB= –FBA, or (c) 3FAB= –FBAAnswer (b)e=×19C1.60 10Q = ne (n =±0、1、2、3…)Electric charge always occurs in multiples of eA conducting sphere has a net charge of −4.8x10−17C. What is the approximate number of excess electrons on the sphere?(a) 100 (c) 300 (e) 500(b) 200 (d) 400Answer: c (-4.8x10-17C/-1.6x10-19C=300 electrons)Question:Question:Two charges of +Q are 1 cm apart. If one of the charges is replaced by a charge of –Q, the magnitude of the force between them is (a) zero(b) smaller(c) the same (d) largerAnswer: cQuestion: A charge of +q is placed 2 cm from a charge of –Q. A second charge of +q is then placed next to the first. The force on the charge of –Q(a) decreases to half its former magnitude (b) remains the same (c) increases to twice its former magnitude(d) increases to four times its former magnitudeAnswer: cTwo positive point charges Q and 2Q are separated by a distance R. If the charge Q experiences a force of magnitude F when the separation is R, what is the magnitude of the force on the charge 2Q when the separation is 2R ?(a) F/4 (c) F (e) 4F(b) F/2 (d) 2FQuestion:Answer: aQuestion: Two charges, one positive and the other negative, are initially 2 cm apart