PHYSICS 1 (LIFE SCIENCES) ELECTRICITY

Transcription

1 THE UNIVERSITY OF SYDNEY PHYSICS 1 (LIFE SCIENCES) ELECTRICITY ELEVENTH EDITION

2 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 nano n 10-9 peta P pico p exa E femto f atto a 10-18

3 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

4 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.

5 iii How to use this text Symbols and constants used in this unit CONTENTS E1 Electric Fields and Charge 1 Objectives 1 Pre-lecture Introduction 2 Lecture Electric charge Forces between charged bodies Electric fields Field lines Electric field and charge Electrostatics in industry and commerce Electrical shielding 8 Post-lecture The fundamental laws of electrostatics Electrostatic deflection Questions 12 E2 ELECTRIC POTENTIAL 13 Objectives 13 Pre-lecture Introduction - electric potential Induced charge 14 Lecture Electric potential Electric field and potential Conductors Electric double layers Capacitance Equipotentials and electric fields 23 Post-lecture Questions Appendix - More on electric double layers 26 E3 ELECTRIC CURRENT 27 Objectives 27 Pre-lecture The nature of electric current 28 Lecture Electric current and charge carriers Current Density Electric current and flow of mass Current and energy flow Resistance and resistivity Example - Electric current in an x-ray machine 38 Post-lecture Correct use of electrical terminology Questions 39 E4 ION DIFFUSION 41 Objectives 41 Pre-lecture Concentration and ph 42 Lecture Electricity in the biological world Diffusion of ions through a semipermeable membrane Nernst equilibrium 48 v vi

6 iv 4-5 Measurement of ph Ion diffusion in biology 50 Post-lecture Membrane potentials and Donnan equilibrium Membrane potentials of living cells Questions 56 E5 ELECTRODES AND ELECTROLYTES 57 Objectives 57 Pre-lecture The interface between materials EMF 58 Lecture & Text What are electrodes? Electrodes and electrolyte Bioelectrodes Batteries Electrolysis 69 Post-lecture Questions 71 E6 MAGNETISM: FIELDS AND FORCES 72 Objectives 72 Lecture Why study magnetism? Magnetic fields Force on a current in a magnetic field Applications of magnetic force Magnetic deflection of electron beams What causes magnetic fields? Forces between current-carrying conductors Magnetic materials 80 Post-lecture Why do iron filings line up along field lines? Questions 82 E7 ELECTROMAGNETIC INDUCTION 83 Objectives 83 Pre-lecture Introduction - Induced EMF 84 LECTURE Voltage induced in a moving conductor Applications of motional EMFs Voltages induced by changing magnetic fields The flux rule Applications of EMFs from changing magnetic fields 90 Post-lecture The flux rule Questions 92 E8 APPLICATIONS 93 Objectives 93 Pre-lecture Electrocardiology Electric shock 97 REVIEW QUESTIONS 103 Answers 107 Answers to review questions 119 Index 125 Basic formulas - Electricity inside back cover

7 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.

8 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 C Permittivity of free space ε F.m -1 Faraday constant F kc.mol -1 Avogadro constant N mol -1 Boltzmann constant k J.K -1 Permeability of free space µ 0 4π 10-7 H.m -1.