题干

粒子回旋加速器的工作原理如图所示,置于真空中的D形金属盒的半径为R,两金属盒间的狭缝很小,磁感应强度为B的匀强磁场与金属盒盒面垂直,高频交流电的频率为f,加速电压为U,若中心粒子源处产生的质子质量为m,电荷量为+e,在加速器中被加速.不考虑相对论效应,则下列说法正确是(   )

A:不改变磁感应强度B和交流电的频率f,该加速器也可加速α粒子

B:加速的粒子获得的最大动能随加速电压U的增大而增大

C:质子被加速后的最大速度不能超过2πRf

D:质子第二次和第一次经过D形盒间狭缝后轨道半径之比为 2 :l

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C,D

同类题3

完型填空。

    Alicia was a young woman who liked to exercise for her health. In fact, she walked five kilometers before 1 every morning, and went swimming once a week at the swimming pool. She didn't smoke and never2. She didn't eat chocolate. She didn't eat sweet and fatty food,3. She weighted herself every day.

    One day Alicia was on her daily walk when she saw a4sitting in a rocking chair under a tree. He looked very old and his hair was white. He looked thin and weak, and his hands were shaking,5 he looked very happy.

    He smiled at her and said, “Good morning! Lovely day, isn't it?”

    He had a wide smile on his6, and his eyes shone with happiness. But Alicia saw that he did not have teeth.

    “Good morning!” replied Alicia. “Yes, it is a lovely day.”

    Alicia thought he7very old and clever. She thought he must be at least 90 years old! She decided to ask him about the8of a happy old age.

    “I hope you don't mind (介意) me 9,” she said, “but what is your secret for being so happy at your age? I hope I can look as happy as you do10I am your age.”

    The man in the rocking chair said, “My secret for11? I smoke twenty packets of cigarettes every week, and drink three bottles of wine every day. I eat hamburgers and chocolates whenever I want. I never eat vegetables. I never walk anywhere and I never play sports. I sit at home every day.”

    Alicia was12. She didn't expect the man to give her an answer like that. She was surprised at how the man got to be so old when he did everything wrong. She thought he should be13and unhappy. Perhaps she was wrong. Maybe, she thought, people could live a long happy life14eating well or doing lots of exercise.

    “How 15 are you?” she asked.

    “It's my birthday today,” said the man. “I'm forty-seven!”