问题 选择题

用一金属窄片折成一矩形框架水平放置,框架右边上有一极小开口.匀强磁场的磁感应强度大小为B,方向垂直于纸面向里,如图所示,框架以速度v1向右匀速运动,一带电油滴质量为m,电荷量为q,以速度v2从右边开口处水平向左射入,若油滴恰能在框架内做匀速圆周运动,则(  )

A.油滴带正电,且逆时针做匀速圆周运动

B.油滴带负电,且逆时针做匀速圆周运动

C.圆周运动的半径一定等于

D.油滴做圆周运动的周期等于

答案

D

题目分析:由于油滴恰能在框架内做匀速圆周运动,所以液滴受的电场力和重力平衡,由右手定则可知,上下两金属片中的上板电势高,场强方向向下,而电场力方向向上,所以液滴带负电;再由左手定则可知,液滴进入场区后受洛伦兹力的方向向上,所以液滴做顺时针方向的匀速圆周运动。AB均错误;根据,而,,解得 选项C错误,D正确。

单项选择题
阅读理解

Did you get a flu shot this year? For the first time, the Centers for Disease Control and Prevention in the US is recommending that all children ages 6 months through 18 years receive the flu vaccine.

Vaccines battle diseases caused by bacteria and viruses. A weakened form of the germ is introduced into the body. The body makes special substances called antibodies to fight the germs. If the actual germs were to attack, the antibodies would fight them.

Because there are many kinds of flu viruses, scientists must create a new vaccine formula (配方) each year. Researchers must make a prediction.

“The flu vaccine is not as effective as the polio (小儿麻痹症) vaccine or the measles(麻疹) vaccine,” says Dr. William Schaffner of Vanderbilt University School of Medicine.

“It’s not a great vaccine, but it is quite a good vaccine.”

But a flu vaccine doesn’t work for everyone. There are certain people who should be careful about taking it. Because the vaccine is produced from eggs, those who are allergic (过敏的) to eggs should not take it. If you have a fever, you should wait until you recover.

Some people are afraid that they might get the flu from the flu shot. Scientist say that it is not possible, because the viruses in the flu shot are inactivated (灭活的). But some minor side effects such as low-grade fever and body aches could occur. If they do, they begin soon after the shot and usually last only one to two days.

60. What is an antibody?

A. An antibody is a vaccine formula

B. An antibody is a germ that is introduced into the body

C. An antibody is a vaccine that fights diseases.

D. An antibody is a special substance to fight germs.

61. Which of the following statements is NOT true?

A. Not everyone is suitable for the flu vaccine.

B. The flu vaccine is not so effective as the measles vaccine

C. Flu viruses create new vaccine formulas each year.

D. People can’t take the flu vaccine at anytime.

62. It can be inferred from the text that _____.

A. scientists should change the vaccine formula each year

B. vaccines can battle diseases caused by bacteria and viruses

C. all children ages 6 months through 18 years can receive the flu vaccine

D. scientists are responsible for the side effects of the flu vaccine

63. Which of the following would be the best title for the text?

A. The flu vaccine        B. Side effects of the flu vaccine

C. How to prevent the flu  D. Not everyone can get the flu shot