问题 计算题

如图为一速度选择器的示意图,a、b为水平放置的平行金属板,a、b相距d,极板长l。两块中央带有小孔的绝缘板紧靠金属板竖直放置,一束具有同一速率的负离子沿水平方向经小孔O进入a、b两板之间。为了测定离子的速率, a、b分别与一输出电压可调的直流电源的正、负极相连,并沿垂直于纸面向里的方向加一磁感应强度为B的匀强磁场。当a、b间电压调为U时离子恰能沿水平直线OO'运动,并从小孔O'射出。若撤消加在a、b间的电压,结果发现离子打在了右挡板上小孔O'下方的P点,测得O'、P间距为。不计重力作用,试由上述条件确定

(1)离子的入射速率v=?

(2)离子的比荷 

答案

(1)(2)

(1)a、b板间电场强度为

            (2分)

射入场区的离子受力平衡

         (2分)

解得:             (1分)

(2)取消电电压后射入的离子作匀速圆周运动,设半径为r,则有

     (2分)

由牛顿第二定律可得

          (2分)

解得: (1分)

阅读理解

阅读理解

       Early in the morning, at noon, or in the evening, in big cities and in the countryside, all across the

United States, you can see Americans running. Men and women, young people and old people are

running. People run everywhere -along thet beaches, throughthe park, down quiet streets or at the gym

(体育馆). Some people even run in their living room.

       Running didn't use to be so popular.  In the 1960s,  runners were mostly athletes and healthy strange

people. In those days, when people saw a runner, they used to say, "Hey, what's the hurry for?" Or they

might say to themselves,  "Is he crazy?" Women almost never ran.  If they did, they might hear a man

shout,  "If I catch you, can you buy me a drink?" But today all this has changed. Men and women of all

ages enjoy running.

       Doctors say many of the health problems in the United States come from these bad habits: eating

too much, smoking cigarettes, and not exercising enough.  Doctors tell us. "Eat less, don't smoke,  and

exercise more. " Running is good exercise because it helps get strong hearts andlungs (肺).  It also helps

most people lose weight.

根据短文内容,判断句子正 (T)误 (F)。 

( )1. Many Americans like running in the morning, at noon, or in the evening.

( )2. Young people run outside and old people run in their living room.

( )3. In the l960s9 running was not very popular. Only athletes and healthy strange people ran.

( )4. Eating too much, smoking cigarettes and exercising caused many health problems.

( )5. Running helps people get strong hearts and lungs and lose weight.

填空题