题干

如图为电动机和发电机模型,其中图____ (选题“甲”或“乙”)为电动机模型,其工作原理是____ ;电动机工作时,能够使线圈平稳,连续不停地转动下去是靠____ 实现的.

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答案(点此获取答案解析)

甲,通电线圈在磁场中受力转动,换向器

同类题2

背景材料:北京时间2015年10月5日下午,2015年诺贝尔生理学或医学奖揭晓,中国药学家屠呦呦获奖。这是中国科学家因为在中国本土进行的科学研究而首次获诺贝尔科学奖,是中国医学界迄今为止获得的最高奖项,也是中医药成果获得的最高奖项。

    1969年中国中医研究院接受抗疟药研究任务,屠呦呦成为课题攻关组组长,开始了征服疟疾的艰难历程。

    当时,青蒿素的提取是一个世界公认的难题。屠呦呦和她的小组搜集了包括青蒿在内的600多种可能对疟疾治疗有效果的中药药方,对其中200多种中草药380多种提取物进行筛查,用老鼠做试验,也没有发现有效结果。屠呦呦和她的同事们熬过了无数个不眠之夜,体会了无数次挫折。

    “后来,我想到可能是因为我在加热的过程中,破坏了青蒿里面的有效成分,于是改为用乙醚提取。我们把青蒿买来先泡,然后把叶子包起来用乙醚泡,直到第191次试验,我们才真正发现了有效成分。经过试验,用乙醚提取的成分,对鼠疟、猴疟的抑制率达到了100%。”屠呦呦说。

    “那时候,她脑子里只有青蒿素,整天不着家,没白天没黑夜地在实验室泡着……”她的老伴儿说。

    后来,屠呦呦和她的同事在青蒿中提取到了一种无色结晶体,一种熔点为156℃~157℃的活性成分,他们将这种无色的结晶体物质命名为青蒿素。

同类题4

阅读下面短文,从短文后各题所给四个选项(A、B、C和D)中,选出可以填入空白处的最佳选项。

    You want to run across the street to catch the bus which is leaving soon? But wait! You'd better not. If a policeman sees you, you'll have to pay a fine (罚款). New traffic laws (交通法) say that if people cross the street when the light is red, they can be fined as much as 50 yuan.

    Traffic accidents killed more than 104,000 people in China last year. Chinese cities have more cars than ever. Drivers and pedestrians (行人) must work together to make the streets safer.

    The law has new rules for drivers and pedestrians. Drivers have to slow down when they are close to crosswalks. If people are in a crosswalk, cars must stop to let them pass.

    There are rules for bus drivers, too. If bus drivers smoke, drink or make phone calls while driving, they can be fined. Buses that carry too many people are also against the law.

    Pedestrians will have to walk more safely under the new law. They must cross streets at crosswalks. Also, they shouldn't climb over the fences (栅栏) along streets.

    Do you bike to school? It's not a good idea to carry classmates on the back. You could be fined 50 yuan. And stay in the bike lane (慢车道) when you're riding. The big roads are for cars and buses. Do you like to ride your uncle's motorbike? When you put on your helmet (头盔), ask him to put one on, too. People on motorbikes must wear helmets, the new law says. If they don't, they'll have to pay 200 yuan. Do you ride in cars often? Don't forget to put on your seat belt (安全带), even if you're going for a short taxi ride. It could save your life.

    If you see a hit-and-run (肇事逃逸), tell the police. They may give you a reward (奖赏). And don't be afraid to help people to the hospital if they are hurt in an accident. Don't worry about money. The new law says that doctors must take care of them even if they can't pay right away.