电学基础Chap(1)

1、返回返回1.1 Circuit TheoryAn electric circuit is a mathematical model that approximates the behavior of an actual electrical system.1.1 Circuit TheoryCircuit theory is a special case of electromagnetic field theory: the study of static and moving electric charges.Three basic assumptions:uElectrical effe

2、cts happen instantaneously through a system.uThe net charge on every component in the system is always zero.uThere is no magnetic coupling between the components in a system.can hold equal, opposite, separated chargeswithininstantaneouslycouplinglThe charge is bipolar, meaning that electrical effect

3、s are described in terms of positive and negative charges.lThe electric charge exists in discrete quantities, which are integral multiples of the electronic charge, 1.602210-19C.lElectrical effects are attributed to the separation of charge and charges in motion.1.2 VOLTAGE ANDSome important charact

4、eristics of electric charge:REVIEW_negative nineteenth power of ten CURRENTcharacteristics bipolarThe separation of charge creates an electric force (voltage)Voltage is the energy per unit charge created by the separation. Electric current is the rate of charge flow.dqdwv dtdqi v =the voltage in vol

5、tsw =the energy in joulesq =the charge in coulombsi =the current in amperesq =the charge in coulombs t =the time in secondsThe ratio of w to q in differential formThe motion of charge creates an electric fluid (current).1.3 THE IDEAL BASIC CIRCUIT ELEMENT An ideal basic circuit element has three att

6、ributes: It has only two terminals, which are points of connection to other circuit components. It is described mathematically in terms of current and/or voltage. It cannot be subdivided into other elements.plus signminus sign1.3 THE IDEAL BASIC CIRCUIT ELEMENTii+v-v+Assign the reference direction f

7、or voltage and current.The reference direction is arbitrary. Once assigned, we must write all the subsequent equations to agree with the chosen references.POSITIVE VALUEvvoltage drop from terminal 1 to terminal 2Or voltage rise from terminal 2 to terminal lipositive charge flowing from terminal 1 to

8、 terminal 2negative charge flowing from terminal 2 to terminal 1NEGATIVE VALUEvvoltage rise from terminal 1 to terminal 2Or voltage drop from terminal 2 to terminal lipositive charge flowing from terminal 2 to terminal 1negative charge flowing from terminal 1 to terminal 2interpretation of reference

9、 direction1.3 THE IDEAL BASIC CIRCUIT ELEMENTi+v-v+-v+v-+-+v-v+ii-+iPassive sign convention: Whenever the reference direction for the current in an element is in the direction of the reference voltage drop across the element, use a positive sign in any expression that relates the voltage to the curr

10、ent. Otherwise, use a negative sign.（无源符号法则）（无源符号法则）R = v / i ，P = v iR= - v / i ，P= - v i+-+ -+-+ -V1001RbvV50V20- v1 +-2R3R4R5R6R7Ri1+ v2 -i2- v3 +i3+ v4 -i4+ -+ -1.4 POWER AND ENERGYdtdwp vidtdqdqdwdtdwp )(p =the power in wattsw =the energy in joulest =the time in secondsp =the power in wattsv =t

11、he voltage in voltsi =the current in amperesPower is the time rate of expending or absorbing energy.Power is the product of the current and the voltage. How to tell whether power is being delivered to the pair of terminal or extracted from it?As positive charges move through a drop in voltage, they

12、lose energy.+-If p0, power is being delivered to the circuitinside the box，if p0, power is being extracted from the circuit inside the box. Assume that a 20V voltage drop occurs across an element from terminal 2 to terminal 1 and a current of 4A enters terminal 2.a) Specify the values of v and i for

13、 the polarity references shown in Fig (a)-(d) v=-20V,i=-4A v=-20V,i=4A v=20V,i=-4A v=20V,i=4Aabsorbing80wC) How much power is the circuit absorbing? P= viP=-viP= -viP= viEXAMPLE b) State whether the circuit inside the box is absorbing or delivering power.REVIEW1.How to assign the reference direction

14、 of voltage and current?2.How to judge the algebraic sign of the real voltage and current?3.State the passive sign convention.4.The blank box is delivering power if the real current flow through a rise in voltage. Yes or no?P=- viYes!plus signminus signterminalsPassive sign conventionreference directionsubsequent