电工学武汉理工大学课件chapter

1、Chapter1 Circuit Elements and Circuit LawsEmphases：Reference direction of voltage, current and electromotive force 2. Characteristic of circuit element3. Kirchhoffs Laws 11.1 Constitute of Circuit1.Constitute of circuit + i - VSource loadSource: generator or batteryLoad: electric light, motor or sto

2、veMiddle joint: switch or wire22. Circuit and Circuit ModelPractical circuit:Circuit model:导线开关电池灯泡+R0R开关E干电池电珠Swireswitchbattery+R0RswitchEbatteryLightSI3_+Rcircuit model: ideal circuit element composedswitchbatterylight+_UEPractical circuit41.2 Reference direction of voltage and current1.Currentde

3、fine: the time rate of change of charge passing through apredetermined area.Value: i =dq / dt (AC) I = q / t (DC)Direction:unit: A, mA, A abSIab 5Reference directions (1) Reference directions for current(2) Reference directions for voltage 6Reference direction: often define direction of positive ele

4、ctric charge (or opposite direction of negative charge).Sometimes factual direction is unknown before analysis in some circuit; direction of alternating current always change, its direction of current is uncertain.Reference direction: any direction of current is supposed before analysis in some circ

5、uit.If direction between reference direction and factual direction is same, value of current is positive; or not, its value is negative.Reference direction of current7Direction： Action in electric field, positive electric charges move from higher potential to lower potential.Reference direction: def

6、ine move direction of positive electric charge.Before analysis in electric circuit, choose any direction as reference direction of voltage.If direction between reference direction and factual direction is same, value of voltage is positive; or not, its value is negative.Unit： V, kV, mV2.Voltage8In f

7、igure relativedirection voltage and currentRelative directionabIUabIUReference conventions for two-terminal elements (R, L, C or . )9 Ohms law states that voltage across many types of conducting materials is directly proportional to the current flowing through the material. Expression:If R=constant

8、, we called R linear resistance. Ohm law is applied in simple circuit.UIR3. Ohms Law10Define symbol in Ohms law:If reference direction in current and voltage is relative directionUIROhms lawIf reference direction in current and voltage is non-relative direction(1)(2)UIRSymbol in Ohms law11List expre

9、ssion in figure (a), (b), (c), (d )by Ohms law. R=?Ohms lawUIR6V2A(a)UIR6V-2A(b)UIR-2A-6V(d)UIR(c)-6V 2AExample1:12Figure aOhms lawUIR6V2A(a)UIR6V-2A(b)Figure b13UIR-2A-6V(d)UIR(c)-6V 2AOhms lawFigure cFigure d14R0EUabR1.3 Work State of Circuit and PowerBase: Ohms LawIn right figure:E: sourceR0: int

10、ernal resistanceR: load resistance15UabR0ERII=E/(R0+R)U=IRor U=E-IR01. Load State16If R0R, then U ER0EUabRILoad state17If U=E-IR0 then UI= EI-I2R0or P = PE - PR0EUabRILoad StateWhere: PE=EI is produced power by source; P= I2R0 is absorbed power by resistor in the source; P=U I is output power by sou

11、rce or absorbed power by load.18Source: factual direction of U and I is opposite, I sends out from “+” term, generate power.Load: factual direction of U and I is same, I flows in from “+” term, absorb power.The symbol for powerp(t)0 the power absorbed by the element.p(t)0 the negative power absorbed

12、 by the element or it is actually generating power and delivering it to some external element.192. Open CircuitR0EU= U0abRI= 0CharacteristicsI = 0U = U0 = EP = 0We define a open circuit as an infinite resistance.203. Short circuitWe define a short circuit as a resistance of zero ohms.R0EabRIScdU = 0

13、I = Is = E / R0P = P = I2 R0Characteristics211.4 Kirchhoffs Laws Above some simple circuits are analyzed using Ohms laws, but some complicated circuits can not be found by Ohms laws. Next Kirchhoffs laws are introduced.I1bI2U1+_R1aI3R3R2+_U222Kirchhoffs laws:1. Kirchhoffs current law(KCL) apply in n

14、ode2. Kirchhoffs voltage law(KVL) apply in loopKirchhoffs laws: basic laws of electrical circuitKirchhoffs laws23Nouns and Concepts1. Branch: Every branch in a circuit. It is consisted of some elements.2. Node: The junction of three or more conductors.Kirchhoffs lawsabcdI2I3Branch current: current i

15、n a branch.Three branches in picture: ab, acb and adbI1 Three branches current in picture: I1、 I2 and I3Two nodes in picture: a、 b24Nouns and Concepts3. loop: closed circuits which is consisted of one, two or more branches.Three loops in picture: adbca、abca and abda.abcdI2I3I1Kirchhoffs lawsb=3n=2l=

16、325Kirchhoffs current lawKCLKirchhoffs lawsacdI2I1I3defines: Currents entering the node as being positiveCurrents exiting a node as being negativebcKCL states: The algebraic sum of all the currents entering any node is zero.26KCL states：the sum of the currents at a node must equal zero. For example

17、node a: I1 + I2 = I3 or I1 + I2 - I3 =0Kirchhoffs lawsKirchhoffs current lawKCLKCL another states: The algebraic sum of all the currents leaving a node is zero, or the algebraic sum of all the currents entering a node must equal the algebraic sum of all the currents leaving the node.27The KCL may be

18、 extended to the supernode:28Kirchhoffs voltage lawKVLKirchhoffs lawsKVL describes: mutual relationship in per conductors voltage of a closed circuit. It is applied in loop. KVL states: The algebraic sum of the voltage around any closed path in a circuit is zero.KVL:29KVL another state : in circuit terms, the sum of all voltages associated with sources must equal the sum of the load voltages, so that the net voltage around a closed circuit is zero.Kirchhoffs laws30Application of KVLKirchhoffs lawsacdI2I1I3bcR1R2R3E2For example : around the closed circuit dabdda