问题 问答题

选做题.在以下两题中选择一题完成.多做不加分

如图所示,足够长的矩形区域abcd内充满磁感应强度为B、方向垂直纸面向里的匀强磁场,现从ad边的中心O点处,垂直磁场方向射入一速度为v0的带正电粒子,v0与ad边的夹角为30°.已知粒子质量为m,带电量为q,ad边长为L,不计粒子的重力.

(1)求要使粒子能从ab边射出磁场,v0的大小范围.

(2)粒子在磁场中运动的最长时间是多少?在这种情况下,粒子将从什么范围射出磁场?

19-B汤姆生用来测定电子的比荷(电子的电荷量与质量之比)的实验装置如图所示,真空管内的阴极K发出的电子(不计初速、重力和电子间的相互作用)经加速电压加速后,穿过A′中心的小孔沿中心轴O1O的方向进入到两块水平正对放置的平行极板P和P'间的区域.当极板间不加偏转电压时,电子束打在荧光屏的中心O点处,形成了一个亮点;加上偏转电压U后,亮点偏离到O′点,(O′与O点的竖直间距为d,水平间距可忽略不计.此时,在P和P′间的区域,再加上一个方向垂直于纸面向里的匀强磁场.调节磁场的强弱,当磁感应强度的大小为B时,亮点重新回到O点.已知极板水平方向的长度为L1,极板间距为b,极板右端到荧光屏的距离为L2(如图所示).

(1)求打在荧光屏O点的电子速度的大小.

(2)推导出电子的比荷的表达式

答案

A、(1)画出从O点射入磁场的粒子运动轨迹的动态圆,能够从ab边射出的粒子的临界轨迹如图所示;

轨迹与dc边相切时,射到ab边上的A点,此时轨迹圆心为O1,则轨道半径r1=L,由qv0B=m

v2
r1
,得最大速度v0=
qBl
m

 轨迹与ab边相切时,射到ab边上的B点,此时轨迹圆心为O2,则轨道半径r2=

L
3
,由qv0B=m
v2
r2
,得最小速度v0=
qBl
3m

 所以粒子能够从ab边射出的速度范围为:

qBL
3m
v0
qBL
m

(2)当粒子从ad边射出时,时间均相等,且为最长时间,因转过的圆心角为300°,所以最长时间:tm=

5
6
T=
5πm
3qB
,射出的范围为:OC=r2=
L
3

B.(1)当电子受到的电场力与洛沦兹力平衡时,电子做匀速直线运动,亮点重新回复到中心O点,设电子的速度为v,则  evB=eE

得  v=

E
B
即   v=
U
Bb

(2)当极板间仅有偏转电场 时,电子以速度v进入后,竖直方向作匀加速运动,加速度为a=

eU
mb

电子在水平方向作匀速运动,在电场内的运动时间为  t1=

L1
v

这样,电子在电场中,竖直向上偏转的距离为  d1=

1
2
a
t21
=
e
L21
U
2mv2b

离开电场时竖直向上的分速度为  v1=at1=

eL1U
mvb

电子离开电场后做匀速直线运动,经t2时间到达荧光屏  t2=

L2
v

t2时间内向上运动的距离为    d2=vt2=

eUL1L2
mv2b

这样,电子向上的总偏转距离为   d=d1+d2=

eU
mv2b
L1(L2+
L1
2
)

可解得  

e
m
=
Ud
B2bL1(L2+L1/2)

阅读理解

The evidence for harmony may not be obvious in some families. But it seems that four out of five young people now get on with their parents, which is the opposite of the popularly held image (印象) of unhappy teenagers locked in their room after endless family quarrels.

An important new study into teenage attitudes surprisingly shows that their family life is more harmonious than it has ever been in the past. “We were surprised by just how positive today’s young people seen to be about their families,” said one member of the research team. “They’re expected to be rebellious(叛逆的) and selfish but actually they have other things on their minds: they want a car and material goods, and they worry about whether school is serving them well. There’s more negotiation(商议) and discussion between parents and children, and children expect to take part in the family decision-making process. They don’t want to rock the boat.”

So it seems that this generation of parents is much more likely than parents of 30 years ago to treat their children as friends.” My parents are happy to discuss things with me and willing to listen to me,” says 17-years-old Daniel Lazall. “I always tell them when I’m going out clubbing. As long as they know what I’m doing, they’re fine with it.” Susan Crome, who is now 21, agrees. “Looking back on the last 10 years, there was a lot of what you could call negotiation. For example, as long as I’d done all my homework, I could go out on a Saturday night. But I think my grandparents were a lot stricter with my parents than that.”

Maybe this positive view of family life should not be unexpected. It is possible that the idea of teenager rebellion is not rooted in real facts. A researcher comments, “Our surprise that teenagers say they get along well with their parents comes because of a brief period in out social history when teenagers were regarded as different beings. But that idea of rebelling and breaking away from their parents really only happened during that one time in the 1960s when everyone rebelled. The normal situation throughout history has been a smooth change from helping out with the family business to taking it over.”

小题1: What is the popular images of teenagers today?

A.They worry about school

B.They dislike living with their parents

C.They have to be locked in to avoid troubles

D.They quarrel a lot with other family members小题2:The study shows that teenagers don’t want to ___

A.share family responsibility

B.cause trouble in their families

C.go boating with their family

D.make family decisions小题3:Compared with parents of 30 years age, today’s parents___.

A.go to clubs more often with their children

B.are much stricter with their children

C.care less about their children’s life

D.give their children more freedom小题4: According to the author, teenage rebellion____.

A.may be a false belief

B.is common nowadays

C.existed only in the 1960s

D.resulted from changes in families

单项选择题