THE UNIVERSITY OF SYDNEY PHYSICS 1 (LIFE SCIENCES) ELECTRICITY ELEVENTH EDITION
SI stands for Système International SI UNITS SI base units Quantity Unit Symbol Mass kilogram kg Distance metre m Time second s Electric current ampere A Temperature kelvin K Luminous intensity candela cd Amount of substance mole mol Derived units with special names Quantity Unit Symbol Equivalent Force newton N kg.m.s -2 Pressure pascal Pa N.m -2 = kg.m -l.s -2 Energy joule J N.m = kg.m 2.s -2 Power watt W J.s -1 = kg.m 2.s -3 Frequency hertz Hz s -l Charge coulomb C A.s Capacitance farad F C.V -1 Conductance siemens S A.V -1 = Ω -1 Magnetic field tesla T Wb.m -2 Electric potential, EMF volt V W.A -1 Magnetic flux weber Wb V.s Resistance ohm Ω V.A -1 Prefixes for units name symbol value name symbol value kilo k 10 3 centi c 10-2 mega M 10 6 milli m 10-3 giga G 10 9 micro µ 10-6 tera T 10 12 nano n 10-9 peta P 10 15 pico p 10-12 exa E 10 18 femto f 10-15 atto a 10-18
THE UNIVERSITY OF SYDNEY PHYSICS 1 (LIFE SCIENCES) ELECTRICITY ELEVENTH EDITION Reprinted with corrections, 1993 Written by IAN FALCONER MICHAEL LARGE Revised and edited by IAN SEFTON
ii Electricity is one of six units for the course PHYSICS 1 (LIFE SCIENCES). Original text by I.S. Falconer and M.I. Large. Revised and edited by I.M. Sefton. Word processing by Jeanne Leppard and I.M. Sefton. Diagrams by M.I. Large, I.S. Falconer and I.M. Sefton. Copyright 1982, 1993, The University of Sydney.
iii How to use this text Symbols and constants used in this unit CONTENTS E1 Electric Fields and Charge 1 Objectives 1 Pre-lecture 2 1-1 Introduction 2 Lecture 2 1-2 Electric charge 2 1-3 Forces between charged bodies 3 1-4 Electric fields 4 1-5 Field lines 5 1-6 Electric field and charge 6 1-7 Electrostatics in industry and commerce 8 1-8 Electrical shielding 8 Post-lecture 10 1-9 The fundamental laws of electrostatics 10 1-10 Electrostatic deflection 11 1-11 Questions 12 E2 ELECTRIC POTENTIAL 13 Objectives 13 Pre-lecture 14 2-1 Introduction - electric potential 14 2-2 Induced charge 14 Lecture 15 2-3 Electric potential 15 2-4 Electric field and potential 16 2-5 Conductors 17 2-6 Electric double layers 19 2-7 Capacitance 21 2-8 Equipotentials and electric fields 23 Post-lecture 24 2-9 Questions 24 2-10 Appendix - More on electric double layers 26 E3 ELECTRIC CURRENT 27 Objectives 27 Pre-lecture 28 3-1 The nature of electric current 28 Lecture 29 3-2 Electric current and charge carriers 29 3-3 Current Density 30 3-4 Electric current and flow of mass 32 3-5 Current and energy flow 34 3-6 Resistance and resistivity 35 3-7 Example - Electric current in an x-ray machine 38 Post-lecture 39 3-8 Correct use of electrical terminology 39 3-9 Questions 39 E4 ION DIFFUSION 41 Objectives 41 Pre-lecture 42 4-1 Concentration and ph 42 Lecture 43 4-2 Electricity in the biological world 43 4-3 Diffusion of ions through a semipermeable membrane 44 4-4 Nernst equilibrium 48 v vi
iv 4-5 Measurement of ph 48 4-6 Ion diffusion in biology 50 Post-lecture 52 4-7 Membrane potentials and Donnan equilibrium 52 4-8 Membrane potentials of living cells 54 4-9 Questions 56 E5 ELECTRODES AND ELECTROLYTES 57 Objectives 57 Pre-lecture 58 5-1 The interface between materials 58 5-2 EMF 58 Lecture & Text 59 5-3 What are electrodes? 59 5-4 Electrodes and electrolyte 59 5-5 Bioelectrodes 64 5-6 Batteries 66 5-7 Electrolysis 69 Post-lecture 71 5-8 Questions 71 E6 MAGNETISM: FIELDS AND FORCES 72 Objectives 72 Lecture 73 6-1 Why study magnetism? 73 6-2 Magnetic fields 73 6-3 Force on a current in a magnetic field 75 6-4 Applications of magnetic force 76 6-5 Magnetic deflection of electron beams 78 6-6 What causes magnetic fields? 78 6-7 Forces between current-carrying conductors 79 6-8 Magnetic materials 80 Post-lecture 81 6-9 Why do iron filings line up along field lines? 81 6-10 Questions 82 E7 ELECTROMAGNETIC INDUCTION 83 Objectives 83 Pre-lecture 84 7-1 Introduction - Induced EMF 84 LECTURE 84 7-2 Voltage induced in a moving conductor 84 7-3 Applications of motional EMFs 86 7-4 Voltages induced by changing magnetic fields 87 7-5 The flux rule 89 7-6 Applications of EMFs from changing magnetic fields 90 Post-lecture 91 7-7 The flux rule 91 7-8 Questions 92 E8 APPLICATIONS 93 Objectives 93 Pre-lecture 93 8-1 Electrocardiology 94 8-2 Electric shock 97 REVIEW QUESTIONS 103 Answers 107 Answers to review questions 119 Index 125 Basic formulas - Electricity inside back cover
v HOW TO USE THIS TEXT This text and its five companion volumes define the course, so they are your primary reference and study material. Four of the six units are accompanied by one video lecture for each chapter of the text. The video lectures and live lectures are supplementary resources, but they do not define the course. Some, but not all, chapters of the text have a section labelled LECTURE which usually follows the same sequence as the video lecture. However if you try to read the book while watching the video lecture - a practice which is not recommended - you may find some discrepancies. Where differences do occur, the text version is to be preferred. (You probably won't even notice the differences unless you constantly refer to the book during the lectures.) Some of the differences between text and videos include the following. There are some changes in symbols used for physical quantities. The text has been changed to achieve consistency throughout the course or to match the most commonly used symbols. There are some references in the video lectures to early versions of the course notes and to other things that are no longer considered to be relevant. Just ignore these if they don't appear in the text. The sequence of material is sometimes a bit different. Some parts of the text such as worked examples and descriptions of demonstrations, are printed in a smaller typeface. In this volume some illustrative material based on the TV lectures is printed in boxes with a double-lined border and can be read independently of the main text. Although such material is useful for understanding, it is not worth trying to remember the details of the examples described. The desirable final state of your knowledge is defined by the objectives, the main text, and the review questions.
vi Symbols and constants used in this unit Quantity Symbol SI unit Symbol Notes Charge. q, Q coulomb C Force. F newton N volt per metre V.m -1 Electric field. Magnitude of field. Component of field in x- direction. E E We use the bold face form E when we want to include direction. E x Surface charge density. σ coulomb per square metre C.m -2 Permittivity. ε farad per metre F.m -1 Relative permittivity. k none none Dimensionless. Also called dielectric constant. Alternative symbol, ε r. Potential energy. U joule J Potential. V volt V Potential difference. V, V EMF. volt V Capacitance. C farad F Current. I ampere A Velocity. v metre per second m.s -1 Drift velocity. v metre per second Magnitude of drift velocity. v m.s -1 Current density. Magnitude of current j j ampere per square metre A.m -2 density. Mass flow rate. µ kilogram per second kg.s -2 Degree of ionisation. z none none A positive or negative integer. Molar mass. M kilogram per mole kg.mol -1 Number density. n (number) per cubic metre m -3 Power. P watt W Resistance. R ohm Ω Conductance. G siemens S Resistivity. ρ ohm metre Ω.m S = Ω -1 Conductivity. γ siemens per metre S.m -1 Concentration. C mole per cubic mol.m metre The common practical unit is mole per litre, mol.l -1. Temperature (absolute). T kelvin K Magnetic field. Magnitude of field. B B tesla T T = Wb.m -2 Magnetic flux. Φ weber Wb Permeability. µ henry per metre H.m -1 H.m -1 = Wb.A -1.m -1 Constant Symbol Value Electronic charge e 1.602 10 19 C Permittivity of free space ε 0 8.854 10-12 F.m -1 Faraday constant F 96.485 kc.mol -1 Avogadro constant N 0 6.022 10 23 mol -1 Boltzmann constant k 1.381 10-23 J.K -1 Permeability of free space µ 0 4π 10-7 H.m -1.