刘剑-电气工程及其自动化专业英语(建筑电气类)Unit1-passage A(6页).doc

-刘剑-电气工程及其自动化专业英语(建筑电气类)Unit1-passage A-第 6 页TextElectric Circuit The diagram of Fig 1.1 illustrates the essential parts of an electric circuit,which consists,in its simplest form, of an energy source and an interconnected energy dissipation or conversion device,known as the load. A practical energy source may take one many forms, depending, for example,on/electro-chemical,electro-magnetic,thermo-electric,photo-electric.,principles,but for the purpose of circuit analysis only two idealized forms are recognized, to one of which all practical sources approximate, They are: the voltage source and the current source. The voltage source maintains a constant terminal voltage irrespective of the current supplied to the load. It is important to appreciate that the voltage may be a function of, for example, time, temperature, pressure etc. It is constant without respect to variation of load.The current source maintains a constant in the load irrespective of the terminal voltage-which, in this case, is determined by the magnitude of the load, As with the voltage source, the generated current may depend on many other factors, but its one essential attribute is its independence of load.The symbols used for these active devices are illustrated in Fig 1.2 (a) and（b）。Also shown on the figure are the arbitrarily chosen positive directions of voltage and current. It should be noted that, conventionally, current flows through the source from the negative to the positive terminal.The transformation from these idealized sources to simulate the characteristics of real sources can be simply effected .The energy, w, expanded in moving a charge q through a potential difference (p.d.) is given by (1.1)hence (1.2)The rate of expenditure of energy is defined as the power p. Hence,in general the power is given by (1.3)and is measured in watts when and i are in volts and amperes, respectively.If power p(t) is expanded for time T, the total energy expanded (or stored) is (1.4) By a method similar to that adopted for energy sources, the load-or passive element of a circuit-may be idealized and defined by its terminal voltage/current relationship. All practical passive devices possess energy dissipative properties, often accompanied by energy-storage properties so that three distinct idealized types are possible. (a)The resistance parameter: A circuit, which dissipates energy but stores none is said to consist solely of resistance. The property is defined by the relationship (1.5)Where R is the resistance in ohms if and are in volts and amperes, respectively, and Eq.1.5 is known as Ohms Law. The corresponding diagram is shown in Fig 1.3(a),which also shows the positive directions of p.d. and current. It should be noted that, unlike an active element, a passive element develops a potential difference in obedience to the current flow so that there is a fall of potential though the element in the direction of the current flow, For this reason the terminal p.d. Is called a potential drop-voltage drop. The element which possesses resistance is termed a resistor.The reciprocal of resistance is conductance designated by the symbol G. Thus, (1.6)the units of G being siemens,or reciprocal ohms.Hence,an alternative form of Ohms Law is: (1.7) The power dissipated,may be written in terms of resistance (or conductance) and voltage or current only；thus, (1.8) If, for example, the voltage applied is constant, i.e. U(t)=U, then (1.9) and the power is also independent of time.(b)The inductance parameter:A circuit is said to possess inductance if it able to store magnetic field energy. The property is defined by the relationship (1.10)Where L is the inductance, the units of which are henrys if and are in volts and amperes, respectively, and t is in seconds. A p.d. of 1V will, therefore, cause the current to change at the rate of 1A/sec in an inductance of 1H, The circuit representation of the inductance parameter is shown in Fig 1.3(b) . The Eq.1.10 may also be written in general integral form: (1.11)The element which possesses inductance is termed an inductor.The power,may be written: (1.12)and is non-zero only when has a value.Hence for a steady current ,but for the current to have been established, has contributed to the stored energy: (1.13)Where is the time taken for the current to build up to .Hence, (1.14)(c)The capacitance parameter:A circuit which is able to store electrostatic field energy is said to possess capacitance. The property is defined in terms of the electric charge stored per unit of potential difference at its terminals, according to the equation: (1.15)Where is the capacitance, the units of which are farads when and are in volts and coulombs, respectively. Hence, a capacitance of 1F stores a charge of 1C for a terminal p.d. of 1V.Combining and Eq.1.15 gives (1.16)With in seconds. Thus ,a current of 1A flows into 1F when the terminal voltage changees at the rate of 1V/s Eq.1.16 may be rewritten in fneral integral form：Eq.1.16: （1.17）The element which possesses capacitance is termef a capacitor,and its circuit representation is illustated in Fig 1.3(c).The power ,may be written (1.18)and is non-zero only when has a value.Hence,for a steady voltage ,say,but for the voltage to have built up on the capacitor, has contributed to the stored energy (1.19)Where is the time taken for the voltage to have built up to .Hence (1.20) Eq.1.10 and Eq.1.16 show that step discontinuities are nit possible in the current through inductance nor in the voltage across capacitance,since such steps would require,respectively infinite voltage and infinite current.The ideas implicit in these restrictions are important in the analysis of circuits containing inductance and capacitance since they enable the initial conditions to be defined.