電氣課件電路理論

1、CIRCUIT THEORYLectured by 李妍Email to Chapter 1 Basic Concepts1.1 Introduction1.2 Charge , Current , Voltage1.3 Power and energy 1.4 The Ideal Basic Circuit ElementCIRCUIT THEORYLectured by liyan1-1 INTRODUCTIONWhy start with circuit theory?The solution of an engineering problem normally proceeds:Fir

2、st, a real-world problem is identified; second, the problem is modeled; third, the model is analyzed; and fourth, the results are applied to the original physical problem. 1. Our goal in circuit theory:To define ideal elements so that when they are connected, the resulting interconnection will quant

3、itatively predict the behavior of the actual system.1.1 Introduction 2. A note to the student:2). Problem solving is an essential part of the learning process. The best way to learn is to solve a lot problems. 1). This course is the foundation on which most other courses in the electrical engineerin

4、g curriculum rest.2). lumped circuit and distributed circuit （集中參數(shù)電路和分布參數(shù)電路）Electrical effects propagate through a circuit at a finite velocity 1). An electric circuit is an interconnection of electrical elements .3. Circuit and its classification To a lumped circuit, we can assume that electric eff

5、ect occurs instantaneously throughout the circuit.A discussion on lumped or distributed circuits:The frequency of signals used to transmit electric power is 50HzThe frequency of the radio signals is on the order of 109HzConclusion: when the dimensions of a circuit is comparable to or even larger tha

6、n the wavelength associated with the frequencies of interest,we can not deal with it as a lumped circuit. Lumped circuit: The spatial dimension does not enter into the circuit model.D 0.01, lumped circuitD 0.01, distributed circuit35KV, distributed circuit.We consider the lumped circuits exclusively

7、 in this course.1. Lumped circuits are simpler to understand and to design.2. The theory of the distributed circuits is based on that of lumped circuits.1)The charge is bipolar (described in terms of positive and negative charges). 2)Charge can neither be created nor destroyed,only transferred. 1. C

8、harge1.2 Charge,current,voltage(電荷、電流、電壓）2.The motion of charges creates current. Convention: the true direction of the current flow is the direction of movement of the positive charges. Electric current is the time rate of change of charge,measured in amperes(A)i: the current in amperes q: the char

9、ge in coulombst: the time in secondsReference direction for current:You can assign it arbitrarily, show it by an arrow.i 參考方向I1 = 1A10V10I1I1 = -1A10V10I1Drill:The total charge entering a terminal is given by Calculate the current at t=0.5s. Solution:At t=0.5, i=31.42mAAn alternative current (ac) is

10、 a current that varies sinusoidally with time. 交流電流A direct current (dc) is a current that remains constant with time. 直流電流1A=1C/1s(秒) 1kA=1000A，1mA=10-3A，1A=10-6A，1nA=109ATerms:3. VOLTAGE(potential difference)Voltage is the energy per unit charge that is created by the separation. V: the voltage in

11、 volt W: the energy in joules q: the charge in coulombsIt is a convention that the true direction of the voltage is from the positive polarity to the negative polarityReference direction for voltage:+實(shí)際方向+實(shí)際方向+（參考方向）U+（參考方向）UU 0U 0, power is being delivered to the elementP0, power is being supplied

12、by the element4. How to decide the direction of power5. The law of conservation of powerThe algebraic sum of power in a circuit , at any instant of time, must be zero: The total power supplied to the circuit must be balance the total power absorbed. 6. EnergyEnergy is the capacity to do work,measure

13、d in joules(J).The electric power utility companies measure energy in watt_hours(Wh).1Wh=3600JDrillA stove element draws 15A when connected to a 120V line, How long does it take to consume 30KJ?Answer: 16.67s1.4 The Ideal Basic Circuit Element “ideal” implies that we will be able to formulate a prec

14、ise mathematical relationship between the terminal voltage and current.“basic” implies that the circuit elements cannot be further reduced or subdivided into other elements. The ideal basic circuit elements form the building blocks for constructing circuit models.Active elements(有源元件）: capable of ge

15、nerating energy such as generators, batteries etc.Passive elements（無源元件）: resistors, capacitors and inductors.Classification:1.3.1 Ideal Independent Sources（理想獨(dú)立電源）An ideal independent source is an active element that provides a specified voltage or current that is completely independent of other ci

16、rcuit variable.Independent voltage source & Independent current source1、獨(dú)立電壓源 Independent Voltage SourceThe ideal independent voltage source maintains its prescribed voltage, independent of its output current.General symbols for a voltage source:Its characteristic:uS+_iu+_USui0maintains its voltaget

17、he current is determined by the loadGraphical characteristic of a dc voltage source: Open_circuited and short_circuited:uS+_iu+_(1) 開路 i=0(2) 理想電壓源不允許短路。Power analysis:i , uS are associated reference direction:pabsorb=uSi psupply= uSii , usare not associated reference direction:psupply= uS i pabsorb

18、= - uSiuS+_iu+_uS+_iu+_2、獨(dú)立電流源 Independent Current SourceThe ideal current source produces its prescribed current, independent of its output voltage.Circuit symbol and its characteristic:ISui0iSiu+_General symbol of a current sourcemaintains its current, the voltage is determined by the loadgraphica

19、l characteristic of a dc current sourceShort_circuited and open_circuited:(2) It can not be open_circuited.(1) short_circuited：i= iS ，u=0 iSiu+_Power analysis:p發(fā)= u is p吸= uisp吸= uis p發(fā)= uisiSu+_iSu+_u , iS are associated, u , iS are not associated, Check Your UnderstandingThe battery is an ideal in

20、dependent voltage source.?Actual model of a direct current sourceRIsActual model of a direct voltage sourceR+Vs-Ideal basic circuit elements do not exist in the physical world; to an actual element, we should analyze its working condition, then set up a suitable circuit model to describe it.Keep in

21、your mind: circuit model is the interconnection of the ideal basic circuit elements. Note:1.3.3 受控電源 CONTROLLED SOURCESAn ideal dependent (or controlled) source is an active element in which the source quantity is controlled by another voltage or current.CCCS：current gain, dimensionlessVCCSg：transco

22、nductance（S）Four types: u1=0 i2=b i1 i1=0i2=gu1CCCSb i1+_u2i2+_u1i1VCCSgu1+_u2i2+_u1i1VCVS：voltage gain, dimensionlessCCVSr：transresistance（） u1=0u2=r i1 i1=0u2= u1VCVSu1+_u2+_u1+_i2i1i2i1CCVSr i1+_u2+_u1+_Controlled sources are active.p吸 = u1i1 + u2i2 = u2i2 =u2 (-u2/R) 0VCVSu1+_u2+_u1+_i2i1R* ，g， ，r is constant，linear controlled sourc