Unit 13 Alternating currentI. Lead in1. What’s an alternating current?2. What’s the most common wave in AC?3. In AC circuits, is the instantaneous power equal to the average power?4. Which portion of power flow is known as active power/reactive power?5. What is the relationship among real, reactive and apparent power?II. Word Study1. accomplish [ə'kʌmpliʃ, ə'kɔm-] vt. 完成；实现；达到1) accomplish one’s aim, a task 达到目的、完成任务2) A system is a group of components that work together to accomplish an objective. 系统就是为实现一个目标而共同工作的一组部件。

3) Set priorities. Accomplish the most important tasks first, and avoid physical overexertion.把握好轻重缓急。

首先完成最重要的任务，并防止身体过度疲劳2. analogous [ə'næləɡəs] adj. 类似的，相似的；可比拟的(to，with)【计算机】模拟的；【生物学】同功的；【自动化】模拟的1）The two processes are not analogous.这两种过程不相似。

2）The present crisis is analogous with the situation immediately before the war. 目前的危机与大战前夕的形势类似。

3. distinct [dis'tiŋkt] adj. 明显的；独特的；清楚的；有区别的1）The footprints are quite distinct; they must be fresh.足迹清晰易辨，一定是不久前留下来的。

2）I had the distinct impression that I was being watched.我很明显地感觉到有人在监视我。

3）The was a distinct sense of embarrassment in the air.周围的气氛中有一种明显的局促不安的感觉。

4. manifest ['mænifest] vt. 证明，表明；显示vi. 显示，出现n. 载货单，货单；旅客名单adj. 显然的，明显的；明白的1）manifest the truth of a statement 证明某说法属实2）manifest fear, hatred, etc 显示恐惧、憎恨等3）She manifested little interest in her studies. 她对学习显得没有什么兴趣。

5. distort [dis'tɔ: t] vt. 扭曲；使失真；曲解vi. 扭曲；变形1）a heap of distorted metal 一堆变了形的金属东西2）a face distorted by pain 因疼痛而扭曲的脸6. reversal [ri'və: səl] n. 逆转；[摄] 反转；[法] 撤销1）a dramatic reversal of her earlier decision 她的突然改变决定2）a reversal of the usual procedures, tendencies, opinions 常规的颠倒、趋势的扭转、意见的转变3)Much of the recent reversal in the global movement of capital , goods and jobs has been directly due to the financial crisis .近来全球资本、商品和就业趋势的逆转，很大程度上是金融危机的直接后果。

III．Language point s1. An alternative current (AC) is an electrical current whose magnitude and direction vary cyclically, as opposed to direct current，whose direction remains constant.译文：直流电流的流动方向是保持不变的，交流电与它不同，交流电是指电流的大小和方向都会作周期性地变化。

说明：as opposed to：与…对照；与…对比e.g.I am here on business as opposed to a holiday. 我在这里是办公事而不是度假。

“v ary” means “change” (改变，变动，变化) e.g.1) Our routine never varies. 我们的常规从无变化。

2) Prices vary with the seasons. 物价随季节而变动。

3) work with varying degrees of enthusiasm 工作积极性有所不同2. The sine wave is the most common wave in AC and sometimes we refer to sine AC as AC in short.译文：正弦波是交流电最常见的波形，有时我们把正弦交流电简称为交流电。

说明：in short 总之；简言之e.g.1) Things couldn’t be worse, financially: in short, they’re bankrupt. 在财务方面已经糟得不能再糟了：总之一句话，他们破产了。

2) In short, a confident company is a successful company.简言之，自信造就成功的公司。

3) It is, in short, a risky business.简言之，这是一项颇具风险的任务。

3. The angular frequency ω is related to the physical frequency f which represents the number of oscillations per second (unit: hertz), by the equation:ω=2译文：角频率与物理频率f即每秒振荡的次数(单位：赫兹)有关，有ω=2；说明：be related to与…有关e.g.1) It pays to be nice to your family. You’ll always be related to them. 对家人好是值得的, 你一辈子都离不开他们。

2) I think that financial success can be related to doing what you love, but it’s not always the case.我认为经济成功和自己喜爱做的事可以有关但也不一定。

3) One thing to remember is that your yearly goal should be related to your life mission. Otherwise, there is a missing link between the lifetime and yearly levels.要记住的一点是，你的年度目标应该和生活目的有关。

否则的话，终生层面和每年层面的连接就断开了。

4. The voltage and current are both sinusoids in AC circuits, however, the instantaneous power, which is still the product of voltage and current, changes with time and is not equal to the average power.译文：但在交流电路中，电压和电流都是正弦量，瞬时功率仍然是电压和电流的乘积，随时间变化而变化的，瞬时功率并不等于平均功率。

说明：be equal to等于e.g.1) The horizontal forces must be equal to each other. 水平力必须彼此平衡。

2) If fun and pleasure are equal to happiness, then pain must be equal to unhappiness. 如果乐趣和愉悦就等于幸福的话，那么痛苦就一定等于不幸。

3) Archimedes principle states that the volume of water displaced must be equal to the volume of the object. 阿基米德定律说明排开的水的体积必等于被放入物体的体积。

5. In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow.译文：在交流电路中，储存能量的元件如电感和电容(只能储存或释放能量)，引起能量流做周期性的往返流动。

说明：such as 像；诸如；例如e.g.1) Wild flowers such as orchids and primroses are becoming rare. 兰花和报春花之类的野花越来越少了。

2) Our discussions were all about issues such as education and climate change.我们讨论的都是像教育和气候变化这样的问题。

3) Indicators of financial stress outside housing, such as delinquencies on car loans and credit cards, are rising.房地产之外的财务压力指标正在上升，比如汽车贷款和信用卡债拖欠。