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Alternating Current (AC) flows one way, then the other way, continually reversing direction (as shown in Fig.3-1 and Fig.3-2). An AC voltage is continually changing between positive (+) and negative(-). The rate of changing direction is called the frequency of the AC, and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second.
An AC supply is suitable for powering some devices such as lamps and heaters, but almost all electronic circuits require a steady DC supply.
Fig.3-1 AC from a Power Supply: This shape is called a sine wave
Fig.3-2 This triangular signal is AC because it changes between positive (+) and negative (-)
Direct Current (DC) always flows in the same direction, but it may increase and decrease.A DC voltage is always positive (or always negative), but it may increase and decrease.Electronic circuits normally require a steady DC supply which is constant at one value (as shown in Fig.3-3). Cells, batteries and regulated power supplies provide steady DC which is ideal for electronic circuits. Lamps, heaters and motors will work with any DC supply.
Fig.3-3 Steady DC: from a battery or regulated powersupply, this is ideal for electronic circuits
An electrical signal is a voltage or current which conveys information, usually it means a voltage.The term can be used for any voltage or current in a circuit.
The voltage-time graph on the Fig.3-4 shows various properties of an electrical signal. In addition to the properties labeled on the graph, there is frequency which is the number of cycles per second. The diagram shows a sine wave but these properties apply to any signal with a constant shape.
Fig.3-4 Various Properties of anElectrical Signals
Amplitude is the maximum voltage reached by the signal. It is measured in volts, V.
Peak voltage is another name for amplitude.
Peak-peak voltage is twice the peak voltage (amplitude). When reading an oscilloscope trace, it is usual to measure peak-peak voltage.
Time period is the time taken for the signal to complete one cycle. It is measured in seconds (s), but time periods tend to be short, so milliseconds (ms) and microseconds (µs) are often used. 1 ms=0.001s and 1 µs=0.000001 s.
Frequency is the number of cycles per second. It is measured in hertz (Hz), but frequencies tend to be high, so kilohertz (kHz) and megahertz (MHz) are often used. 1 kHz=1000 Hz and 1 MHz=1000000 Hz.Frequency=1/time period, and time period= 1/frequency.
Another value used is the effective value of AC. This is the value of alternating voltage or current that will have the same effect on a resistance as a comparable value of direct voltage or current will have on the same resistance.
positive ['pɔzətiv] adj. 正的
frequency ['fri:kwənsi] n. 频率
measure ['meʒə] v. & n. 测量
device [di'vais] n. 装置，设备
label ['leibl] n. & v. 标签；贴标签于……
amplitude ['æmplitju:d] n. 振幅
period ['pɪərɪəd] n. 时期，周期
oscilloscope [ɔ'siləskəup] n. 示波器
millisecond ['milisekənd] n. 毫秒
microsecond ['maikrəusekənd] n. 微秒
Alternating Current 交流电
Direct Current 直流电
power supply 电源
electronic circuit 电子电路
electrical signal 电信号
peak voltage 峰值电压
peak-peak voltage 峰-峰值电压
effective value 有效值
1. The rate of changing direction is called the frequency of the AC, and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second.
译文：这种方向变化的速率称为交流信号的频率，单位是赫兹，它表示一秒内周期性变化的次数。
hertz (Hz)：n. 赫兹（电学频率单位），赫（兹），每秒周数【频率单位】，【大写】赫兹（德国物理学家）
cycles per second：每秒循环数。
2. The voltage-time graph on the Fig.3-4 shows various properties of an electrical signal.
译文：图3-4的电压-时间图显示了电信号的各种特性。
voltage-time graph：电压时间曲线。
3. The diagram shows a sine wave but these properties apply to any signal with a constant shape.
译文：该图显示了一个正弦波，但这些特性适用于具有恒定形状的任何信号。
sine wave：n.正弦波。
cosine wave：n.余弦波。
4. Time period is the time taken for signal to complete one cycle. It is measured in seconds (s), but time periods tend to be short so, milliseconds (ms) and microseconds (µs) are often used. 1 ms =0.001 s and 1 µs=0.000001 s.
译文：周期是信号完成一个周期所需要的时间。周期的单位是秒，但是周期往往非常短，所以常用毫秒（ms）和微秒（µs）来表示。1 ms =0.001 s，1 μs=0.000001 s。
milliseconds (ms)：n. 毫秒。
microseconds (µs)：micro-second n. 微秒。
5. 1 kHz=1000 Hz and 1 MHz=1000000 Hz.
译文：千赫（kHz）和兆赫（MHz）。
6. Frequency=1/time period, and time period=1/frequency.
译文：频率=1/周期，周期=1/频率。
time period：周期。
1.Write T (True) or F (False) beside the following statements about the text.
a. Direct Current (DC) flows one way, then the other way, continually reversing direction.
b. Alternating Current (AC) always flows in the same direction.
c. Amplitude is the minimum voltage reached by the signal.
d. Peak-peak voltage is twice the peak voltage (amplitude).
e. The effective value of AC is the value of alternating voltage or current that will have the same effect on a resistance as a comparable value of direct voltage or current will have on the same resistance.
2. Fill in the missing words according to the text.
a. An AC voltage is continually changing _____ between and _____.
b. Electronic circuits normally require a steady DC supply which is _____ one value.
c. Cells, batteries and regulated power supplies provide _____ which is ideal for electronic circuits.
d. In addition to the properties labeled on the graph, there is frequency which is the number of _____ per second.
e. When reading an oscilloscope trace, it is usual to measure _____ voltage.
3. Match the following terms to appropriate definition or expression.
