GENERAL PHYSICS PHYS 2114
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This 0 page Class Notes was uploaded by Kendrick Wilderman on Sunday November 1, 2015. The Class Notes belongs to PHYS 2114 at Oklahoma State University taught by Flera Rizatdinova in Fall. Since its upload, it has received 19 views. For similar materials see /class/232918/phys-2114-oklahoma-state-university in Physics 2 at Oklahoma State University.
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Date Created: 11/01/15
PHYS 2114 LECTURE 1 CH20 General information about PHYS21 14 quot II Instructor Prof Flera Rizatdinova III Syllabus and course information can be found here I http hepOokstateedufleraPhys2l 14 El Course format I Lectures I will appreciate your comments and suggestions on the format of lectures I Quizzes Twice a week 4 multiple choice questions on the material to be discussed on the lecture I Labs you will need to pick up a PHYS 2 74 Lab Manual in your first lab class 8182009 General information about PHYS21 14 II D Homeworks will be assigned weekly via WebAssign httpwwwwebassignnet I Always due online by 7 759 pm Friday No extensions Assigned problems will be listed on the course website but must be submitted on WebAssignwhere you will also be able to check cumulative grades including discussion problems and exams Each assignment will consist of 8 7 0 problems and each assignment will have the same weight D Information on user ID and password will be given on Thursday s lecture 8182009 Tentative plcm for today s lec rure ll El Electric charge I Coulomb s law CI The Electric field 8182009 Charging Experiments 1 Rods that haven t been rubbed 5 Plastic Plastic Plastic rubbed Glass rubbed with with silk wool lt Observations 2 l Plastic rods rubbed with At increased distance No force is observed Plastic rubbed between unrubbed rods 4mm wool wool repel each other Glass rod rubbed with silk 4 attracts plastic rod rubbed with wool Increased distance forces are decreased 8182009 Charging Experiments 3 n 1 Rodthathad 2 The metal been charged Charged 5351 sphere acquires Metal plast1c charge Paper plastic moo This sphere acquires 1333 Metal More Observatlons Charged l The charge from a plastic rod rubbed with wool Metal rod PlaStiC can be Transferred to a metal sphere 2 After the transfer the plastic rod does not attract papen 3 A 2nol metal sphere connected by a metal rod Charged acquires the rod s charge Plastic 4 A 2nd metal sphere connected by a plastic rod does not acquire the rod s charge This sphere remams Metal 4 Metal neutral Plastic rod 8182009 Electric Charge and Atoms An atom with atomic number Z ThBPUCleusgexagg r ted for consists of a small but massive Clarlly contain posmve promns39 nucleus of charge Ze surrounded by a cloud of Z electrons each with much less mass and with a charge of e 10 10 m The nucleus contains Z protons with charge e and N neutrons with charge 0 The atomic mass number of the atom is A ZN The electron cloud is negatively charged Charge q NpeNee NpNee for neutral atoms q0 NpNeZ TABLE 151 Protons and electrons Particle Mass kg Charge Proton 167 x 10quot27 3 8182009 Electron 911 X 10 31 e Atoms can have a net charge if Np Ne where NpZ is the number of protons and Ne is the number of electrons in the atom If a neutral atom loses one electron it will have a net positive charge of qe because NpNe1 If a neutral atom gains one electron it will have a net negative charge of qe because NpNeI Positive and Negative Ions A positive ion will net charge q 2 A negative ion with net charge 7 e 8182009 Charging by Friction n Friction causes the breaking of molecular bonds and can result in a separation of charges in a formerly neutral molecule creating a positive and a negative molecular ion Rubbing a plastic rod with wool or a glass rod with silk produces such charge separation effects Electrically neutral molecule Atoms Bond g Friction These bonds were broken b friction Posttive y Negative molecular molecular ion ion This halfol39 the molecule lost an electron as the bond broke This half or the molecule gained an extra electron as the bond broke 8182009 Electric Charge III Electric force is one of the fundamental forces a Friction tension normal forces that we discussed in mechanics are based on electric interactions III Electric charge what is that We do not know III We know about electric charge that n There are two types of them positive and negative n Like charges repel opposite attract ti Charge is quantized the elementary charge is charge of the electron or proton they are the same in the magnitude but of opposite sign a Charge is a conserved quantity net charge in a closed region remains constant 8182009 Coulomb s low El The force beTween Two chorges ocTs along The line ioining Them wiTh The mogniTude proporTionoI To The producT of The charges and inversely proporTionoI To The square of The disTonce beTween Them s k F 611 12 r2 Coulomb s law Tm Here the product qlqz is positive so F12 is in the The unit vector 1 always points away from ql kqlqz A same direction F12 2 quot a as 39 r a 39 r 11 12 f ll GD iquot i r i a 41 1312 92 f aeH Here the charges have opposite signs so 1 qlqz lt 0 and 1312 points opposite Copyrgmo zocv Pearson mam inc ptbhshing as Pearsnn AddsanM esiev Units of the electric charge In El SI unit of charge is coulomb C named after the French physicist Charles Augustin de Coulomb n 1C625 gtlt lOl8e n le 16 xioWc II SI value of the constant k 90 x109 Nm2C2 8182009 Using Coulomb s Law n l Coulomb s Law applies only to point charges 2 Strictly speaking Coulomb s Law applies only to electrostatics nonmoving charges However it is usually OK provided vltltc 3 Electrostatic forces can be superposed Linear superposition gt gt Fl20nj F onj 13 net Em onj 8182009 Example Sum of Two Forces F F Two 10 nC charged particles are 2 cm apart on the x axis qquot qquot 4392 mm 1 What is the net force on a 10 nC 0 1 2 particle midway between them lm a 2 What is the net force if the charged q 5 Fact 1 particle on the right is replaced by a q g 10 nC charge 6 1 xmm gi t Met lon3 zon3 lFllF0 Fnet Flon3 F20n3 lF i lFi 2F i18 x 10393 N 8 9 I 10x10 2m2 90 X 104 N 8182009 Example Point of Zero Force In Two positively charged f20quot3VFM particles a1 and q23q1 are A placed 10 0 cm apart Where F2 Fiona zona other than infinity could another 139 I ctquot qz BLMEIOn3 charge q3 be placed so as to 6 l6 21 Only ifq3 is somewhere along the line between ql and 72 can the forces add to zero experience no net force Need force vectors to be colinear so location must be on x axis Need force vectors to be in opposite directions so location must be between 0 and d Need force vectors equal in magnitude so FKq1q3x2Kq2q3dx2 Therefore dx23x2 or xdI i3 10 cml ii 732 x100 cm2732 366 cm x100 cmO732l 366 cm which does not satisfy the 2nol criterion 8182009 Example Three Charges 1 Three charges with q1 50 nC q2 q330nc 9999quot 50 nC and q3 30 nC are placed at the corners of a 10 cm x 5 cm rectangle as shown What is the net force on q3 due to the I 39 I 39 I 39 I I100 cm I I 39 I 39 I l I I other two char es g ql 50nC39 39 q250nC 2 Fl 0n3 KQ1q3r13 90x109 NmZC250gtlt10398 C30gtlt10398 C010 m2 135gtlt10393 N onn3 Kq2q3 332 90gtlt109 N mZC25Ogtlt10398 C3Ogtlt10398 CO10 m2005 m2 108gtlt10393 N 8182009 Example Three Charges 2 Etet Z F ongose f Eon3jF20nEine Fx 108gtlt10393 ml 483x10394 N IE 3 3 10 Fy 135gtlt10 N108gtlt10 N 125 384gtlt10394 N 2quotsz Fy2 617gtlt10394 N F An bn385 x 8182009 The electric field In III Define the electric field as the force per unit charge The electric eld at any point is the force per unit charge that would be experienced by a charge at point Mathematically Right at this point the electric field is described by the vector Eli That means a point charge q placed here vyould I experience an electric Spree qE 1 E electric field q l l x Cupyrigm 2007 Pearson Education Ina publishing as Pearsan AddisonWesley fil The electric field permeates all of space kG 1 Charges interact with the electric field inducing a nonzero value of E subiect X 1390 OverhereJartheriirom the charge producing III Does E depend on q the charge that W23 l39 nrceq 42 ma detects it No because the force is E emd39rw m39 The electric field is a continueus entilyiso q 82gggggvlvl ixi rs cvcrywhcreWcust ll wright 2007 u E The Field of c Poin r Charge V F A 3r field of a pomt charge H G H qtest r A Cnpyrigy u aaaaaaaaaaaa arson Edumlion m publmhing as FameI Ad lsawWaley 8182009 Question n gt gt 7 7 r 7 An electron q e m me is placed at the position shown by the black clot The force on the electron is CI Zero b To the right c To the left ol Insufficient information to tell 8182009 Example Calculating the Electric Field y A 10 nC charged particle is located at the 10m 9 3 origin Points 1 2 and 3 have xy 450001 iC coordinates in cm of 10 01 and 11 90000 MC E3 respectively gt Determine the electric field E at each of these points Ell 2 f 4760 r 10 nC 90000 NC 1 cm 8182009 Question 2 m Which system has the largest electric field at the location of the numbered point I I QC 0 W A 8182009
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