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1、1-1 INTRODUCTION,An electric circuit is an interconnection of electrical elements. Active elements: capable of generating energy such as generators, batteries etc. Passive elements: resistors, capacitors and inductors.,Why start with circuit theory? The solution of an engineering problem normally pr
2、oceeds: 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.,1-2 BASIC CONCEPTS,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 Id
3、eal 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,An alternative current (ac) is a current that varies sinusoidally with time. 时变电流,1. 电流：CURRENT,Electric current is the time rate of change of charge, measured in amperes (A).,A direct current (
4、dc) is a current that remains constant with time. 直流电流 1A=1C/1s(秒) 1kA=1000A，1mA=10-3A，1A=10-6A，1nA=109A,Andre-Marie Ampere (1775-1836) 安培,Assigning reference directions 电流的参考方向 To specify the current in a conductor, we need both a reference direction and numerical value, which can be positive or ne
5、gative.,2. 电位 Voltage of Potential,Alessandro Antonio Volta (1745-1827) 伏特,Voltage reference directions 电位的参考极性 参考点：,Voltage (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V). Ua=W/q V，kV，mV，V,3. 电压 VOLTAGE,Uab=Ua-Ub，point a is at a pote
6、ntial of Uab volts higher than point b. Voltage reference directions,Keep in mind that electric current is always through an element and that electric voltage is always across the element or between two points.,Check Your Understanding What the difference between Uab and U ?,U=Us+RI U=UsRI；,4. 电功率 P
7、ower,Power is the time rate of expending or absorbing energy, measured in watts (W).,P=UI 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.,(a)：P=UI0， Power absorb P=UI0， Power supply,(b)：P=UI0， Power
8、supply,(c): P=UI0， Power supply,In a source set, the current reference direction is 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
9、 direction.,Example：The electron beam in a TV picture tube caries 1015 electrons per second. As a design engineer, determine the voltage U0 needed to accelerate the electron beam to achieve 4W. Solution：,1-2 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 cir
10、cuit element with resistance approaching infinity. R=0，短路 Short circuit: a circuit element with resistance approaching zero.,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 U=10I+100I2,Georg Simon Ohm (1787
11、-1845)，欧姆,G=1/R mhos or siemens (S),P=UI=RI2=U2/R=GU2,Check Your Understanding Uab= ? Uad=? Uac=? Ucd=?,Ua-R1I1+E1=Ub；Uab=R1I1-E1； Uad=R1I1-E1-R3I3-E3； Uac=R1I1-E1+E2-R2I2； Ucd=R2I2-E2-R3I3-E3；,二、电感器 INDUCTORS,(t)=Li(t),Joseph Henry (1797-1878)， 亨利,The voltage-current characteristics of an inductor,
12、The energy stored in the inductor:,三、电容器 CAPACITOR,Circuit-theory definition of a capacitor: q(t)=Cu(t),法拉（F），1F=10-6F，1pF=10-12F,Michael Faraday (1791-1867)， 法拉弟,The voltage-current characteristics of an capacitor:,The energy stored in the capacitor:,1-3 基尔霍夫定律 KIRCHHOFFS LAW,一、 NODES, BRANCHES, AN
13、D LOOPS,1A branch represents a single element such as a voltage source or a resistor. In other words, branch represents any two-terminal element. 2A node is the point of connection between two or more branches. 3A loop is any closed path in a circuit.,A network with b branches, n nodes, and l indepe
14、ndent loops will satisfy the fundamental theorem of network topology：b=l+n-1,二、基尔霍夫电流定律 Kirchhoffs Current Law (KCL),Gustav Robert Kirchhoff (1824-1887)， 基尔霍夫/克西霍夫,Kirchhoffs current law (KCL) states that the algebraic sum of currents entering a node is zero. The sum of the currents entering a node
15、is equal to the sum of the currents leaving the node. Kirchhoffs current law (KCL) states that the algebraic sum of currents entering a closed boundary is zero.,Check Your Understanding:,Find the current Id Id=Ia-Ib-Ic=1-2-3=-4(A),Find the current i3 i3=i2-i1=2t-10sint (A),三、基尔霍夫电压定律 Kirchhoffs Volt
16、age Law (KVL),Kirchhoffs voltage law (KVL) states that the algebraic sum of all voltages around a closed path (or loop) is zero.,adcfa：Uad+Udc+Ucf+Ufa=0 UaUd+UdUc+UcUf+UfUa=0 U=0 adcfa：Uad=-R3I3+E3；Udc=R4I4；Ucf=R6I6；Ufa= E6； R3I3+R4I4+R6I6=E6E3 Udf=? R3I3+E3+UdfE6=0；Udf=E6+R3I3E3 Or: R4I4+R6I6Udf=0；Udf=R4I4+R6I6,Example: Find the current I .,Solution: R3I+E1+R1I+R2IE2=0,例：R1=1，R2=2，R3=3，E3
