Introductory General Physics II
Introductory General Physics II PHYS 1302
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This 46 page Class Notes was uploaded by Kayley Corwin on Saturday September 19, 2015. The Class Notes belongs to PHYS 1302 at University of Houston taught by Staff in Fall. Since its upload, it has received 10 views. For similar materials see /class/208345/phys-1302-university-of-houston in Physics 2 at University of Houston.
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Date Created: 09/19/15
Chapter 19 Electric Charges Forces and Fields E eld lines E eld lines q b c Overview of Chapter 19 Electric Charge Insulators and Conductors Coulomb s Law The Electric Field Electric Field Lines Shielding and Charging by Induction 191 Electric Charge The effects of electric charge were first observed as static electricity DD 05 After being rubbed on a a piece of fur an amber a rod acquires a charge and can attract small 0 objects c 191 Electric Charge Charging both amber and glass rods shows that there are two types of electric charge like charges repel and opposites attract a b 191 Electric Charge All electrons have exactly the same charge the charge on the proton has the same magnitude but the opposite sign Magnitude of an Electron39s Charge e e 160 X 10 19C SI unit coulomb C 191 Electric Charge The electrons in an atom are in a cloud surrounding the nucleus and can be separated negatively charged electron cloud positively charged nucleus 191 Electric Charge When an amber rod is rubbed with fur some of the electrons on the atoms in the fur are transferred to the amber 191 Electric Charge We find that the total electric charge of the universe is a constant Electric charge is conserved Electric charge is quantized in units of e The atom that has lost an electron is now positively charged it is a positive ion The atom that has gained an electron is now negatively charged it is a negative ion 192 Insulators and Conductors Conductor A material whose conduction electrons are free to move throughout Most metals are conductors lnsulator A material whose electrons seldom move from atom to atom Most insulators are nonmetals 192 Insulators and Conductors 10 If a conductor carries excess charge the excess is distributed over the surface of the 7 conductor 192 Insulators and Conductors 11 Semiconductors have properties intermediate between conductors and insulators their properties change with their chemical composition Photoconductive materials become conductors when light shines on them 193 Coulomb s Law 12 Coulomb s law gives the force between two point charges m lqzl 72 F E k SI unit newton N k 899 gtlt 10gNm2C2 The force is along the line connecting the charges Attractive if the charges are opposite Repusive if the charges are like 193 Coulomb s Law The forces on the two charges are action reaction forces a 13 193 Coulomb s Law If there are multiple point charges the forces add by superposition b Treat forces as charges as vector sum figure out overall direction and magnitude of force 14 194 The Electric Field 15 Definition of the electric field E i0 81 unit NC Here q0 is a test charge it serves to allow the electric force to be measured E Ek r2 194 The Electric Field 16 If we know the electric field we can calculate the force on any charge 11h The direction of the E e force depends on the imitcoi sign of the charge in the direction of the field for a positive charge opposite to it for a 3930 MC negative one quotl39ll 195 Electric Field Lines 17 Electric field lines are a convenient way of visualizing the electric field Electric field lines 1 Point in the direction of the field vector at every point 2 Start at positive charges or infinity 3 End at negative charges or infinity 4 Are more dense where the field is stronger 195 Electric Field Lines 18 E field lines q V a Lines from positive charges point away lines from negative charges point towards Direction defined as where a positive charge would move 195 Electric Field Lines 19 Combinations of charges Use the afore rules to determine how these look E eld lines a b c 195 Electric Field Lines fa A parallelplate capacitor consists of two conducting plates with equal and opposite charges Here is the electric field 20 196 Shielding and Charge by Induction 21 Excess charge on a conductor is free to move Charges will move so that they are as far apart as possible This means that excess A charge onaconductor resides on its surface x iij 196 Shielding and Charge by Induction 22 When electric charges are at rest the electric field at the center of the conductor is zero This is shielding E field lines 196 Shielding and Charge by Induction 23 The electric field is stronger where the surface is more sharply curved E field lines Chapter 20 Electric Potential and Electric Potential Energy Overview of Chapter 20 Electric Potential Energy and the Electric Potential Energy Conservation The Electric Potential of Point Charges Equipotential Surfaces and the Electric Field Capacitors and Dielectrics Electrical Energy Storage 201 Electric Potential Energy and the Electric Potential The electric force has a potential energy associated with it It takes work to move an electric charge perpendicular to an electric field W L0Ed As usual the change in potential energy is the negative of the work AU w 1701311 201 Electric Potential Energy and the Electric Potential Sine we define the electric field it is useful to define the electric potential potential energy per unit of charge Definition of Electric Potential V All W AV 7 L70 bio SI unit joule coulomb volt V The electron volt is a unit of energy 19V 160 x 10 10C1 V 160 x 104 201 Electric Potential Energy and the Electric Potential R Distance m The electric field is related to how rapidly the potential is changes over distance Electric potential V O l l 4 54 T 44 As E 44 mm H r l v w w L 202 Energy Conservation In general for a mass moving from A to B where potentiakinetic energy are conserved vaZ LIA mv132 LIB For the electric force U qV so that m032 vaz LIA 113 14124 WA VB 202 Energy Conservation A free charge in an electric field will move to region of lowest potential energy AU is positive 1Positive charges accelerate in the direction of decreasing electric potential AV positive 2 Negative charges accelerate in the direction of increasing electric potential AV negative E 10 AV 203 The Electric Potential of Point Charges Potential energy between 2 charges is 5 s 6 W 10 I 71 The difference in potential energy between points A and B is kqoq him U U A B 7 A TB 203 The Electric Potential of Point Charges Therefore the electric potential of a point charge is k Vq If shown here for a positive and negative charge respectively e n a ElgarKl PO 203 The Electric Potential of Point Charges The electric potential of a group of point charges is the algebraic sum of the potentials of each charge gags l 200 400 1 0 204 Equipotential Surfaces and the Electric Field Lines of equal electric potential equipotential are shown by green lines Electric field is perpendicular to these equipotential ines Mi 5 5 0 z m 20 V 15V 10 V 10 5 O 5 10 x m 11 204 Equipotential Surfaces and the Electric Field For two point charges lnf5 I I 6 quotII 9 g 36 1 a 204 Equipotential Surfaces and the Electric Field There are electric fields inside the human body the body is not a perfect conductor so there are also potential differences An electrocardiograph R Ventricular ria depolarization p I e h depolarife ciori P T electrical actIVIty Q S Ventricular repolarization O O b 205 Capacitors and Dielectrics A capacitor is two conducting plates separated by a finite distance A Q 1T E d W v w v v v v v v v i Qi39 f7 i A 14 205 Capacitors and Dielectrics The capacitance relates the charge to the potential difference SI unit coulomb volt f farad F 205 Capacitors and Dielectrics A simple type of capacitor is the parallelplate capacitor It consists of two plates of area A separated by a distance d i d i By calculating the electric field created by the charges IQ we find that the A capacitance of a parallel plate capacitor is 8014 d C l i 16 205 Capacitors and Dielectrics The general properties of a parallelplate capacitor that the capacitance increases as the plates become larger and decreases as the separation increases are common to all capacitors 32 d 205 Capacitors and Dielectrics A dielectric is an insulator when placed between the plates of a capacitor it gives a lower potential difference with the same charge due to the polarization of the material This increases the capacitance quot 9 l 2 D ZO Illlllllllllllllllll llllllllllllllllllll e a b c 205 Capacitors and Dielectrics The polarization of the dielectric results in a lower electric field within it the new field is given by dividing the original field by the dielectric constant K 2 E K E Therefore the capacitance becomes Q Q KQZK V V0K V0 C0 205 Capacitors and Dielectrics TABLE 201 Dielectric constants Substance Dielectric constant K The dielectric Water 804 constant is a Neoprene property of the mbber 67 material here are PYrex glass 56 some examples Mm 54 Paper 3 Mylar 31 Te on 21 Air 100059 Vacuum 1 20 205 Capacitors and Dielectrics If the electric field in a dielectric becomes too large it can tear the electrons off the atoms thereby enabling the material to conduct This is called dielectric TABLE 202 Dielectric strengths breakdown the Di ctric eld at WhICh thls Substance strent Vm happens Is called the dielectric Mica 100 X 106 strength Te on 60 X 106 Paper 16 X 106 Pyrex glass 14 X 106 Neoprene rubber 12 X 106 Air 30 x 106 1 206 Electrical Energy Storage By considering how much energy it takes to move an increment of charge AQ from one plate to the other we can find the total energy stored in the capacitor u Qvav QV 1 2 ECV Q2 2 22 206 Electrical Energy Storage The energy stored in a capacitor can be put to a number of uses a camera flash a cardiac defibrillator and others In addition capacitors form an essential part of most electrical devices used today If we divide the stored energy by the volume of the capacitor we find the energy per unit volume this result is valid for any electric field LIE electric energy density electr1c energy 1 E2 7 50 volume 23
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