问题 选择题

如图所示,在竖直平面一圆形区域内存在着磁感应强度为B的匀强磁场,B的方向垂直纸面,O点是圆形区域的圆心.一带电粒子(不计重力)从A点沿AO方向入射,速度为v0,偏转60°之后从B点出射.现把圆形区域的磁场改为竖直方向的匀强电场E,使带电粒子仍以原速度沿AO方向入射从B点出射,则(  )

A.E=

4
3
Bv0

B.E=

2
3
Bv0

C.E=Bv0

D.E=

3
4Bv0

答案

根据几何关系得,带电粒子在匀强磁场中做匀速圆周运动的半径r=

3
R.

因为qvB=m

v02
r
,解得
3
R=
mv0
qB
①.

现把圆形区域的磁场改为竖直方向的匀强电场E,

竖直位移y=

3
2
R,水平位移x=
3
2
R

t=

x
v0
=
3R
2v0

y=

1
2
at2=
1
2
qE
m
9R2
4v02
=
3
2
R
,解得qER=
4
3
9
mv02

联立①②两式得,E=

4
3
Bv0.故A正确,B、C、D错误.

故选A.

阅读理解

Do you know that women’s brains are smaller than men’s? The average women’s brain weighs 10% less than men’s. Since researches have shown that the bigger the brain, the cleverer the animal, men must be more intelligent than women. Right? Wrong. Men and women always score similarly on intelligence tests, despite the difference in brain size. Why? After years of study, researchers have concluded that it’s what’s inside that matters, not just the size of the brain. The brain consists of “grey matter” and “white matter”. While men have more of the latter, the amount of “thinking” brain is almost exactly the same in both sexes.

It has been suggested that smaller brain appears to work faster, perhaps because the two sides of the brain are better connected in women. This means that little girls tend to learn to speak earlier, and that women can understand sorts of information from different sources at the same time. When it comes to talking to the boss on the phone, cooking dinner and keeping an eye on the baby all at the same time, it’s women who come out on top every time.

There are other important differences between two sexes. As white matter is the key to spatial(空间的) tasks, men know better where things are in relation to other things. “A great footballer always knows where he is in relation to the other players, and he knows where to go,” says one researcher. That may explain one of life’s great mysteries—why men refuse to ask for directions … and women often need to!

The differences begin when fetuses(胎儿) are about nine weeks old, which can be seen in the action of children as young as one. A boy would try to climb a barrier (障碍物) before him or push it down while a girl would attract help from others. These brain differences also explain the fact that more men take up jobs that require good spatial skills, while more women speech skills. It may all go back to our ancestors, among whom women needed speech skills to take care of their babies and men needed spatial skills to hunt, according to one research. www. .com

If all this disappoints you, it shouldn’t. “The brain changes throughout our lives according to what we do with it.” says a biologist.

小题1:Which of the following is true according to the first paragraph?

A.Women’s brain is 10% less than men’s

B.Grey matter plays the same role as white matter.

C.Grey matter controls thinking in the brain.

D.Both sexes have the same amount of white matter.小题2:What can we infer from the second and third paragraphs?

A.Women prefer doing many things at a time.

B.Men do better dealing with one job at a time.

C.Women do not need to tell directions.

D.Men have weaker spatial abilities.小题3:Which of the following do you agree with according to the fourth paragraph?

A.Young boys may be stronger than young girls.

B.More women take up jobs requiring speech skills.

C.Women may have stronger feelings than men.

D.Our ancestors needed more spatial skills.小题4:What is the writer’s attitude in writing this passage?

A.Defensive.

B.Persuasive.

C.Supportive.

D.Objective.

单项选择题