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电气技术专业英语 朱一纶 主编 中国电力出版社 Unit 2 Basic Components 课件制作: 吴岱曦 Index • Text – 1. – 2. – 3. – 4. Resistor Capacitor Inductor Impedance • Reading materials – 1. Resistor Color Codes – 2. Domestic power plugs and sockets – 3. Battery charger • Exercises – 1. – 2. – 3. – 4. Put the Phrases into English Put the Phrases into Chinese Sentence Translation Translation Text 1. Resistor • Suppose that some material is connected to the terminals of an ideal voltage source as shown in Fig2.1(a),if the resulting current i(t) is always directly proportional to the voltage for any function v(t), then the material is called a linear resistor, or resistor for short. Fig 2.1 the relationship of voltage and current (a) Connected circuit (b) i(t) is directly proportional to v(t) • Since voltage and current are directly • proportional for a resistor, there exists a proportionality constant R, called resistance. The amount of current flowing in a resistor is directly proportional to the voltage across it and inversely proportional to the resistance of the resistor. This is Ohm law and can be expressed as a formula: v(t)=Ri(t) • The units of resistance (volts per ampere) are referred to as ohms, and are denoted by the capital Greek letter omega, Ω. A plot of voltage versus current for a (linear) resistor is given in Fig 2.1(b). • Resistors are used to limit current flowing to a device, thereby preventing it from burning out, as voltage dividers to reduce voltage for other device, as transistor biasing circuits, and to serve as circuit loads. • Fig2.2 (a) shows the different resistor symbols that are used in circuit diagrams. The rectangular box is used throughout Europe, while the zig-zag line is more common in Japan and the USA. Fig 2.2 Resistors 2. Capacitor • A capacitor is an electronic device for temporarily store electrical energy. Capacitors can be found in almost any complex electronic circuit. There are many different types of capacitor but they all work in essentially the same way. • A simplified view of a capacitor is a pair of metal plates separated by a gap in which there is an insulating material known as the dielectric. This simplified capacitor is also chosen as the electronic circuit symbol for a capacitor is a pair of parallel plates as shown in Fig2.3(a). • Some capacitors’ capacitance can be adjusted, so they are variable capacitors (Fig 2.3(b)). Fig 2.3 capacitors • If voltage is applied to the capacitor terminals, charge flows in and collects on one plate. Meanwhile, current flows out of the other terminal, and a charge of opposite polarity collects on the other plate. • The magnitude of the net charge Q on one plate is proportional to the applied voltage V. Thus, we have Q=CV, in which C is the capacitance. • A capacitor will block dc current, but appears to pass ac current by charging and discharging. It develops an ac resistance, known as capacitive reactance, which is affected by the capacitance and ac frequency. The formula for capacitive reactance is XC=1/(2πfC C), with units of ohmsΩ. 3. Inductor • An inductor is an electrical device, which can temporarily store electromagnetic energy in the field about it as long as current is flowing through it. The inductor is a coil of wire that may have an air core or an iron core to increase its inductance. • The circuit symbols for inductors are shown in Fig 2.4(a). Some inductors’ inductance can be adjusted; a powered iron core in the shape of a cylinder may be adjusted in and out of the core in such inductors, so they are variable inductors . Fig 2.4 inductors • An inductor tends to oppose a change in electrical current, it has no resistance to dc current but has an ac resistance to ac current, known as inductive reactance, this inductive reactance is affected by inductance and the ac frequency and is given by the formula XL=2πfL L, with units of ohms. 4. Impedance • Impedance (symbol Z) is a measure of the overall opposition of a circuit to current, in other words: how much the circuit impedes the flow of current. It is like resistance, but it also takes into account the effects of capacitance and inductance. Impedance is measured in ohms, symbol Ω. • Impedance is more complex than resistance because the effects of capacitance and inductance vary with the frequency of the current passing through the circuit and this means impedance varies with frequency! The effect of resistance is constant regardless of frequency. • The term 'impedance' is often used for simple circuits which have no capacitance or inductance - for example to refer to their 'input impedance' or 'output impedance'. This can seem confusing if you are learning electronics, but for these simple circuits you can assume that it is just another word for resistance. • Four electrical quantities determine the impedance (Z) of a circuit: resistance (R), capacitance (C), inductance (L) and frequency (f). • Impedance can be split into two parts: Z=R+jX • Resistance R (the part which is constant regardless of frequency). • Reactance X (the part which varies with frequency due to capacitance and inductance). This is the End of the Text Reading materials 1. Resistor Color Codes • Components are coded with colors to identify their value and tolerance,here takes the resistor as an example. • The values of the resistor are calculated from the color of the bands (see Tab 2.1) . The values of the colors are shown in Table 2.1. The first band is the tens values. The second band gives the units; the third band is a multiplying factor, the factor being 10’s band value. • The fourth band gives the tolerance of the resistor. No band implies a tolerance of ± 20%, a silver band means the resistor has a tolerance of ± 10% and a gold band has the closest tolerance of ± 5%. • For a 5-band resistor, the first band is the hundreds values. The second band gives the tens and the third band gives units. The forth band is a multiplying factor, the factor being 10’s band value. • The colors brown, red, green, blue, and violet are used as tolerance codes on 5band resistors only. All 5-band resistors use a colored tolerance band. Table 2.1 The values of the colors Fig 2.5 Resistor Color Codes • Fig 2.6 gives some examples of color code resistor. Resistor in Fig 2.6(a) colored BrownGreen-Grey-Silver-Red would be 1.58 Ω with a tolerance of ± 2% • resistor in Fig 2.6(b) colored Yellow-VioletOrange-Gold would be 47 kΩ with a tolerance of ± 5%. 2. Domestic power plugs and sockets • In most countries, household power is single-phase electric power, in which a single phase conductor brings alternating current into a house, and a neutral returns it to the power supply. • Domestic power plugs and sockets (Fig 2.7) are devices that connect the home appliances and portable light fixtures commonly used in homes to the commercial power supply so that electric power can flow to them. • Many plugs and sockets include a third contact used for a protective earth ground, which only carries current in case of a fault in the connected equipment. • Power plugs are male electrical connectors that fit into female electrical sockets. They have contacts that are pins or blades that connect mechanically and electrically to holes or slots in the socket. • Plugs usually have a phase or hot contact, a neutral contact, and an optional earth or Ground contact. Many plugs make no distinction between the live and neutral contacts, and in some cases they have two live contacts. The contacts may be steel or brass, either zinc, tin or nickel plated. • Power sockets are female electrical connectors that have slots or holes which accept the pins or blades of power plugs inserted into them and deliver electricity to the plugs. • Sockets are usually designed to reject any plug which is not built to the same electrical standard. Some sockets have one or more holes that connect to pins on the plug. Fig 2.7 Domestic power Various Plugs & Sockets 3. Battery charger • A battery charger is a device used to put energy into a secondary cell or (rechargeable) battery by forcing an electric current through it. • The charge current depends upon the technology and capacity of the battery being charged. For example, the current that should be applied to recharge a 12 V car battery will be very different from the current for a mobile phone battery. • A simple charger works by connecting a constant DC power source to the battery being charged. The simple charger does not alter its output based on time or the charge on the battery. This simplicity means that a simple charger is inexpensive, but there is a tradeoff in quality. • Typically, a simple charger takes longer to charge a battery to prevent severe overcharging. Even so, a battery left in a simple charger for too long will be weakened or destroyed due to overcharging. • A trickle charger is a kind of simple charger that charges the battery slowly, at the self-discharge rate. A trickle charger is the slowest kind of battery charger. Leaving a battery in a trickle charger keeps the battery "topped up" but never over-charges. • The output of a timer charger is terminated after a pre-determined time. Timer chargers were the most common type for high-capacity Ni-Cd cells in the late 1990s.Often a timer charger and set of batteries could be bought as a bundle and the charger time was set to suit those batteries. • If batteries of lower capacity were charged then they would be overcharged, and if batteries of higher capacity were charged they would be only partly charged. • An intelligent charger is that it’s output current depends upon the battery's state. An intelligent charger may monitor the battery's voltage, temperature and/or time under charge to determine the optimum charge current at that instant. • Charging is terminated when a combination of the voltage, temperature and/or time indicates that the battery is fully charged. This is the End of the Reading materials Exercises • Ready to go? 1. Put the Phrases into English • 1)一个理想电压源 • 2)比例常数 • 3)欧姆定律 • 4)电子器件 • 5)可变电容 Show the Answer • 6)阻挡直流 • 7)感抗 • 8)电磁能 • 9)交流电频率 • 10)输入阻抗 Show the Answer 2. Put the Phrases into Chinese • 1)linear resistor • 2)be inversely proportional to • 3)circuit diagram • 4)simplified capacitor • 5)pass ac current Show the Answer • 6)capacitive reactance • 7)as long as • 8)powered iron core • 9)variable inductors • 10)take into account Show the Answer 3. Sentence Translation • 1)Suppose that some material is connected to the terminals of an ideal voltage source. • 1)假设在某一器件的两端加上一个理想电 压源。 • 2) Resistors are used as voltage dividers to reduce voltage for other device. • 2)电阻可用于分压,减小其他器件的电压 。 • 3) There are many different types of capacitor but they all work in essentially the same way. • 3)电容的种类很多,但它们的基本工作原 理都是一样的。 • 4) The magnitude of the net charge Q on one plate is proportional to the applied voltage V. • 4)一块极板上聚集电荷的多少取决于所加 的电压。 • 5) Impedance is more complex than resistance because the effects of capacitance and inductance vary with the frequency of the current passing through the circuit. • 5)阻抗比电阻复杂,因为电容和电感(对 电流的阻碍)作用是会随着流过电路的电 流的频率变化而变化的。 4.Translation • Standard-type resistors usually maintain their value regardless of external conditions, such as voltage, temperature, and light. These types of resistors are referred to as linear resistors. • There are other types of resistors referred to as nonlinear, whose resistance varies with temperature (thermistor 热敏电 阻),voltage(varistor变阻器) and light. Show the Answer This is the End of the Exercises Good work everyone !!! (习题答案仅供参考) That is the End of Lecture Thank you