问题 计算题

(18分)如图所示,相距为d、板间电压为U的平行金属板M、N间有垂直纸面向里、磁感应强度为B0的匀强磁场;在POy区域内有垂直纸面向外、磁感应强度为B的匀强磁场;POx区域为无场区.一正离子沿平行于金属板、垂直磁场射入两板间并做匀速直线运动,从H(0,a)点垂直y轴进入第Ⅰ象限.

(1)求离子在平行金属板间的运动速度;

(2)若离子经OP上某点离开磁场,最后垂直x轴离开第Ⅰ象限,求离子在第Ⅰ象限磁场区域的运动时间;

(3)要使离子一定能打在x轴上,则离子的荷质比应满足什么条件?

答案

(1)(2)(3)

题目分析:(1)粒子在平行板内做匀速直线运动,有,又

解得离子在平行板内的速度为:

(2)如图为离子在第一象限内的运动轨迹图,由几何关系可得,轨迹半径为

轨迹对应的圆心角为 ;运动的周期为:

运动时间为:

(3)要使粒子一定能打到x轴上,离子在磁场中运动的最小半径为r2,由几何关系

解得:

 可得:

阅读理解

阅读理解。

     Popeye the Sailor first became a popular cartoon in the 1930s.The sailor in that cartoon ate lots of

spinach to make him strong. People watched him, and they began to buy and eat a lot more spinach.

Popeye helped sell 33 percent more spinach than before! Spinach became a necessary part of many

people's diets. Even some children who hated the taste began to eat the vegetable.

     Many people thought that the iron in spinach made Popeye strong, but this is not true. Spinach does

not have any more iron than any other green vegetable.

     People only thought spinach had a lot of iron because the people who studied the food made a

mistake. In the 1890s, a group of people studied what was inside vegetables. This group said that

spinach had ten times more iron than it did. The group wrote the number wrong, and everyone accepted

it.

     Today, we know that the little iron there is in spinach cannot make a difference in how strong a person

is.However, spinach does have something else which the body needs-folic acid.

     It is interesting to point out that folic acid can help make a person strong. Maybe it was really the folic

acid that made Popeye strong all along.    

1. A good title for this reading passage is______.

A. Popeye the Sailor

B. The Truth About Spinach

C. A Mistake with Numbers

D. Folic Acid Makes You Strong

2. Why did many people eat spinach after they saw Popeye the Sailor?

A. They thought spinach made them strong.

B. They thought Popeye was funny.

C. Spinach had a lot of iron.

D. People liked folic acid.

3. A research group told people that spinach______.

A. made Popeye strong

B. was a green vegetable

C. had less iron than other green vegetables

D. had more iron than other green vegetables

4. The reading passage says that perhaps Popeye got his strength from______.

A. iron

B. folic acid

C. spinach

D. exercise

5. Folic acid is ______.

A. something in food

B. a vegetable

C. dangerous

D. a certain kind of spinach

单项选择题