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在一个显像管里,电子枪释放出电子,从静止开始经电势差为U0的电场加速之后,电子沿水平方向从南到北运动.该处地磁场在竖直方向上的分量向下,磁感应强度大小为 B,已知电子的电荷量为 e,质量为 m,重力不计.试求:

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    Many people who work in London prefer to live outside it, and to go in to their offices or schools every day by train, car or bus, even though this means they have to get up early in the morning and reach home late in the evening. 
    One advantage of living outside London is that houses are cheaper. Even a small flat in London without a garden costs quite a lot to rent. With the same money, one can get a little house in the country with a garden of one's own.
    Then, in the country one can really get away from the noise and hurry of busy working lives. Even though one has to get up earlier and spend more time in trains or buses, one can sleep better at night and during weekends and on summer evenings, one can enjoy the fresh, clean air of the country. If one likes gardens, one can spend one's free time digging, planting, watering and doing the hundred and one other jobs which are needed in a garden. Then, when the flowers and vegetables come up, one has got the reward together with those who have shared the secret of nature.
    Some people, however, take no interest in country things: for them, happiness lies in the town, with its cinemas and theatres, beautiful shops and busy streets, dance-halls and restaurants. Such people would feel that their life was not worth living if they had to live it outside London. An occasional (偶尔的) walk in one of the parks and a fortnight's (two weeks) visit to the sea every summer is all the country they want: the rest of the country they are quite prepared to spend with those who are glad to get away from London every night.