问题 问答题

如图所示,所以O为圆心,R为半径的圆形区域内,有一个水平方向的匀强磁场,磁感应强度大小为B,方向垂直于纸面向外;竖直平行放置的极板A、K相距为d,AK之间的电压可以调节,S1、S2为A、K极板上的两个小孔,且S1、S2和O三点在垂直于极板的同一直线上,OS2=R;质量为m、电量为q的正离子从S1进入电场后,自S2射出并进入磁场区域,不计重力和离子进入电场时的初速度,问:

(1)为使正离子射出磁场时的速度的方向与进入时重直,A、K之间的电压应为多大?

(2)粒子在磁场中的运动时间多长?

答案

(1)正离子在电场中做初速度为零的加速直线运动

由动能定理得qU=

1
2
mv2

∵正离子以速度v进入匀强磁场,做匀速圆周运动Bqv=m

v2
r

正离子离开磁场时的速度与进入时垂直,故r=R   ③

解得A、K间的电压U=

B2R2q
2m

(2)离子进入磁场后洛仑兹力提供向心力,由牛顿第二定律得:Bqv=m

v2
R

离子在磁场中做匀速圆周运动的周期为:T=

2πR
v

离子在磁场中的运动时间为:t=

1
4
T⑥

解④⑤⑥式得:t=

πm
2Bq

答:(1)为使正离子射出磁场时的速度的方向与进入时重直,A、K之间的电压应为

B2R2q
2m

(2)粒子在磁场中的运动时间为::t=

πm
2Bq

阅读理解

D

Everyone would like to be remembered. A few people even try to do something that no one else in history has ever tried to do. These people sometimes have their names recorded(记录)in the Guinness Book of Records.

Often it takes a group of people to do something unusual. One such team paddled(用桨划)a bathtub(浴缸)145 kilometers for 24 hours. This group had 13 members. They paddled their bathtub on a river near Ashford, Great Britain on May 28 and 29. 1983. A month later, a racing team of hospital bed pushers in Avon, Great Britain, pushed a hospital bed 16 kilometers in just 50 minutes! However, the distance(距离)record for hospital bed pushing is still held by the men of Bruntsfield Bedding Center. This team from Edinburgh pushed a hospital bed 5,203 kilometers in 1979. It took the team 35 days to cover the distance.

Another group of record makers held the highest dinner party in the world. Nine people from Sydney,Australia, climbed Mountain Huascaran on June 28, 1989. The climbers carried a dinner table. chairs, and a meal up the mountain. When they reached the top, they all put on warm suits for dinner.

People all over the world are trying hard to do something unusual to make a record. It should be understood, however, that now not all the records will be kept in the Guinness Book of Records.

小题1: According to the passage, what record was made near Ashford in May, 1983?

A.A record for paddling a bathtub.

B.A record for climbing a mountain.

C.A record for pushing a hospital bed

D.A record for swimming across a river.小题2:The word cover in the passage means _______.

A.start

B.go through

C.keep

D.know小题3:What was unusual about the highest dinner party held by the nine people in the passage?

A.The people who held the dinner party all came from Sydney.

B.Only nine people held the highest dinner party.

C.They had a meal on the mountain.

D.They carried a dinner table and chairs up the mountain to hold the dinner party.小题4:Which of the following shows the right order(顺序)of what happened in the passage?

a. A group of 13 members paddled a bathtub on a river near Ashford.

b. A team from Edinburgh made a distance record for hospital bed pushing.

c. A group of nine people from Sydney had a dinner party on the top of Mountain Huascaran.

d. A racing team pushed a hospital bed 16 kilometers in just 50 minutes in Avon.

A.a-b-c-d

B.d-b-c-a

C.a-d-b-c

D.b-a-d-c小题5: Which of the following is mentioned(提及)in the passage?

A.Hospital bed pushing was a popular sport in Edinburgh.

B.People living in Ashford usually used a bathtub to carry things.

C.Two different records for hospital bed pushing were given.

D.Hospital bed pushing was held twice a year in Avon, Great Britain.

问答题 简答题