1. （2005天津卷）By the time Jane gets home，her aunt_____for London to attend a meeting.
A. will leave
B. leaves
C. will have left
D. left
［解析］答案为C。从gets home可知Jane还未回到家，等到Jane在将来的某个时间回到家以前，她的aunt将已经赴伦敦参加一个会议去了，故用将来完成时。
2. （2008四川卷）The telephone_____，but by the time I got indoors，it stopped.
A. had rung
B. was ringing
C. rings
D. has rung
［解析］答案为B。进门之前电话铃一直在响，但当我进门的时候，它停止了。but 前面分句的动作是先发生的，而且延续的时间较长，故用was ringing。
3. （2008山东卷）By the time he realizes he_____into a trap，it’ll be too la te for him to do anything about it.
A. walks
B. walked
C. has walked
D. had walked
［解析］答案为C。主句用了一般将来时，根据时态一致原则排除B和D。根据句意“截止到他意识到的时候，他已经走进了一个陷阱”可知，空档最好填现在完成时。
4. （2009北京卷）When I talked with my grandma on the phone，she sounded weak，but by the time we_____up，her voice had been full of life.
A. were hanging
B. had hung
C. hung
D. would hang
［解析］答案为C。当我与祖母通电话时，她听起来很虚弱，但是当我们挂断电话的时候，她的声音已经充满活力。by the time后的主句用了过去完成时，因此空档处最好用一般过去时。
5. （2011辽宁卷）By the time Jack returned home from England，his son_____from college.
A. graduated
B. has graduated
C. had been
D. had graduated
［解析］答案为D。截止到Jack从英国返家，他的儿子已经大学毕业了。回家用了一般过去时，大学毕业发生在过去的过去，故用过去完成时。
by the time意为“到……时候（为止）”，是一个介词短语，在句中经常起连词作用，引导时间状语从句。它的主要用法如下：
1. 如果从句中的谓语动词是一般过去时，那么主句中的谓语动词通常用过去完成时，表示“截止到从句动作发生时，主句动作已经完成，即过去的过去”。例如：By the time he was fourteen years old，Einstein had learned advanced mathematics all by himself. 到14岁的时候，爱因斯坦就已经自学完了高等数学。
By the time he returned home，the rain had stopped. 等他回到家时，雨已经停了。
2. 如果从句中的谓语动词为一般现在时（表示将来），那么主句中的谓语动词常为将来完成时，表示“截止到将来某一时间为止，主句动作将已经完成”。例如：
By the time John finishes his homework，his classmates shall have left to play football. 到约翰做完作业时，她的同学将已离开去伦踢足球了。
By the time you get back，I shall have finished the work. 到你回来时，我将已经把活做完了。
3. 如果主句动作不强调已经完成，只是说明过去某个时候的状况，此时主句不用完成时，而应根据实际需要选用适当的时态，此时by the time相当于when。例如：He was out of breath by the time he reached the top. 登上顶端时，他气喘吁吁。
By the time the bus came，so many people were waiting that it was a free fight to get on it. 公共汽车来的时候，等车的人很多，成了一场挤车的混战。
［能力拓展训练］
1. By the time you arrive home，I_____，so please don’t make any noise when you come in.
A. shall have been sleeping
B. shall have slept
C. shall sleep
D. shall be sleeping
2. By the time you arrive in London，we_____in Europe for two weeks.
A. shall stay
B. have stayed
C. will have stayed
D. have been staying
3. Hurry up！By the time we get to the church，the marriage service_____.
A. had started
B. will have started
C. has started
D. must have started
4. ―Did you see Xiao Li at the party？
―No，_____by the time I arrived
A. she’d left
B. she’s left
C. she was left
D. she must leave
5. By the time I saw the angry expression on his face，I_____what I had to face.
A. knew
B. have known
C. would know
D. had known
Key：1-5 DCBAA
It 的用法总结 一. 代词: 1)指代上文 2)指代this/that 3)指代未知性别的婴儿或孩子或身份不明的人 4）指代时间/地点/天气/温度/距离/环境 What does “it” refer to? 1.It gets dark very early in the winter. 2.What will you call it if it’s a boy? 3.I love spring---It’s a wonderful time of the year. 4.It’s less than 100 kilometres from here to Jinan. 5.When the factory closes, it will mean 500 people losing their jobs. 6.What’s this? It’s a cat. 7.It has snowed much this year. 注意：it, one和that, those作替代词的区别 (1)The book in your bag is very interesting. Can I borrow ? (2) The book in your bag is more interesting than ______ on the desk. (3) The weather here is much colder than____ in Nanjing. (4) The books are free. You can take____ free of charge. (5) The books in the bag are better than _____ on the desk. it 表示特指，与前面指代的名词是同一个 one 表示泛指，指代单数可数名词，与指代的名词同类但不同一个 that表示特指，相当于the+名词单数，与指代的名词同类但不同一个those表示特指,相当于the+复数可数名词, 与指代的名词同类但不同一个
感官动词和使役动词 默认分类2010-05-28 23:14:26 阅读46 评论0 字号：大中小订阅 使役动词，比如let make have就是3个比较重要的 have sb to do 没有这个用法的 只有have sb doing.听凭某人做某事 have sb do 让某人做某事 have sth done 让某事被完成（就是让别人做） 另外： 使役动词 1.使役动词是表示使、令、让、帮、叫等意义的不完全及物动词,主要有make(使,令), let(让), help(帮助), have(叫)等。 2.使役动词后接受词,再接原形不定词作受词补语。 He made me laugh. 他使我发笑。 I let him go. 我让他走开。 I helped him repair the car. 我帮他修理汽车。 Please have him come here. 请叫他到这里来。 3.使役动词还可以接过去分词作受词补语。 I have my hair cut every month. 我每个月理发。 4.使役动词的被动语态的受词补语用不定词,不用原形不定词。 (主)He made me laugh. 他使我笑了。 (被)I was made to laugh by him. 我被他逗笑了。 使役动词有以下用法： a. have somebody do sth让某人去做某事 ??i had him arrange for a car. b. have somebody doing sth.让某人持续做某事。 ??he had us laughing all through lunch. 注意：用于否定名时，表示“允许” i won't have you running around in the house. 我不允许你在家里到处乱跑。 ******** 小议“使役动词”的用法 1. have sb do 让某人干某事 e.g:What would you have me do? have sb/sth doing 让某人或某事处于某种状态，听任 e.g: I won't have women working in our company. The two cheats had the light burning all night long. have sth done 让别人干某事，遭受到 e.g:you 'd better have your teeth pulled out. He had his pocket picked. notes: "done"这个动作不是主语发出来的。 2.make sb do sth 让某人干某事 e.g:They made me repeat the story. What makes the grass grow?
南昌大学物理实验报告 课程名称：_____________ 大学物理实验 实验名称：_______________ 惠斯通电桥 学院：___________ 专业班级： 学生姓名：_________ 学号： 实验地点：___________ 座位号： 实验时间：第11周星期4上午10点开始
、实验目的: 1. 掌握电桥测电阻的原理和方法 2. 了解减小测电阻误差的一般方法 、实验原理: (1) 惠斯通电桥原理 惠斯通电桥就是一种直流单臂电桥，适用于测中值电阻，其原理电路如图 7-4所示。若调节电阻到合适阻值时, 可使检流计 G 中无电流流过，即 B 、D 两点的电位相等，这时称为“电桥平衡”。电桥平衡，检流计中无电流通过， 相当于无BD 这一支路，故电源 E 与电阻R ,、R x 可看成一分压电路；电源和电阻 R 1 上面两式可得 R 2 桥达到平衡。故常将 R 、R 2所在桥臂叫做比例 臂,与R x 、R S 相应的桥臂分别叫做测量臂和比 较臂。 V B C 点为参考，贝y D 点的电位V D 与B 点的电位V B 分别为 R 2 R S R S V D R X 因电桥平V B V D 故解 R 2、R S 可看成另一分压电路。若以 R x 为 E 待测电阻，则有 R>< R X R S 上式叫做电桥的平衡条件，它说明电桥平衡时，四个臂的阻值间成比例关系。如果 1 10,10 1等)并固定不变，然后调节 金使电
(2)电桥的灵敏度
n R S R S 灵敏度S 越大，对电桥平衡的判断就越容易，测量结果也越准确。 此时R s 变为R s ，则有：R x R2 R s ，由上两式得R x . R s R s 三、 实验仪器： 线式电桥板、电阻箱、滑线变阻器、检流计、箱式惠斯通电桥、待测电阻、低压直流电源 四、 实验内容和步骤： 1. 将箱式电桥打开平放，调节检流计指零 2. 根据待测电阻(线式电桥测量值或标称值)的大小和 R 3值取满四位有效数字原则，确定比例臂的取值，例如 R 为数千欧的电阻，为保证 4位有效数字，K r 取 3. 调节F 3的值与R ＜的估计 S _____ S 的表达式 R S R S S-i S 2 _____________________ ES R i R 2 R s R x 1 R E % R i R 2R X Rg 2 R x R s R 2 R - R E 2 R R s R x (3) 电桥的测量误差 电桥的测量误差其来源主要有两方面，一是标准量具引入的误差, 二是电桥灵敏度引入的误差。为减少误差传递, 可采用交换法。 交换法：在测定R x 之后，保持比例臂 R -、R 2不变，将比较臂 R s 与测量臂R x 的位置对换，再调节 R s 使电桥平衡，设 电桥的灵敏程度定义: R i
it的用法练习题（一） 1.It took us over an hour _____________along the street. A. walk B. to walk C. walking D. walked 2.I think it a great honor ________to visit your country. A. to invite B. inviting C. having invited D. to be invited 3.Many people now make_____________ a rule to buy cards for their friends before Christmas. A. themselves B. it C. that D. this 4._____is very clear to everyone that he's round and tall like a tree. A. This B. What C. That D. It 5.In the United States, bus travel doesn't cost much as train travel,_____________? A. don't they B. does it C. do they D. doesn't it 6.Someone is at the door, who is_____________? A. this B. that C .it D. he 7.—It is raining cats and dogs. —_____________ . A. So it is B. So is it C. Neither it is D. Neither is it 8.—My home is in that tall building over there. —_______________? A. Can it see B. Can see it C. Can be seen it D. Can it be seen 9. _____________raining hard for 3 hours without stopping. A. It is B. It was C. It has been D. It had been 10.—Has the boy got his bicycle now? —Yes, the police gave_____________. A. him to him B. it to it C. it to him D. him to it 11.—Boy, —It is, looks like spring is coming soon.
初中阶段主要句型的用法 1. It's time to (for) .... 表“时间到了；该干…的时间了”之意。 It's time to go home. It's time for school. 注意：to的后面接动词短语，而for的后面接名词。 2. It's bad (good) for ... 表示“对…有害（有益）的”含义。 Please don't smoke. It's bad for your health. Please take more exercise. It's good for your health. 3. be late for (school) 是“上学（迟到）”之意。for后面还可以接meeting或class。He was late for school this morning. Don't be late for class, please. 4. had better do / not do sth. 表示“最好做/不做某事”之意。 You had better put on the coat when you go out. It's cold outside. We had better stop to have a rest. 注意：用had better时，后面一定要直接跟动词原形，决不能加to do sth. 否定一定用在第二个动词之前，是动词不定式的否定式。 5. be (feel) afraid of ... 表示“恐怕”，“害怕…”之意。 He is afraid of snakes. Mary feels afraid of going out alone. 6. enjoy doing sth. 是“喜欢（爱好）做某事”之意，doing是动名词作动词enjoy 的宾语。enjoy有欣赏之意。 Are you enjoy living in Beijing? Do you enjoy listening to music? 7. stop doing sth. 停止做某事 stop to do sth. 停下来做某事 It's time for class. Stop talking, please. When she saw me, she stopped to talk with me. 8. Let (make) sb. do 让（使）某人做某事。 Let's go to school. Father made his son clean the room again. 注意：当make sb. do sth. 用于被动语态时，第二个动词前面就一定要加to 了。如： His son was made to clean the room again. he was made to work twelve hours a day in the past. 9. like to do / like doing sth. 是“喜欢做某事”之意。 like to do sth. 是表示比较具体的喜欢做某事；like doing sth. 是表示宠统的喜欢做某事。如： I like to swim in the swimming pool. 我喜欢在游泳池中游泳。（喜欢的具体的地方游泳） I like swimming. 我喜欢游泳（只讲喜欢这项运动） 10. ask (tell) sb. to do. (not to do) sth. 请（让）某人（不）做某事 Jim asked his mother to help him with his lessons. When did you tell him not to shout loudly?
感官动词 1.see, hear, listen to, watch, notice等词，后接宾语，再接省略to的动词不定式或ing形式。前者表全过程，后者表正在进行。句中有频率词时，以上的词也常跟动词原形。 注释:省略to的动词不定式--to do是动词不定式，省略了to,剩下do，其形式和动词原形是一样的，但说法不同。 see sb do sth 看到某人做了某事 see sb doing sth 看到某人在做某事 hear sb do sth 听到某人做了某事 hear sb doing sth 听到某人在做某事 以此类推... I heard someone knocking at the door when I fell asleep. (我入睡时有人正敲门,强调当时正在敲门) I heard someone knock at the door three times. (听到有人敲门的全过程) I often watch my classmates play volleyball after school. (此处有频率词often) （了解）若以上词用于被动语态，须将省略的to还原： see sb do sth----sb be seen to do sth hear sb do sth----sb be seen to do sth 以此类推... We saw him go into the restaurant. → He was seen to go into the restaurant. I hear the boy cry every day. → The boy is heard to cry every day. 2.感官动词look, sound, smell, taste, feel可当系动词，后接形容词。 He looks angry. His explanation sounds reasonable. The cakes smell nice.
期末复习提纲（十） Un it 10 By the time I got outside, the bus had already left. 重点词组与句型： 1. take a shower 2.get in the shower 3.get up 4.get outside 5.get to school 6.1 eave sth.+ 地点 7.get back to sp. 8.start teach ing 9.wake up late 10.be late for 11.go off 12.wait for https://www.doczj.com/doc/a315551506.html,e out 14.have some breakfast 15.run off 16.get there 17.on time https://www.doczj.com/doc/a315551506.html,e by 19.give sb. a ride 20.bri ng sth. to sp. 21.break dow n 22.show up 23. get dressed 24.stay up all night 25. invite sb. to do sth. / invite sb. to sp. 26. by the time 27.happa n to sb. 28. a costume party 29.thousa nds of 30.h un dreds of * (31.flee from sp. 32. April Fool ' s DOyB.stop doing sth. 34.as …as one can 35. fool sb. 36.ask sb. to do sth. 37.both…and 38.an hour later 39. get married 40.play a joke on sb) 40. By the time I got up, my brother had already gotte n in the shower. 41. By the time she got up, her brother had already gone into the bathroom. 42. By the time she went outside, the bus had already gone. 43. By the time she got to class, the teacher had already started teach ing. 44. Whe n she got to school, she realized she had left her backpack at home. 45. By the time I got back to school, the bell had rung. 46. My alarm clock didn ' t go off. 47.1 took a quick shower, and had some breakfast, and the n ran off to the bus stop. * (48.Have you ever forgotte n to bring your homework to school? 49.Wells was so convincing that hun dreds of people believed the story. 50.She was thrilled, because she wan ted to get married. 语法： 过去完成时。 练习： 一、根据句子意思和所给首字母提示，用适当单词完成句子。 1. Bad luck. I missed the early bus because I o ______ this morni ng. 2. Perhaps he ' ll dress himself up as a clown in the c _________ party. 3. You must be e ________ a fter swimmi ng for along time. Come and have a rest.
肇 庆 学 院 肇 庆 学 院 电子信息与机电工程 学院 普通物理实验 课 实验报告 级 班 组 实验合作者 实验日期 姓名: 学号 老师评定 实验题目： 惠斯通电桥测电阻 实验目的： 1．了解电桥测电阻的原理和特点。 2．学会用自组电桥和箱式电桥测电阻的方法。 3．测出若干个未知电阻的阻值。 1．桥式电路的基本结构。 电桥的构成包括四个桥臂（比例臂R 2和R 3，比较臂R 4，待测臂R x ），“桥”——平衡指示器（检流计）G 和工作电源E 。在自组电桥线路中还联接有电桥灵敏度调节器R G （滑线变阻器）。 2．电桥平衡的条件。 惠斯通电桥(如图1所示)由四个“桥臂”电阻（R 2、R 3、R 4、和R x ）、一个“桥”（b 、d 间所接的灵敏电流计）和一个电源E 组成。b 、d 间接有灵敏电流计G 。当b 、d 两点电位相等时，灵敏电流计G 中无电流流过，指针不偏转，此时电桥平衡。所以，电桥平衡的条件是：b 、d 两点电位相等。此时有 U ab =U ad ，U bc =U dc ， 由于平衡时0=g I ，所以b 、d 间相当于断路，故有 I 4=I 3 I x =I 2 所以 44R I R I x x = 2233R I R I = 可得 x R R R R 324= 或 43 2R R R R x = 一般把 K R R =3 2称为“倍率”或“比率”，于是 R x =KR 4 要使电桥平衡，一般固定比率K ，调节R 4使电桥达到平衡。 3．自组电桥不等臂误差的消除。 实验中自组电桥的比例臂（R 2和R 3）电阻并非标准电阻，存在较大误差。当取K=1时，实际上R 2与R 3不完全相等，存在较大的不等臂误差，为消除该系统误差，实验可采用交换测量法进行。先按原线路进行测量得到一个R 4值，然后将R 2与R 3的位置互相交换（也可将R x 与R 4的位置交换），按同样方法再测 一次得到一个R ’ 4值，两次测量，电桥平衡后分别有： 43 2R R R R x ?= ' 42 3R R R R x ?= 联立两式得： ' 44R R R x ?= 由上式可知：交换测量后得到的测量值与比例臂阻值无关。 4．电桥灵敏度 电桥灵敏度就是电桥偏离平衡状态时，电桥本身的灵敏感反映程度。在实际测量中，为了便于灵敏度 I 2 I x c
英语中感官动词的用法 一、感官动词 1、感官动词(及物动词)有：see/notice/look at/watch/observe/listen to/hear/feel(Vt)/taste(Vt)/smell(Vt) 2、连缀动词(含感官不及物动词) be/get/become/feel/look/sound/smell/taste/keep/stay/seem/ appear/grow/turn/prove/remain/go/run 二、具体用法： 1、see, hear, smell, taste, feel,这五个动词均可作连系动词，后面接形容词作表语，说明主语所处的状态。其意思分别为"看/听/闻/尝/摸起来……"。除look之外，其它几个动词的主语往往是物，而不是人。 例如：These flowers smell very sweet.这些花闻起来很香。 The tomatoes feel very soft.这些西红柿摸起来很软。 2、这些动词后面也可接介词like短语，like后面常用名词。 例如：Her idea sounds like fun.她的主意听起来很有趣。 3、这五个感官动词也可作实义动词,除look(当"看起来……"讲时)只能作不及物动词外,其余四个既可作及物动词也可作不及物动词,此时作为实义动词讲时其主语一般为人。 例如:She smelt the meat.她闻了闻那块肉。 I felt in my pocket for cigarettes.我用手在口袋里摸香烟。 4、taste, smell作不及物动词时,可用于"t aste / smell + of +名词"结构,意为"有……味道/气味"。 例如:The air in the room smells of earth.房间里的空气有股泥土味。 5、它们(sound除外)可以直接作名词,与have或take构成短语。 例如：May I have a taste of the mooncakes?我可以尝一口这月饼吗？taste有品位、味道的意思。 例如：I don’t like the taste of the garlic.我不喜欢大蒜的味道。 She dresses in poor taste.她穿着没有品位。 look有外观，特色的意思，例：The place has a European look.此地具有欧洲特色。 feel有感觉，感受的意思，watch有手表，观察的意思。例：My watch is expensive.我的手表很贵。 6、其中look, sound, feel还能构成"look / sound / feel + as if +从句"结构，意为"看起来/听起来/感觉好像……"。 例如：It looks as if our class is going to win.看来我们班好像要获胜了。 7、感官动词+do与+doing的区别： see, watch, observe, notice, look at, hear, listen to, smell, taste, feel + do表示动作的完整性，真实性；+doing 表示动作的连续性，进行性。 I saw him work in the garden yesterday.昨天我看见他在花园里干活了。(强调"我看见了"
大学物理实验教程预习思考题、分析题参考答案
大学物理实验教程预习思考题、分析题参考答案 分享 作者：雪已被分享2次评论(0)复制链接分享转载举报 为节省大家时间，特从网上搜相关答案供大家参考！（按咱做实验顺序） ２.用模拟法测绘静电场 【预习思考题】 1．用电流场模拟静电场的理论依据是什么？模拟的条件是什么？用电流场模拟静电场的理论依据是：对稳恒场而言，微分方程及边界条件唯一地决定了场的结构或分布，若两种场满足相同的微分方程及边界条件，则它们的结构也必然相同，静电场与模拟区域内的稳恒电流场具有形式相同的微分方程，只要使他们满足形式相同的边界条件，则两者必定有相同的场结构。模拟的条件是：稳恒电流场中的电极形状应与被模拟的静电场中的带电体几何形状相同；稳恒电流场中的导电介质是不良导体且电导率分布均匀，并满足σ极>>σ介以保证电流场中的电极（良导体）的表面也近似是一个等势面；模拟所用电极系统与被模拟电极系统的边界条件相同。
2．等势线和电场线之间有何关系？ 等势线和电场线处处相互垂直。 3．在测绘电场时，导电微晶边界处的电流是如何流动的？此处的电场线和等势线与边界有什么关系？它们对被测绘的电场有什么影响？ 在测绘电场时，导电微晶边界处的电流为0。此处的电场线垂直于边界，而等势线平行于边界。这导致被测绘的电场在近边界处受边界形状影响产生变形，不能表现出电场在无限空间中的分布特性。 【分析讨论题】 1．如果电源电压增大一倍，等势线和电场线的形状是否发生变化？电场强度和电势分布是否发生变化？为什么？ 如果电源电压增大一倍，等势线和电场线的形状没有发生变化，但电场强度增强，电势的分布更为密集。因为边界条件和导电介质都没有变化，所以电场的空间分布形状就不会变化，等势线和电场线的形状也就不会发生变化，但两电极间的电势差增大，等势线的分布就更为密集，相应的电场强度就会增加。 2．在测绘长直同轴圆柱面的电场时，什么因素会使等势线偏离圆形？
1.感官动词用法之一：see, hear, listen to, watch, notice等词，后接宾语，再接动词原形或ing形式。前者表全过程，后者表正在进行。句中有频率词时，以上的词也常跟动词原形。 I heard someone knocking at the door when I fell asleep. (我入睡时有人正敲门) I heard someone knock at the door three times. (听的是全过程) I often watch my classmates play volleyball after school.(此处有频率词often) 若以上词用于被动语态，后面原有动词原形改为带to不定式： We saw him go into the restaurant. →He was seen to go into the restaurant. I hear the boy cry every day. →The boy is heard to cry every day. 2.感官动词用法之二：look, sound, smell, taste, feel可当系动词，后接形容词： He looks angry. It sounds good. The flowers smell beautiful. The sweets taste sweet. The silk feels soft. I felt tired. They all looked tired. 这些动词都不用于被动语态。如：The sweets are tasted sweet.是个病句。注意：如果加介词like,则后不可接形容词，而接名词或代词：
实用英语10000 句！ Dinner is on me. 晚饭我请。 You ask for it! 活该！ You don't say! 真想不到！ Get out of here! 滚出去！ How come… 怎么回事，怎么搞的。 Don't mention it. 没关系，别客气。 It is not a big deal! 没什么了不起！ thousand times no! 绝对办不到！ Who knows! 天晓得！ Have a good of it.玩的很高兴。 Don't let me down. 别让我失望。 It is urgent. 有急事。 Can I have this. 可以给我这个吗？ It doesn't take much of you time. 这不花你好多时间。 Drop it! 停止！Bottle it! 闭嘴！ There is nobody by that name working here.这里没有这个人。Easy does it. 慢慢来。 Don't push me. 别逼我。 Come on! 快点，振作起来！ What is the fuss？吵什么？ Still up？还没睡呀？ It doesn't make any differences. 没关系。 It is a deal！一言为定！ Take a seat! 请坐！
Here ye! 说得对！ It can be a killer. 这是个伤脑筋的问题。 Don't take ill of me. 别生我气。 It's up in the air. 尚未确定。 I am all ears. 我洗耳恭听。 Right over there. 就在那里。 Get an eyeful. 看个够。 Here we are! 我们到了！ I lost my way. 我迷路了 Say hello to everybody for me. 替我向大家问好。Not precisely! 不见得，不一定！ That is unfair. 这不公平！ We have no way out. 我们没办法。 That is great! 太棒了！ You are welcome! 别客气！ I'm bored to death. 我无聊死了。 Bottoms up! 干杯！ Big mouth! 多嘴驴！ Can－do. 能人。 Don't play possum! 别装蒜！ He neither drinks nor smokes. 他既不喝酒也不抽烟。 Make it up! 不记前嫌！ Watch your mouth. 注意言辞。 Any urgent thing？有急事吗？
A B C D G R 1 R 2 R s R x E I 1 I 2 3.8 惠斯通电桥 【实验简介】 惠斯通电桥是一种精确测量电阻的装置。 电桥法测电阻的实质是将被测电阻与已知电阻相比较，从而确定待测电阻的阻值。制造高精度的电阻比较容易，因此电桥法测电阻容易获得较高的测量精度。 直流电桥主要分为单臂电桥（又称惠斯通电桥）和双臂电桥（又称开尔文电桥）。 单臂电桥用于测量中值电阻（10Ω－106 Ω），双臂电桥用于测量1Ω以下的低值电阻。 【实验目的】 1.了解惠斯通电桥测电阻的原理，掌握用惠斯通电桥测电阻的方法。 2.了解电桥的灵敏度，学习合理选择实验条件，减小系统误差。 【预习思考题】 1. 调电桥平衡的技巧是什么？ 2. 使用检流计有哪些注意事项 3. 调节电桥平衡的过程中，如何使用保护开关K 和滑线变阻器r ？ 4. 惠斯通电桥测电阻系统误差产生的原因主要有哪三种？ 【实验仪器】 检流计、电阻箱、干电池、滑线变阻器、保护开关、待测电阻、导线、开关。 【实验原理】 1. 电桥的基本原理和平衡条件 惠斯通电桥的基本电路如图 3.8.1所示。电阻1R 、2R 、s R 、x R 组成电桥的四个桥臂，C 、D 两点间接有检流计将两点的电位直接进行比较，当两点电位相等时，检流计G 中无电流通过，我们称电桥达到平衡。这时有 x s R I R I R I R I 212211,== 由上两式得出电桥的平衡条件： 平衡时，若已知1R 、2R 、s R 则可求得： （3.8.1） 用惠斯通电桥测电阻x R x 时，首先要调电桥的平衡。本实验中1R 、2R 、s R 均用电阻箱 x s R R R R = 21s x R R R R 1 2= 图3.8.1
一. 基本用法 1. 用作人称代词，代替上文提到过的事物，或事物（同名同物）。The train has arrived. It arrived half an hour ago. I have a car, so you can borrow it. one 指代单数可数名词，同名异物。He has an MP4, but I will buy one tomorrow. that特指单数可数名词或不可数名词。The population of China is larger than that of Japan. The weather of Chongqing is hotter than that of Beijing. 2. 用来代替提示代词this, that。—What’s this?—It’s a knife. —Whose watch is that?—It’s mine. 3. 指代不明性别的人。—Who is knocking at the door? —It’s me. It is a baby. 4. 指环境情况、时间、日期、天气、温度、季节、距离、价格、速度等。It was very noisy at the very moment. —What time is it? —It’s eight o’clock. It often rains in summer here. It is a long way to the school. It is 29 ℃ today. It is five kilometers away. 5. 作形式主语。It is not easy to finish the work in two days. It is no use helping him solve the problem. It is a pity that you didn’t read the book. 6. 作形式宾语。I think it no use arguing with him. I found it very interesting to study English. He made it clear that he was not interested in that subject. 7. 用于强调结构。①It is/was + 被强调部分+ that/who + 原句剩余部分It was Xiaoming whom(that)I met in the street last night. It was in the street that I met Xiaoming last night. It ws last night that I met Xiaoming in the street. It was I who met Xiaoming in the street last night. Was it in the street that you met her? ②特殊疑问词+is/was+it+that+原句剩余部分Who was it that broke the window? What is it that you want me to do? Where was it that he bought the bike?③对until进行强调要把not放在前面。It was not until yesterday that I met my old friend.It was not until he told me that I knew it.④对as,since引导的原因状语从句，要把as,since变成because.It was becaude he got up late that he missed the first bus.⑤判断是否是强调句的方法：It is/was…that/who去掉，看句子是否完整，完整的是强调句。It is there that accidents often happen. It is clear that not all the boys like football. ⑥注意强调句与主语从句、定语从句、状语从句的区别It is a good idea that occurs to me suddenly（强调句）. It is a good idea that we play basketbll after school(主语从句). It is in a small factory that my brother is going to work（强调句）. It is a small factory where my brother is going to work.（定语从句）It wa s five o’clock when the plane landed at the airport(状语从句). It was at five o’clock that the plane landed at the airport（强调句）. 二. 含有it的句型 1.It is time (for sb.) to do sth. It is high/about time that sb. did/should do sth.(虚拟语气) It is time for us to have a rest. It is high time that we took/should take action. It’s about time that we went to the airport. 2. It is + adj. +of / for sb. + to do sth. 形容词为brave, clever, careful, hopeless, kind, good, naughty, nice, silly,stupid,foolish, wise等对人的品质或性格进行评价的词, 一般用 of. It is very kind of you to help me. 3. It is + adj. + that + sb. + (should) do sth.（虚拟语气）, 能用于这个句型的形容词有strange, wonderful, natural, good、proper, right, wrong, well, fortunate, important, necessary, useless, likely, probable, impossible等, 可省去should而直接用动词原形。如：It is important that we should pay close attention to grain. It is natural that he（should）say so. 4. It is no use/ good + doing sth. It is no use trying again. It is no good arguing with your leaders. 5. It is said/reported/announced/ (well) known … that… It is said that he does well in English. 6. It is/was +一段时间+ since … It is three years since I met him in Beijing. It is a long time since I lived in that small mountain village with these farmers.It is five months since I arrived in New York. It is five months since I was in New York. 7.It will be+一段时间+ before …主句的时态可用一般将来时或一般过去时。It will be several years before we meet again. It was not long before they set out for the front. 8. It is /was/will be the first/second/third … time that … It is the first ti me that I have been here. It was the second time that he had seen the film. 9.It is up to sb. to do sth … 应由某人做某事。It is up to you to decide whether we start or not. 10. sb. hate/like/dislike/love/enjoy/resent/appreciate/don’t mind/be fond of/feel like/see to等+it+宾语从句。 I hate it that you can swim so well. I enjoy it when you help someone who is in trouble. 11. It takes sb. some time/ some money to do sth. It took him five years to draw a horse well. 12. 主语+vt.+it+n./adj.+to do sth. I think it hard for you to do the task on you own. 13. 主语+vt.+it+n./adj.+宾语从句。I think it important that you should attend the meeting.