电气工程及其自动化专业英语-Chapter-1课件.pptx
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1、Section 1 Current and Voltagen Textn New Words and Expressionsn Exercisesn EndSection1 Current and Voltage Two variables u(t) and i(t) are the most basic concepts in an electric circuit, they characterize the various relationships in an electric circuit. Charge and Current The concept of electric ch
2、arge is the underlying principle for explaining all electrical phenomena. Also, the most basic quantity in an electric circuit is the electric charge. Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C). Section1 Current and Voltage We know fro
3、m elementary physics that all matter is made of fundamental building blocks known as atoms and that each atom consists of electrons, protons, and neutrons. We also know that the charge e on an electron is negative and equal in magnitude to 1.602101019C, while a proton carries a positive charge of th
4、e same magnitude as the electron. The presence of equal numbers of protons and electrons leaves an atom neutrally charged. Section1 Current and Voltage We consider the flow of electric charges. A unique feature of electric charge or electricity is the fact that it is mobile; that is, it can be trans
5、ferred from one place to another, where it can be converted to another form of energy. When a conducting wire is connected to a battery (a source of electromotive force), the charges are compelled to move; positive charges move in one direction while negative charges move in the opposite direction.
6、This motion of charges creates electric current. It is conventional to take the currentSection1 Current and Voltageflow as the movement of positive charges, that is, opposite to the flow of negative charges, as Fig.l-1 illustrates. This convention was introduced by Benjamin Franklin (l706l790), the
7、American scientist and inventor. Although we now know that current in metallic conductors is due to negatively charged electrons, we will follow the universally accepted convention that current is the net flow of positive charges. Thus, Electric current is the time rate of charge, measured in ampere
8、s (A). Mathematically, the relationship among current i, charge q, and time t is Section1 Current and Voltage The charge transferred between time t0 and t is obtained by integrating both sides of Eq. (1-1). We obtain The way we define current as i in Eq. (1-l) suggests that current need not be a con
9、stant-valued function, charge can vary with time in several ways that may be represented by different kinds of mathematical functions. tqiddtttiq 0d (1-1)(1-2)Section1 Current and Voltage Voltage, Energy, and Power To move the electron in a conductor in a particular direction requires some work or e
10、nergy transfer. This work is performed by an external electromotive force (emf), typically represented by the battery in Fig.l-1. This emf is also known as voltage or potential difference. The voltage uab between two points a and b in an electric circuit is the energy (or work) needed to move a unit
11、 charge from a to b; mathematically Section1 Current and Voltage 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 (l745l827), who invented the first voltaic battery. Thus, Volta
12、ge (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V). qwuabdd(1-3)Section1 Current and Voltage Fig.l-2 shows the voltage across an element (represented by a rectangular block ) connected to points a and b. The plus (+) and minus (-) sign
13、s are used to define reference direction or voltage polarity. The uab can be interpreted in two ways: point a is at a potential of uab volts higher than point b; the potential at point a with respect to point b is uab. It follows logically that in general Fig.1-2 Polarity of voltage uab baabuu(1-4)S
14、ection1 Current and Voltage Although current and voltage are the two basic variables in an electric circuit, they are not sufficient by themselves. For practical purposes, we need to know power and energy. To relate power and energy to voltage and current, we recall from physics that power is the ti
15、me rate of expending or absorbing energy, measured in watts (W). We write this relationship as twpdd(1-5)Section1 Current and VoltageWhere p is power in watts (W), w is energy in joules (J), and t is time in seconds (s). From Eq. (1-1), Eq. (1-3), and Eq. (1-5), it follows that Because u and i are g
16、enerally function of time, the power p in Eq. (1-6) is a time-varying quantity and is called the instantaneous power. The power absorbed or supplied by an element is the product of the voltage across the element and the current through it. If the power has a plus sign, power is being delivered to or
17、 absorbed by the element. If, uip (1-6)Section1 Current and Voltageon the other hand, the power has a minus sign, power is being supplied by the element. But how do we know when the power has a negative or a positive sign? Current direction and voltage polarity play a major role in determining the s
18、ign of power. It is therefore important that we pay attention to the relationship between current i and voltage u in Fig.1-3(a). Fig.1-3 Reference polarities for power using the passive sign convention Section1 Current and VoltageThe voltage polarity and current i direction must conform with those s
19、hown in Fig.1-3(a) in order for the power to have a positive sign. This is known as the passive sign convention. By the passive sign convention, current enters through the positive polarity of the voltage. In this case, p = ui or ui 0 implies that the element is absorbing power. However, if p = -ui
20、or ui 0, as in Fig.1-3(b), the element is releasing or supplying power. Section1 Current and Voltage In fact, the law of conservation of energy must be obeyed in any electric circuit. For this reason, the algebraic sum of power in a circuit, at any instant of time, must be zero This again confirms t
21、he fact that the total power supplied to the circuit must balance the total power absorbed. From Eq. (l-7), the energy absorbed or supplied by an element from time t0 to time t is 0p(1-7)tttpw 0d (1-8)ReturnNew Words and Expressionscurrentn. 电流 alternating current 交流电流 direct current 直流电流 current de
22、nsity 电流密度 active current 有功电流 reactive current 无功电流voltagen. 电压 voltage drop 电压降落 voltage gain 电压增益 voltage divider 分压器 voltage stabilizer 稳压器New Words and Expressionsvariable n. 变量;a.可变的,可调的,易变的 complex variable 复变量 dependent variable 因变量 independent variable 自变量 input variable 输入变量 charge n. 电荷,充
23、电;v. 充电,带电,起电 charge distribution 电荷分布 charge particle 带电粒子 test charge 试验电荷 positive charge 正电荷 negative charge 负电荷New Words and Expressionscoulomb n. 库仑(电荷的单位) coulomb meter 库仑计circuit n. 电路 circuit diagram 电路图 analogue circuit 模拟电路 short circuit 短路 open circuit 开路 active circuit 无源电路 passive circ
24、uit 无源电路underlying a. 基本的,根本的 潜在的,在下(面)的,下伏的New Words and Expressionselectron n. 电子 magnituden. 大小,尺寸,数量,数值electromotivea. 电动的,起电的 electromotive force 电动势 amperen. 安,安培(电流的单位) ampere meter 安培计 ampere turn 安匝 volt ampere 伏安 integratev. 积分,求积分workn. 功 New Words and Expressionspotential a.势的,位的,电位的;n.势
25、(能),位(能), 电势(位,压) potential difference 电位差,电势差 earth potential 地电势(位) potential energy 势(位)能 reference potential 参考(基准)电位rectangular a. 矩形的,直角的,正交的joulen. 焦尔(能量、热量、功的单位)voltn. 伏特(电压、电位、电势的单位) electron volt 电子伏特 New Words and Expressionsvoltaic a. 动流的,电压的,电镀的,伏打(式)的 voltaic battery伏打电池conventionn. 习惯
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