【文章內(nèi)容簡(jiǎn)介】 荷引起的，但我們將遵循通用的慣例，即把電流看作是正電荷的單純的流動(dòng)。于是電流就是電荷的時(shí)率，它是以安培為單位來(lái)度量的。從數(shù)學(xué)上來(lái)說(shuō)，電流i、電荷q以及時(shí)間t之dqi= (11)間的關(guān)系是：dtThe charge transferred between time t0 and t is obtained by integrating both sides of Eq.(11).We obtain 從時(shí)間t0到時(shí)間t所移送的電荷可由方程（1－1）兩邊積分求得。我們算得：tq= 242。tidt (12)The way we define current as i in Eq.(1l)suggests that current need not be a constantvalued function, charge can vary with time in several ways that may be represented by different kinds of mathematical functions 我們通過(guò)方程（1－1）定義電流的方式表明電流不必是一個(gè)恒值函數(shù)，電荷可以不同的方式隨時(shí)間而變化，這些不同的方式可用各種數(shù)學(xué)函數(shù)表達(dá)出來(lái)。電壓，能量和功率To move the electron in a conductor in a particular direction requires some work or energy work is performed by an external electromotive force(emf), typically represented by the batterydwin emf is also known as voltage or potential = (13)The voltage uab between two dqpoints a and b in an electric circuit is the energy(or work)needed to move a unit charge from a to b。mathematically 在導(dǎo)體中朝一個(gè)特定的方向移動(dòng)電荷需要一些功或者能量的傳遞，這個(gè)功是由外部的電動(dòng)勢(shì)來(lái)完成的。圖1－1所示的電池就是一個(gè)典型的例子。這種電動(dòng)勢(shì)也被稱為電壓或電位差。電路中a、b兩點(diǎn)間的電壓等于從a到b移動(dòng)單位電荷所需的能量（或所需做的功）。數(shù)學(xué)表達(dá)式為：where w is energy in joules(J)and q is charge in coulombs(C).The voltage uab is measured in volts(V), named in honor of the Italian physicist Alessandro Antonio Volta(l745～l827), who invented the first voltaic , Voltage(or potential difference)is the energy required to move a unit charge through an element, measured in volts(V).式中w是單位為焦耳的能量而q是單位為庫(kù)侖的電荷。電壓Uab是以伏特為單位來(lái)度量的，它是為了紀(jì)念意大利物理學(xué)家Alessandro Antonio Volta而命名的，這位意大利物理學(xué)家發(fā)明了首個(gè)伏達(dá)電池。于是電壓（或電壓差）等于將單位電荷在元件中移動(dòng)所需的能量，它是以伏特為單位來(lái)度量的。 shows the voltage across an element(represented by a rectangular block)connected to points a and plus(+)and minus()signs are used to define reference direction or voltage uab can be0 2 interpreted in two ways: ①point a is at a potential of uab volts higher than point b。②the potential at point a with respect to point b is follows logically that in general 圖1－2顯示了某個(gè)元件（用一個(gè)矩形框來(lái)表示）兩端a、b之間的電壓。正號(hào)（＋）和負(fù)號(hào)（－）被用來(lái)指明參考方向或電壓的極性，Uab可以通過(guò)以下兩種方法來(lái)解釋。1）在Uab伏特的電位中a點(diǎn)電位高于b點(diǎn)，2）a點(diǎn)電位相對(duì)于b點(diǎn)而言是Uab，通常在邏輯上遵循uab=uba (14)Although current and voltage are the two basic variables in an electric circuit, they are not sufficient by practical purposes, we need to know power and relate power and energy to voltage and current, we recall from physics that power is the time rate of expending or absorbing energy, measured in watts(W).We write this relationship as 雖然電流和電壓是電路的兩個(gè)基本變量，但僅有它們兩個(gè)是不夠的。從實(shí)際應(yīng)用來(lái)說(shuō)，我們需要知道功率和能量。為了把功率和能量同電壓、電流聯(lián)系起來(lái)，我們重溫物理學(xué)中關(guān)于功率是消耗或吸收的能量的時(shí)率，它是以瓦特為單位來(lái)度量的。我們把這個(gè)關(guān)系式寫(xiě)成：dwp= (15) dtWhere p is power in watts(W), w is energy in joules(J), and t is time in seconds(s).From Eq.(11), Eq.(13), and Eq.(15), it follows that 式中p是以瓦特為單位的功率，w是以焦耳為單位的能量，t是以秒為單位的時(shí)間，從方程（1－1）、（1－3）和（1－5）可以推出p=ui (16)Because u and i are generally function of time, the power p in Eq.(16)is a timevarying quantity and is called the instantaneous power absorbed or supplied by an element is the product of the voltage across the element and the current through the power has a plus sign, power is being delivered to or absorbed by the , on the other hand, the power has a minus sign, power is being supplied by the how do we know when the power has a negative or a positive sign？由于u和i通常是時(shí)間的函數(shù)，方程（1－6）中的功率p是個(gè)時(shí)間變量于是被稱為瞬時(shí)功率，某一元件吸收或提供的功率等于元件兩端電壓和通過(guò)它的電流的乘積。如果這個(gè)功率的符號(hào)是正的，那么功率向元件釋放或被元件吸收。另一方面，如果功率的符號(hào)是負(fù)的，那么功率是由元件提供的。但我們?nèi)绾蔚弥螘r(shí)功率為正或?yàn)樨?fù)？Current direction and voltage polarity play a major role in determining the sign of is therefore important that we pay attention to the relationship between current i and voltage u in (a).The voltage polarity and current i direction must conform with those shown in (a)in order for the power to have a positive is known as the passive sign the passive sign convention, current enters through the positive polarity of the this case, p = ui or ui ﹥ 0 implies that the element is absorbing , if p =ui or ui ﹤ 0, as in (b), the element is releasing or supplying ，電流的方向和電壓的極性起著主要的作用，這就是我們?cè)诜治鰣D1－3（a）所顯示的電流i和電壓u的關(guān)系時(shí)特別謹(jǐn)慎的重要原因。為了使功率的符號(hào)為正，電壓的極性和電流的方向必須與圖1－3（a）所示的一致。這種情況被稱為無(wú)源符號(hào)慣例，對(duì)于無(wú)源符號(hào)慣例來(lái)說(shuō)，電流流進(jìn)電壓的正極。在這種情況下，p＝ui或ui0，表明元件是在吸收功率。而如果p＝－ui或uiIn fact, the law of conservation of energy must be obeyed in any electric this reason, the algebraic sum of power in a circuit, at any instant of time, must be zero 事實(shí)上，在任何電路中必須遵循能量守恒定律。由于這個(gè)原因，任一電路中在任何瞬間功率的代數(shù)和必須等于零229。p=0 (17)This again confirms the fact that the total power supplied to the circuit must balance the total power Eq.(l7), the energy absorbed or supplied by an element from time t0 to time t is 這再一次證明了提供給電路的功率必須與吸收的功率相平衡這一事實(shí)。從方程（1－7）可知，從時(shí)間t0到時(shí)間t被元件吸收或由元件提供的功率等于t w=242。pdt (18)tSection2 An electric circuit is simply an interconnection of the are two types of elements found in electric circuits: passive elements and active active element is capable of generating energy while a passive element is of passive elements are resistors, capacitors, and most important active elements are voltage or current sources that generally deliver power to the circuit connected to 。我們發(fā)現(xiàn)電路中存在有兩種元件：無(wú)源元件和有源元件。有源元件能夠產(chǎn)生能量而無(wú)源元件卻不能，無(wú)源元件有電阻、電容和電感器等。最重要的有源元件是通常向與它們相連的電路釋放能量的電壓和電流源。Independent sourcesAn ideal independent source is an active element that provides a specified voltage or current that is pletely independent of other circuit independent voltage source is a twoterminal element, such as a battery or a generator, which maintains a specified voltage between its voltage is pletely independent of the current through the symbol for a voltag