问题 问答题

电子自静止开始经M、N板间(两板间的电压为u)的电场加速后从A点垂直于磁场边界射入宽度为d的匀强磁场中,电子离开磁场时的位置P偏离入射方向的距离为L,如图所示.求匀强磁场的磁感应强度.(已知电子的质量为m,电量为e)

答案

电子在M、N间加速后获得的速度为v,由动能定理得:

 

1
2
mv2-0=eu

电子进入磁场后做匀速圆周运动,设其半径为r,则:

  evB=m

v2
r

电子在磁场中的轨迹如图,由三角形相似得:

  

L
1
2
L2+d2
=
L2+d2
r

由以上三式得:B=

2L
L2+d2
2mu
e

答:匀强磁场的磁感应强度为

2L
L2+d2
2mu
e

完形填空

Today, people want to keep healthy through sports and exercise. But some of them have some wrong ideas about it.

 “A plate of chicken is a good meal before games because it has much energy.”

 In fact, the best meal before games should have carbohydrates (碳水化合物). Food like potatoes, bread, bananas are rich in it, but chicken and meat are not.

“The best time to exercise is early in the morning.”

Morning is a good time to exercise,but it may not be good for you.If afternoon or evening is OK for you,and you enjoy the feeling of getting healthier,you can choose (选择)any time to exercise.So there is no fixed time for you to exercise.

“If you drink water when you exercise,you’ll feel tired.”

You must have some water during breaks (在休息期间) when you exercise. And after exercising, you must

have enough water. If you don’t drink enough water, then you may feel tired.

 “ Exercising every day is quite important.”

 Wrong. Too much exercise is bad for our health. You need to give your body a day of rest.

小题1:Before games it is good for you to eat ___________.

A.bananas

B.chicken

C.meat

D.beef小题2:The underlined word “fixed” probably means (意思是) ___________.

A.重要的

B.困难的

C.固定的

D.简单的小题3:Which of the following is TRUE according to the passage?

A.Don’t drink water when you exercise, or you’ll feel tired.

B.Evening is the best time to exercise because it’s always cool.

C.You should eat enough meat because they can give you energy before games.

D.Exercising too much is not good for us.小题4:Which of the following will be the best title (标题)?

A.Results (结果) of sports and exercise.

B.Best time for sports and exercise.

C.Reasons for sports and exercise.

D.Wrong ideas about sports and exercise.

单项选择题