Chapter 1 Basic concepts.ppt

1、Chapter 1 Basic concepts,要求深刻理解与熟练掌握的重点内容有： 1. 理想元件与电路模型概念,线性与非线性的概念。 2. 电压、电流及其参考方向的概念。 3. 电阻元件、电感元件、电容元件,电压源、电流源和受控源的伏安关系及功率的计算。 4. 基尔霍夫电流定律和基尔霍夫电压定律。 要求一般理解与掌握的内容有： 5. 时变与非时变的概念。 难点：参考方向，受控源，功率计算。,1-1 Introduction,1-2 Basic Concepts and System of Units,1-3 Charge and Current,1.4 Voltage,1.5 Power

2、 and Energy,1-6 Circuit Elements,1-9 Summary and Review,1-1 Introduction,instrumentation,仪表,topology,拓扑,The schematic diagram,the International System of Units (SI),国际单位制,the General Conference on Weights and Measures,国际计量大会,原理图,1. Terms,electron,电子,proton,质子,neutron,an external electromotive force

3、(emf),外电动势,potential difference,电位差,instantaneous power,瞬时功率,中子,First, 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.,2.Why start with circuit theory?,The solution of an engineeri

4、ng problem normally proceeds:,1-2 Basic Concepts and System of Units,1.2.1 System of Units,1.2 .2 Basic Concepts,An electric circuit is an interconnection of electrical elements.,Active elements: capable of generating energy, such as generators, batteries etc.,Passive elements: resistors, capacitors

5、 and inductors,Ideal circuit elements: 理想电路元件 Lumped parameter elements 集中参数元件 Ideal independent source 理想电源元件 Ideal load 理想负载元件 Ideal coupling elements 理想耦合元件 Lumped parameter elements & distributed elements 集中参数与分布参数,Charge is an electrical property atomic particles of which matter consists ,measu

6、red in coulombs(C),An electron e=-1.6021019C In 1 C of charge ,there are 1/(1.60210-19)=6.241018 electrons 3.The law of conservation of charge,Note,1-3Charge and Current1.3.1Charge 电荷,Electric current is the time rate of change of charge, measured in amperes (A).,1A=1C/1s(秒) 1kA=1000A，1mA=10-3A，1A=1

7、0-6A，1nA=109A,1.3.2 Current 电流：,An alternative current (ac) is a current that varies sinusoidally with time. 时变电流,Andre-Marie Ampere (1775-1836) 安培,A direct current (dc) is a current that remains constant with time. 直流电流,To specify the current in a conductor, we need both a reference direction and n

8、umerical value, which can be positive or negative.,I 0 实际方向与参考方向相同 I0 实际方向与参考方向相反,1.3.3 Current reference directions 电流的参考方向,Voltage (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V). Uab=W/q V，kV，mV，V,Alessandro Antonio Volta (1745-1827

9、) 伏特,1.4Voltage1.4.1 Voltage of Potential 电位,Potential reference polarity 电位的参考极性(零电位点） 参考点：,Let Ub=0,Ua=U,Uab=U=Ua-Ub，point a is at a potential of Uab volts higher than point b.,1.4.2 Voltage 电压,Voltage reference directions,Keep in mind that electric current is always through an element and that el

10、ectric voltage is always across the element or between two points.,Check Your Understanding What the difference between Uab and U ?,Uab= U=US+RI Uab =-Uba=U U=USRI,Power is the time rate of expending or absorbing energy, measured in watts (W).,Instantaneous power,1.5 Power and Energy,In dc circuit :

11、P=IU,Power unit :W，kW，1MW106W,When the current enters through the positive terminal of an element, p=+ui. If the current enters through the negative terminal, p=-ui.,p0 Power absorb P0 Power supply,According to law of conservation of energy, we have,In a source set, the current reference direction i

12、s directed out of the + polarity marking (or the first subscript) of the voltage. In a load set, the current reference direction is directed into the + polarity marking (or the first subscript) of the voltage reference direction.,from EQ (1-1),the energy absorbing or supplied by an element from time

13、 t0 to t is,Unit is joules(J),1Wh=3600J,Example：The electron beam in a TV picture tube carries 1015 electrons per second. As a design engineer, determine the voltage U0 needed to accelerate the electron beam to achieve 4W. Solution：,1-6 Circuit Elements,(Chapter 6),Passive elements (无源元件）,Active ele

14、ments (有源元件）,Resistors电阻,Inductors电感,Capacitors电容,Independent source 独立电源,dependent source 受控源,Voltage source 电压源,current source 电流源,CCCS CCVS VCVS VCCS,Summary:,1. Current,2. Voltage,3. Potential,4. Power,2. P0 absorb or expend power(吸收功率） P0 supply power(发出功率）,Emphasis重点,1.current and voltage refe

15、rence direction,An ideal independent source is an active element that provides a specified voltage or current that is completely independent of other circuit variable.,1.6.1 Ideal Independent Sources,1、理想电压源 Ideal Voltage Source,The ideal voltage source maintains its prescribed voltage, independent

16、of its output current.General symbol for a voltage source,graphical characteristic of a dc voltage source:,the current is determined by the load （负载） U=0，Short circuit,maintains its voltage,The ideal current source produces its prescribed current, independent of its output voltage. General symbol of

17、 a current source,2、独立电流源 Ideal Current Source,graphical characteristic of a dc current source,maintains its current,the voltage is determined by the load（负载）,An ideal dependent (or controlled) source is an active element in which the source quantity is controlled by another voltage or current.,1.6.

18、2、 受控电源 CONTROLLED SOURCES,CCCS：A current-controlled current source ：current gain, dimensionless,VCCS： A voltage-controlled current source g：transconductance（S）,VCVS： A voltage-controlled voltage source ：voltage gain, dimensionless,CCVS：A current-controlled voltage source： r :transresistance（）,Examp

19、le：R1=1，R2=2，R3=3，=3， US3=3V. Find open-circuit voltage U22（开路电压）,Solution：22开路：Io=0，Io=0，,1.6.3电阻、电感、电容元件Resistors, Inductors and Capacitors,一、电阻元件 Resistors,R=，开路/断路 Open circuit: a circuit element with resistance approaching infinity.R R=0，短路 Short circuit: a circuit element with resistance appro

20、aching zero.R0,Ohms law states that the voltage across a resistor is directly proportional to the current flowing through the resistor.,u(t)=Ri(t) U=RI，I=GU,Georg Simon Ohm (1787-1845)，欧姆,P=UI=RI2=U2/R=GU2,Check Your Understanding Uab= ? Uad=? Uac=? Ucd=?,Ua-R1I1+E1=Ub；Uab=R1I1-E1； Uad=R1I1-E1-R3I3-

21、E3； Uac=R1I1-E1+E2-R2I2； Ucd=R2I2-E2-R3I3-E3；,二、电感器 Inductors,(t)=Li(t) -(Wb)(韦伯） L-(H）亨利 I-(A)(安培）,graphical characteristic（Wb-A),The voltage-current characteristics of an inductor,The energy stored in the inductor:,三、电容器 Capacitors,Circuit-theory definition of a capacitor: q(t)=Cu(t),C-法拉（F），1F=10

22、-6F，1pF=10-12F q-库仑（C） V-伏特（V),Michael Faraday (1791-1867)， 法拉弟,线性电容,（C-V 曲线),The energy stored in the capacitor:,The voltage-current characteristics of an capacitor:,小结,Ideal Current Source (IS ,iS) (1)Current 不随 Load 变化而变化。 (2)Voltage 随 Load 变化而变化。,2.Ideal Voltage Source (IS ,iS) (1)Voltage 不随 Loa

23、d 变化而变化。 (2)Current 随 Load 变化而变化。,3.Controlled sourses 受控电源,4.Resistor Ohms law,5.Inductor,6.Capacitor,1.9 Summary,An electric circuit consists of electrical elements connected together. The International System of Units(SI) is the international measurement language, which enables engineers to communicate their results. From the six principal units, the units of other physical quantities can be derived. Current is the rate of charge flow. Voltage is the energy required to move 1C of charge thr