电路基础理论Chapter 1

Preface•Place of Electrical Circuits in Modern Technology IntroductionThe design of the circuits has 2 main objectives:1)To gather,store,process,transport,and present information.2)To distribute and convert energy between various forms.The study of circuits provides a foundation for areas of electrical engineering such as:•Communication system •Computer system •Control system •Electronics •Electromagnetic •Power systems •Signal processing•Motivation for doing this course •About the courseCircuit TheoryCircuit AnalysisCircuit Synthesis Circuits(given)Excitation (given)Response(unknown)Circuit AnalysisWhat we emphasize on,Since it provides the foundation forunderstanding the interaction of signalsolution.Circuits(unknown)Excitation (given)Response(given)Circuit synthesis(design)In contrast to analysis,a design problem may have nosolution or several solutions,Resistance circuits analysisDynamic circuits analysisSinusoidal steady stateThe course includes3parts:•Reference Books1)Fundamentals of Electric Circuits Charles K Alexander,Matthew N O Sadiku清华大学出版社2)The Analysis and Design of Linear Circuits Roland E.Thomas,Albert J.Rosa—2nd ed3)Electrical Engineering Principles and Applications Allan R.Hambley---2nd ed4)电路分析基础李瀚荪第三版5)电路邱关源第四版6)Electric Circuits Joseph Edminister,Mahmood Nahvi-----3rd edChapter 1 Fundamental KnowledgeCircuit and circuit model•Actual electrical component:a battery or a lightbulbActual electrical componentIdeal circuitcomponentEmphasize the main characterNeglect the left character•Ideal circuit component: amathematical model of an actualelectric component.R1VsRsCircuit model:A commonly used mathematical model for electric system.Lumped elements Lumped circuiti2-V+i1i1=i2V is certain Actual scale of the circuit is much smaller than the wavelength relating to the running frequency of the circuit.Circuit Type:•Linear----Nonlinear•Time invariant----Time variant •Passive----Active•Lumped----DistributiveCircuit Variablesdtdq i n Electric current is the time rate of change of charge, measured in amperes (A).A direct current (DC)is a current thatremains constant with time. (I)An alternating current (AC)is a current that varies sinusoidally with time.SortReference directioni >0 means the real direction isisame to the reference directioni <0 means the real direction isopposite to the reference directionCircuit VariablesVoltage (or potential difference) is the energy required to move a unit charge through an element, measured in volts(V). dqdw v Reference direction or voltage polarity -V +V>0 means the real polarity is sameto the reference polarity V<0 means the real polarity isopposite to the reference polaritypassive sign convention -V +iPassive sign convention is satisfied when current enters through the positive polarity of the voltage.Unless otherwise stated, we will follow thepassive sign convention throughout this course.Circuit VariablesPower is the time rate of expending or absorbing energy. Measured in watts(W)dtdw p =vi dtdq v dt dw p vdq dw ===∴= P=VI in a DC circuitusing passive signconvention Power absorbed = -Power suppliedReference polarities for power using passive sign conventionP > 0 absorbing powerP < 0 releasing or supplyingpowerExamplesLaw of conservation of energy must be obeyed in any electric circuit.∑=0p Power absorbed = -Power suppliedEnergy is the capacity to do work, measured in joules(J) ⎰⎰==t t tt vidt pdt w 00The energy absorbed or supplied by an element from time t0 to time t isCircuit ElementsPassive elements:resistors,capacitors,and inductorsActive elements:source,operational amplifiersVoltage and Current SourcesThe most important active elements are voltage or current sources that generally deliver power to the circuit connected to them. There are two kinds of sources: independent and dependent sources.An ideal independent source is an active element that provides a specified voltage or current that is completely independent of other circuit variables.Symbols for independent voltage source Symbols for independent voltage sourceNote:▪ 2 or more voltage sources with different value are not permissible to be connected in parallel▪ 2 or more current sources with different value are not permissible to be connected in series▪V oltage sources connected in series is equivalent to one voltage source▪Current sources connected in parallel is equivalent to one current source▪ A voltage source connected to any branch in parallel is equivalent to itself▪ A current source connected to any branch in series is equivalent to itselfAn ideal dependent(or controlled)source is an active element in which the source quantity is controlled by another voltage or current.Symbols for a) dependent voltage sources b) dependent current sources There are a total of fourvariations, namely:1.A voltage –controlled voltagesource (VCVS)2. A current –controlled voltagesource (CCVS)3. A voltage –controlled currentsource (VCCS)4. A current –controlled currentsource (CCCS)V1V1μVCVSV1V1g VCCS I1I1αCCCS I1V1γI1CCVSWhat is the difference between independent and dependent sources?ResistorsThe circuit element used to model the current –resisting behavior of a material is the resistor.Resistance is the capacity of materials to impede the flow of current.The resistance R of an element denotes its ability to resist the flowof electric current;it is measured in ohms(Ω)Symbol: R11ki u i ut1t2ui i u i ut1t2uiLinear Time InvariantLinear Time variant Nonlinear Time Invariant Nonlinear Time Variant Open Circuit Short CircuitLinear Resistor:The resistance of the idea resistor is constant and its value doesnot vary over time.The relation between voltage and current.(V AR)vV=Ri(passive sign convention)i-------Ohm’s LawSince the value of R can range from zero to infinity,it is important that we consider the two extreme possible value of R:R=0-------is called a short circuit;V=0;R=∞------is called an open circuit,I=0;Conductance G is the reciprocal of the resistance, measured in siemens (s)Power : P=vi (passive sign convention) always absorbs power from the circuit Other methods of expressing :G i G v vi p Rv R i vi p 2222======RG 1=About nonlinear resistor。